Introduction to Vintage
Electronic Equipment Restoration


There are those among us who collect vintage electronic equipments who have a desire not only to admire the looks of their equipments, but to use them as well. Many of these find the thought of restoring their equipments to be intimidating. The fact is that electronic restoration of vintage equipments can be a very satisfying activity which is not difficult to learn and relatively inexpensive to perform. There is a certain thrill to seeing a piece of equipment working again for the first time in many decades which is unparalleled in most people's experience. This document is dedicated to those who wish to experience that thrill.


I wish to acknowledge the encouragement, criticism, and helpful suggestions of Mike Grimes and Mary Caruth. It would be impossible to acknowledge the contributions of all the people who participated in my education, but special mention should be made of my father James McCarty who got me started in electronics with a Heathkit transistor trainer and taught me many mechanical skills as well as cultivating my confidence to effect repairs, and my mother Jimmie McCarty who has always had faith in me and who, when I was three, helped me convince my father to purchase an encyclopedia from which I learned to read.


This document contains information related to the restoration and use of vintage electronic equipments. While issues related to physical restoration are touched upon, the emphasis is electronic restoration; cosmetic restoration is not the focus. The intended audience is any and all who wish to collect vintage equipments, and restore them to a useful condition, while preserving their vintage flair. Though the emphasis is electronic restoration, particular attention is given to restoration without extensive knowledge of electronics. Additionally, safety issues are discussed, and some practical information related to disassembly and reassembly of equipments are given. The reader is presumed to have read and understood the "Introduction to Components" by this author, and to have basic knowledge of the use of a digital multi meter (DMM) and basic soldering and mechanical skills. Where special skills or uses of other test equipment are required, they are presented.

This document does not cover advanced topics like radio alignment, or rebuilding of damaged switches and the like, nor does it cover trouble shooting techniques. It is intended to aid the newcomer to vintage equipment restoration. It is hoped, however, that even the seasoned restorer may notice his memory being jogged on occasion and find this document useful.

Although every effort has been expended to make this document as complete as an introduction can be, it is only an introduction. It is not possible to foresee and cover every eventuality one may encounter during attempting restoration or use of vintage electronic equipments. Neither the author nor the Vintage Radio and Phonograph Society (VRPS) assumes any liability for any damages, consequential or otherwise, which may result from performing any of the activities described in this document. Some of these activities are inherently dangerous. Where possible, recommendations have been made concerning due caution which must be taken by the reader when performing repairs, but by no means does this mean that the precautions mentioned are sufficient to ensure safety. This material is presented solely as educational and instructive. It is the reader's responsibility to make a determination of what actions to take, and the reader fully indemnifies the author and the VRPS from any and all liability.


One may wonder how it is possible to repair electronic equipments without significant knowledge of electronics. It is certainly true that those who repair electronic equipments undergo extensive training today, as indeed in the past when the equipments under consideration were new. However, by mastering a few simple mechanical skills, like soldering, and with just the knowledge of how to use a DMM and a little knowledge of how to select modern replacements for defective components, one can, with a high likelihood of success, restore vintage equipments to a useable condition. The reasons for this are simple, though not obvious at first sight.

When electron tube equipment first became available, it was sold sans vacuum tubes. At that time, the cost of the vacuum tubes often exceeded the cost of the rest of the equipment, including assembly and shipping costs. As a consequence, in order to improve sales, manufacturers often took a "modular approach" in which a basic radio could be improved, incrementally, by connecting additional modules improving performance, either through enhanced sensitivity, selectivity, or power output capabilities. To these ends, tubes were made socketed, so they could easily be installed "after market" by the end user, whether for basic functionality, or for enhanced performance.

There is another reason tubes were socketed. The vacuum tube is essentially a fancy incandescent light bulb. As we all know, these light bulbs have a certain lifetime, after which they wear out. That is why light bulbs have been and remain today socketed so they may be replaced by the end user. In their heyday, vacuum tubes were the most likely components to fail in electronic equipments, requiring them to be easily replaceable after market, long after they ceased to be a serious expense concern.

When some component other than a vacuum tube became defective, it was not expected that the end user be able to replace, or indeed even to find, the defective component. For this, specialized knowledge of electronics was needed. This was, and is, because equipment failure is normally caused by a "single point of failure", since the likelihood of two components failing at precisely the same moment is very low. Now, a failed component may result in damage to other associated components, but a single point failure is the most common root cause. For someone to make a living troubleshooting and repairing failing equipment meant being able, quickly and accurately, to locate the failed component, ascertain which if any other components have been damaged, and install suitable replacements. A "shotgun approach", in which components are replaced wholesale, or perhaps tested wholesale and replaced as found defective, was not efficient and profitable. Today, the situation is significantly different with regards to restoration of vintage equipments.

As mentioned, in the past the most likely cause of failure in electron tube equipments was the vacuum tube; that is no longer true. The second most likely cause in the past was the paper capacitor. It is now number one. There were two causes of failure common for paper capacitors. One was internal corrosion due to the inevitable residual water present in the paper used as a dielectric reacting with the metallic foils present, which takes place regardless of whether the capacitor has voltage on it. The other was electrolysis taking place in the oil when the capacitor has voltage on it. Both result in irreversible damage to the capacitor. Ordinary paper capacitors had a design lifetime of about five years, and premium paper capacitors had a design lifetime of fifteen years. As a consequence, all paper capacitors are now at least sixty years beyond their expected lifetimes.

By actual count, the author has found that ninety seven per cent of paper capacitors are unacceptably "leaky", and it is a certainty that the remaining three per cent will soon become so if put back into service. Wholesale replacement of all paper capacitors is the necessary action today, in contrast with the past. For one thing, they are almost certainly bad, and for another, in the past only another paper capacitor would be available. Today's capacitors are much more reliable and have much longer expected lifetimes, on the order of fifty years.

After paper capacitors, electrolytic capacitors come next as likely to be defective in vintage equipments. Something like half of those manufactured after WWII can be restored to service through a process called "reforming", though this document does not cover that. Practically all of those manufactured before WWII cannot be reformed. Wholesale replacement of electrolytics is the preferred procedure today. Often, for cosmetic purposes, the original component is left physically installed, but should be electrically isolated from the circuit.

The next most likely defective component type is the carbon composition resistor. Perhaps one third of them will be found to have increased in value to outside their acceptable tolerance. Each resistor should be measured, and suitable replacements installed for those outside their tolerance. Vintage carbon resistors have a tendency to change values when heated. After replacement of a component, one should alway retest any carbon composition resistors attached to the same solder lug. One or more may be found to have "gone bad" due to being heated.

Defective tubes, while still an issue, are not a major cause of equipment failure. One will find a tube with shorts or open filament in perhaps one in ten equipments, or about two per cent of bad tubes. Even when equipment has a bad tube(s), simply replacing the bad tube(s) is not likely to repair the equipment, unlike in the past.

It is a fact borne out by experience that replacing all paper capacitors, electrolytic capacitors, and carbon composition resistors which are out of tolerance will bring most vintage electronic equipments to an operating state electronically, and that this is the most efficient means of doing so today. This procedure may be carried out without specialized troubleshooting knowledge or knowledge of electronics.

Necessary Equipment and Supplies

The tools necessary for electronic restoration of vintage equipments are neither extensive, nor especially expensive. Most of the investment required for electronic restoration is in labor, which is free (grin). However, do not scrimp on tools. Buy the best you can afford. It is better to have a few quality tools than a large number of shoddy ones. When need arises, more may be purchased, or borrowed. Start small, and expand as necessary.

The first "tool" one needs for working on vintage equipments is an adequate work area. Vintage equipments are not noted for being "miniaturized", and one needs adequate work space with adequate lighting. The bench should be illuminated from above with a diffuse bright source of light, and smaller lights which can be aimed directly into chassis should also be available. Small goose-neck lamps and flashlights are useful in this regard. Flashlights with plastic, non conductive, cases are preferred. The work bench should be made of insulating material, as well as the flooring and preferably also the seating. Along with this should come adequate storage space for the other tools, components, and "work in progress" which is waiting for some specialized part or just more time before being worked on again. Shelves mounted to the wall behind the work bench will be found very useful. The work bench should be situated such that easy exit can be made from the room it is in, without having to pass by the bench. This will be important during "smoke testing". Be sure to put tools back where they belong after use. The rule is, when getting out another tool, put back two you aren't using. This helps to prevent clutter on the work bench, and to keep your tools where you can find them.

[photo of workbench]

A certain investment should be made in basic tools. These include screwdrivers, both flat end and Philips. One should get a set of "jeweller's" screwdrivers, and small, medium, and large "regular" screwdrivers. Get a good needle nosed pliers with jaws being smooth and not more than about two inches (4 cm) long, and some tweezers which have sharp points. The tweezers may be found in the cosmetics department of larger stores; don't use the ones with "angled" tips. Expect to spend $15 or so for good pliers, and $5 for the tweezers. Get some lead trimmers, again about $10. Get a set of nut drivers, or a speed handle and a set of small sockets. Don't scrimp here. Spending $15 is about right. One set of regular slip joint pliers will occasionally (not often) be handy, as well as one "vice grip" type pliers. A small vice is a nice touch, but purchase may be deferred until it is needed. Get one which has both "soft" (plastic) and "hard" jaws.


Another "tool" for the workbench is a small dustpan and whisk broom or brush, along with a few empty (and cleaned!) tuna fish or shallow pet food cans for use as trash receptacles on the bench. These should be used to receive solder splashes, cut off bits of wire, and small components which have been removed. Useful for cleaning off bits of dust etc. from the chassis is a basting brush, which looks somewhat like a paintbrush. A toothbrush, brass bristle brush, non conductive ceramic scrub pads, or sand paper help with more stubborn dirt or corrosion. The ceramic scrub pads (usually green in color) are also helpful for cleaning the pins of miniature and loctal tubes. Repeatedly plunge the pins into the pad, twist the tube gently, and remove.


Some somewhat more specialized tools necessary include a soldering pencil of about 30 W along with a holder and sponge. Don't scrimp on the holder. Get a nice heavy one with a guard around the soldering pencil tip and a place to hold the sponge. About $15 for the pencil and $5-$10 for the holder is about right. Get some solder, so-called 60/30 tin lead, with rosin core. Do not use modern lead free ROHS compliant solder, as it is immiscible with vintage solder. NEVER use plumber's solder or acid flux on electronic equipments. Get a solder sucker and solder wick. Be sure to get a solder sucker with a guard surrounding the moving plunger. If you use one with no guard, I guarantee that one day you will smack yourself in the face when you release the plunger, and wish you had one with a guard. A decent solder sucker is $10 or so.

[soldering stand iron and solder sucker]

Get a good wire stripper for removing insulation from wire. Expect to spend $15 to $25 for a decent wire stripper. Get some wire in various colors with insulation rated for the equipment you intend to use it in. Get both stranded and solid wire. Usually 18 AWG is about right. Get some good electrical tape (not friction tape) and some heat shrink tubing in various colors and diameters. Colors were used in vintage equipments to indicate uses of wires, viz. yellow for filament, green for signal, red for B+ or other high voltage, and black for B-.

[wire stripper and wires]

Get a good DMM. Fluke makes several good ones. I like the 73 series Flukes. Another good make is EXTECH. Expect to spend $50 or somewhat more for a meter. A case can be made both for auto-ranging and for non auto-ranging meters. Get whichever you prefer. The abilities to measure voltages to 1000 VDC and 700 VAC, and resistance up to 20 meg ohms are required. The probes should be rated to 1000 VDC and 700 VAC, or better. Don't use a cheap meter on high voltage equipment. Sometimes you are trusting your life to your meter. Get a good one.

The ability to measure current is nice, but not necessary, and it is not necessary to have a "true RMS" capability. There is a saying that "a man can never have too many meters". Well, that's not quite true, but having more than one is sometimes a help. However, buying the second meter is something that can be put off.


You'll want some power strips. Get more than one, or get a really long one. However, don't "daisy chain" the power strips. Each one gets plugged into its own outlet, not into another power strip. Get an isolation transformer. This is a transformer with equal input and output voltages used for purposes of isolating power sources from Earth ground. This helps prevent shocks, even when working on transformer sets.

Buy several "clip leads" in various colors. These are insulated flexible wires with mini-gator clips on the ends. These are indispensable for temporary hookups, extending speaker wires, and connecting test equipment. The colors help in identifying what is being connected to what. Store them by clipping them to a shelf bracket as shown in the photograph, so they don't tangle.

[clip leads]

Get some terminal strips and suitable replacement components. Lay in a stock of various values of resistors and capacitors. It is much more expensive to buy components in single piece lots. Radio Shack (in the USA) sells resistor selections in both 1/4 W and 1/2 W power handling capabilities. These are good value for the money.

Then there are some ultra special tools, that is tools which must be fabricated. Some of them you'll discover only as you need them, but some of them can be anticipated. These include wood blocks of various sizes to use as chassis supports (DON'T use conductive cans for this), "third hands", which can be clothes pins mounted on wooden blocks, and a dim bulb. A dim bulb is a necessity, though they cannot be purchased. One can be made for $5 to $20 depending upon how fancy one wishes to be. As an adjunct to a dim bulb one may want a variac. A variac is not a substitute for a dim bulb, nor is the dim bulb a substitute for a variac. Given a choice, I choose a dim bulb over a variac, but both are useful. Expect to spend $50 for a decent variac. Purchase of a variac may be deferred, but construction of a dim bulb is a necessity. If you feel the need for more guidance, see the information in the appendix. Here you see my dim bulb, isolation transformer, and variac.

[dim bulb] [isolation transformer] [variac]

Another necessary tool is the discharge tool. This is a 10 K ohm 10 W resistor mounted on an insulated handle at least four inches long, like a popsicle stick. One lead of the resistor should protrude beyond the end of the stick 1/2 inch or so, and the other should have a clip lead attached to it. This is used to discharge high voltage capacitors. Attach the clip lead to the terminal at or near ground potential, and with one hand behind your back, and the other holding the handle of the tool, discharge the capacitor by touching all other terminals in turn with the bare lead. This discharge tool is good for up to 600 V. If you need to discharge higher voltages, then a higher power resistor is necessary to withstand the voltage. The one I use is inside my condenser checker, a Sprague TO-6, so I can't show it to you.

[discharge tool]

There is one other class of tools. These are the tools which some newcomers to repair of vintage equipments think they need, but do not. One common initial impression is that one needs a tube tester in order to repair tube equipment. This is untrue, for a few reasons. One is that bad tubes are not the usual cause of malfunction in vintage equipment, as mentioned above. Another is that, while a tube tester may be able definitively to say that a tube will not work, if it has an open filament and simply cannot provide any emission, a tube tester cannot definitively say that a tube will work in any given circuit. The best test of a tube is the circuit in which it is intended to work.

The only real reason for tube testers, except very expensive laboratory equipment used for characterizing tubes, was to sell tubes. Since we are no longer in the business of convincing customers that they need to replace "weak" but serviceable tubes in advance of failure, the need for tube testers is debatable at best. One can make a case for a tube tester being used to check for shorts, since that test is quick and easy to perform, and can catch weak shorts that only develop when the tube is hot. These can be found by using the tubes in the equipment, however, and the best check is to replace the tube with another known to be good. If the behavior of the equipment improves, then the tube needs replacement. It should not necessarily be discarded, as it may prove serviceable in another equipment.

Another is the oscilloscope. The author has an oscilloscope, and has used it a few times with vintage equipment, but the frequency of use is certainly less than once a year, and it has rarely revealed anything about vintage equipment which could not have been discovered with other, less expensive and simpler to use, test equipment.

Feasibility Evaluation

Before beginning restoration of a piece of vintage equipment, one should make a preliminary investigation to ascertain the feasibility of restoration. It is unfortunately the case that the beginner (and some not so beginners) sometimes starts work on a piece of equipment, to find out only after having invested significant labor that there is a difficult or expensive to procure part requiring replacement, forcing him to abandon the effort. The feasibility evaluation should ideally be made before purchase. There are several simple checks one can make on a piece of equipment which can quickly identify certain common serious impediments to restoration. When one is contemplating purchase of an equipment, it is a good idea to have an ohmmeter along.

One should engage all the senses when making the feasibility evaluation. What is the general first impression? Generally ok shape, or has it been sitting in a leaky barn for fifty years? Carefully turn all controls and switches. Do any bind? Is there any resistance? Are there any unusual noises, like scraping? Do the indicators, like the dial pointer, move smoothly? Sniff the equipment. Is there an unusual odor, like animal urine? If the set has a power transformer, sniff it. Is there an acrid odor? An acrid odor from any transformer is usually bad news, since they are expensive. Has any "goo" leaked out of a transformer? Is there evidence of significant overheating, like scorch marks, melts, or burns on the inside of the case? Examine the speaker cone. Small tears or holes are not a problem, but large holes or a warped cone result in a costly repair; re coning costs about $10 per inch of speaker diameter.

If you have an ohmmeter, check the speaker field coil (if present) and voice coil for continuity, as well as the secondary of the output transformer, by unplugging the speaker cable, if the equipment has one. Speaker field coils are 400 ohms to 1000 ohms for AC/DC sets, and 2000 ohms to 4000 ohms for transformer sets, with an occasional 10000 ohms coil in a transformer set. For transformer sets, check the primary of the power transformer by measuring the prong to prong resistance of the power cord with the set turned on (but not plugged in!). It should be on the order of five to fifteen ohms or so. Replacement power transformers cost $50 to $150, and often exceed the value of the equipment to replace. If the chassis is easily removed, check the primary of the output transformer for continuity. Output transformers are less expensive than power transformers, generally, but also cost tens of dollars.

Is the chassis in generally good condition? Do any of the tube sockets look charred, or burnt? Are there coils of wire visible, like transformers, antenna coils, or speaker field coils, which appear to be gnawed or chewed? Are the IF transformer "cans" in generally good shape, or are some of them leaning, indicating excessive force and possibly a replacement or rebuild?

If any of these tests fails, then consider possibly purchasing the equipment for use as an "organ donor" at some time. If it isn't likely that you'll want parts from it, and there is a likelihood that the repair will cost more than the set is worth; don't purchase it, unless it's "just like the one grandpa owned".

A brief word about the selection of the first equipment for restoration. It is suggested that one chose an AC/DC radio receiver which passes the above tests, and which has five tubes, being either 12SA7, 12SK7, 12SQ7, 50L6GT, and 35Z5GT, or 12BE6, 12BA6, 12AV6 (or 12AT6) 50C5 (or 50B5), and 35W4. Choose one which has some cosmetic issues, like pieces missing from the case, bad cracks, etc. This means that you will get it very cheaply, and learner's mistakes won't cost much. Pick a single band receiver, that is AM only, no FM or short wave bands. It is suggested to get a floating ground not hot chassis receiver. Using an ohmmeter, turn on the radio (unplugged) and measure the prong to prong resistance. It should be about 120 ohms. Measure the resistances from each prong to the chassis. They should be 200 K ohms or more.

Equipment Disassembly

While not strictly part of electronic restoration, one generally must disassemble equipment from its case before being able to begin electronic work, so it is reasonable to pass along some recommendations for disassembly. This is a good time to start the service notes for the equipment. Make a folder, and label it with the set model and any other necessary identification. Start the service notes with the date of acquisition, and anything noted during the feasibility evaluation. Actually, if there is a delay between acquisition and beginning work, create the folder as soon as reasonable, and make your initial notes. Each time you begin work on the equipment, make another dated entry in your notes indicating what work was done, filling it in as you proceed, keeping them accurate and complete.

During disassembly, update your notes with annotated drawings of how things go back together. Make your notes as if you were not going to complete the work, but rather handing the set off to someone else to finish. When you wind up putting the equipment aside for six months, and come back to it, you will be that someone else who isn't familiar with the equipment. Some "before" photographs may come in useful later.

This is also a good time to begin initial physical cleaning of the chassis and case. It might be a good idea to begin disassembly away from the work bench, perhaps outside entirely. Remove any large amounts of dust, grime, dead insects and spiders, wasps nests, etc. before putting the chassis on the work bench. Watch out for the smell of rodent urine. This is an indication that hanta virus may be present. Take care. Wear eye protection and a pollen mask to prevent inhalation of dust particles.

As you remove small parts, like screws, knobs, pointers, buttons, and the like, put them into small bottles, like pill bottles, or small plastic bags, like zipper closable sandwich bags. Put small slips of paper in the containers indicating what equipment they came from. Small plastic and wooden parts may be put together in a single container, but metal parts should be in separate containers from plastic and wooden parts, or from other painted metal parts, to avoid scratches. All the small containers should be put into one large, say gallon size, plastic bag, which should be marked or have a slip of paper in it indicating what equipment the parts came from. Larger parts, like cases of tabletop radios, should be put into a box which then contains everything but the chassis. If the chassis is set aside, then the box and chassis should be kept in proximity.

If screws of different diameters or lengths are removed, make a drawing in the notes indicating which are long, etc., unless it is obvious where they go back. Remember that you are describing this for someone who did not see where they came from in the first place. If washers are removed, make a drawing indicating where they came from, and on which side of the hole they go. As knobs which differ are removed, make a drawing of which knob went on which control shaft. Sometimes a band switch knob differs only by having a small dot, etc. Look carefully.

If wires are being desoldered or otherwise disconnected in order to remove the chassis, then a careful drawing indicating where they came from should be made. It is often helpful to put onto the wires small gummed stickers with numbers or other indicators like B+ or GND, etc. as to which wire is which and where it came from. Small bits of masking tape wrapped around the wire, then stuck to itself to form a small "flag" can be useful in this way. If the flag is made large enough, it may be written on. It may also be useful to tag the connector it came from. Small paint pens are available at arts and crafts stores which may be used to put small colored paint dots on wires and corresponding terminals.

Concerning drawings and photographs, neither is a substitute for the other. A well made drawing omits detail not necessary to convey the desired information, and may include detail which would be concealed in a photograph. Conversely, sometimes a photograph captures a detail which was omitted from all drawings. Sometimes, wires get disconnected inadvertently during bench work. A drawing is not going to show you where wires which were not intentionally disconnected came from, but a photo just might.

If large or heavy objects are being removed, like speaker boards from a console radio, remove the screws starting with the lowest, and work your way up. When only the top few screws remain, support the object from the bottom with one hand while removing the remaining screws with the other, to prevent splintering the supports. Don't be afraid to enlist help from an assistant if it turns out you need three hands to do some of the disassembly.

One commonly encountered challenge when removing chassis from cases is knobs which are stuck on their shafts, and must be removed. Look for small headless screws (set screws) in the knobs, and be sure that they are loosened. If the knob is stubborn, then a small piece of thin soft cloth, like a handkerchief or piece of a T-shirt, may be slid behind the knob, wrapped around the shaft, and used to pull the knob off. NEVER use a screwdriver as a pry bar for removing knobs or other "stuck" items from a chassis or case. You will inevitably do damage. Cultivate a relaxed and patient approach. Applying any significant force to a screwdriver will eventually result in a slip, causing a scratch or gouge. Using a screwdriver as a pry bar will result in chips, cracks, or dents in the case, not to mention doing damage to the screwdriver.

Before removing the chassis from the case, it is a good idea to fully mesh any tuning condensers which may be present, to prevent bending their plates, and to maintain them in this state unless the receiver is being actually used. As the chassis is removed from the case, take care and proceed slowly, watching carefully for entangled parts, wires, etc., and rotate the chassis as necessary to prevent tearing, breaking, or otherwise disturbing any protruding components. It is occasionally necessary to remove some internal bracing wood blocks, or other parts of the case, before the chassis can be fully removed. After the case is fully removed, cut a piece of thin cardboard to fit over the speaker cone, and attach it to the frame of the speaker with tape or small bits of wire put through holes to match those on the speaker frame. This reduces the likelihood of speaker cone punctures.

Reassembly is generally just a reversal of the disassembly steps. Be careful when reinstalling screws not to get them "cross threaded". This is most easily done by gently inserting the screw into the hole, and turning it counter clockwise, as if removing it, feeling the screw moving out of the hole until it suddenly jumps into the hole as the threads engage. Then turn the screw in slowly and gently to verify that the threads are properly engaged, before tightening. If multiple screws are used to hold an object on, then thread the screws only just fully in, less than finger tight, but do not tighten them until all are in, so the multiple holes can all be aligned by gently wiggling the object, then take a second pass and tighten. Do not over tighten screws, especially in wood, and extra especially in plastic. Just 1/8 to 1/4 turn past finger tight is usually enough. As you tighten, you'll feel a sudden increase in resistance; that's the time to stop. When reinstalling heavy items, support them from below with one hand while putting in the top few screws, and then work your way down, tightening the screws not quite finger tight, then make a second pass as described above.

A Word about Safety

Now that the chassis is on the work bench, the next step is some further preliminary tests to discover whether there be any serious impediment to successful restoration. Some of these tests involve connecting to the power line. Since line power is potentially lethal, a short word about safety is in order.

There are two kinds of people who should not work around high voltage. The first is the one who fears high voltage. If the thought of a piece of equipment on your workbench having 700 VAC present gives you the willies, then you probably should not perform the tests described in the next section. Hand off the equipment to someone else to perform these tests. When the preliminary out of case tests are finished, and have passed, then you can do the actual work of restoration, which does not involve powering the equipment and the presence of potentially lethal voltages.

The second kind is the one who does not respect high voltage. If the thought of a piece of equipment on your workbench having 700 VAC present doesn't give you pause to consider, then you probably should not perform the tests described in the next section. Do as advised above. Of course, since you don't respect high voltage, you also probably won't take this advice. Please don't read the rest of this document.

Ok, now we have the third kind of person still reading, the one who thinks "Wow! Seven hundred volts! Is that safe?". No, it is not safe. It is dangerous. It can kill. Think about that. So is driving an automobile. Thousands of people get killed every year driving automobiles. A safety expert once commented that the most dangerous activity that people routinely engage in is starting their automobiles. The fact that something is dangerous does not mean that one should fear it. However, anyone who drives without caution is a fool, and so is anyone who deals with high voltage without caution.

Any voltage over 24 VAC or 40 VDC should be considered potentially dangerous. That's why your doorbell, and your thermostat, run on 24 VAC. Unfortunately, tubes (with a few exceptions) require at least 45 VDC for their plate supplies, and that's the ones with low voltage requirements. Most tubes are much more comfortable with 250 VDC to 400 VDC for their plate supplies. Most transformer sets have 500 VAC to 700 VAC present during normal operation. The voltages are somewhat higher during the testing, due to the lack of a load on the transformer.

Furthermore, even if an equipment is turned off, and unplugged, under certain conditions it may retain high voltage for hours or even days. Some television sets can retain high voltages for weeks in an unpowered state.

Both breathing and heartbeat are regulated by minute electrical currents flowing in the body. If these electrical currents get disrupted, say by an electrical shock, then breathing and heartbeat cease. So, it's a good idea to prevent shocks, especially those passing current through the heart or through the head, which controls breathing. Furthermore, muscles may contract violently and involuntarily when they come in contact with high voltage, resulting in cuts, bruises, or other consequential damage. Since the muscles which contract the hands are stronger than those which open them, one may be unable to let go of a source of high voltage.

I recall once as a boy I was on a relative's farm, and saw a spider walking on a bare wire which looked like a clothes line running from the house. I struck the wire with my hand, thinking to knock the spider off. Well, that wire was the high voltage wire for the electric fence used to contain a small herd of cattle, and my hand involuntarily grasped the wire. I found that I could not release the wire. I was stuck. After a moment's thought, I realized that I could break the circuit by jumping from the ground. So, I strained to release the wire and pull my hand back, and when I jumped, my hand sprang from the wire.

At this point, you should be ready to listen to advice about how to deal with high voltage with due caution. At least, one hopes so.

When one is about to begin work on equipment which may have high voltage present, one should remove all unnecessary jewelry. Jewelry is normally made from metal, which is conductive, so remove all rings, bracelets, necklaces, watches, etc. before beginning work. This is a good idea even when dealing with low voltage. Auto mechanics always do the same when dealing with automobiles, because inadvertent contact with battery voltage, 12 VDC, may result in very high currents, which may weld a ring to the source, and heat it to very high temperatures.

Equipment which uses high voltage should ALWAYS be presumed to have LETHAL VOLTAGES present until it has been verified safe. TURNING EQUIPMENT OFF IS NOT ENOUGH. As mentioned above, HIGH VOLTAGE MAY LINGER. Furthermore, if the equipment is still plugged in, then potentially lethal line voltage is present, at least up to the switch. Also, SWITCHES HAVE BEEN KNOWN TO FAIL "ON". ALWAYS consider that HIGH VOLTAGE IS PRESENT IF EQUIPMENT IS PLUGGED IN. ALWAYS TAKE HIGH VOLTAGE PRECAUTIONS when handling equipment until you have verified that it is UNPLUGGED, and have verified by ACTUAL MEASUREMENT that the high voltage has been drained. When making these checks, always take care to NOTE THE FUNCTION AND RANGE THE METER IS SET TO. Also take note that CAPACITORS CAN "RECOVER CHARGE". That means that, even when the capacitor has been discharged to a low voltage on its terminals, it may have residual energy stored in it, and the voltage may begin to rise after any load is removed.

A reading of 0 V is not always zero volts. Especially BE WARY of having your METER SET TO THE PROPER FUNCTION. A meter set to an AC voltage range may measure 0 V when in contact with 300 VDC, and vice versa a meter set to measure VDC may indicate none present when there is high AC voltage present. Then there is the range factor to consider. You may think your meter is telling you it has 300 mV present, when it is indicating 300 V. This is especially something to consider if you have an auto ranging meter. YOU MAY NOT KNOW what range the meter has selected. ALWAYS VERIFY YOUR METER'S SETTINGS.

The MOST IMPORTANT high voltage precaution one can take is simply NEVER to COME INTO CONTACT with it. Don't put body parts near it, and you won't get shocked. However, one must always prepare for the worst. In the event of an accidental shock, one wants to keep the shock away from the heart and head. So, when approaching equipment which may have high voltage present, ALWAYS KEEP ONE HAND BEHIND YOUR BACK, and your HEAD BACK FROM the equipment. An inadvertent shock is then less likely to have permanent consequences. RESIST THE URGE TO TOUCH THE CHASSIS when approaching areas of the chassis where high voltage may be present. Current, to flow, needs two points of contact. Simply contacting a high voltage point does not cause current to flow unless there is another point of contact.

Get and use test equipment rated for the voltages present in the equipment you are going to restore. DON'T USE CHEAP METERS which may not withstand the high voltages present. Air is an excellent insulator, good to about 30,000 V per inch. The insulation on clip leads should not be considered adequate insulation for high voltage. Make sure NOT TO TOUCH ANY CLIP LEADS you have connected to high voltage points when high voltage may be present. KEEP AIR between your body and anything not adequately insulated. Note that some TVs have high voltage points with about 30,000 V present, so keep at least two inches of air between you and those points.

When measuring voltages when high voltage may be present (even if measuring a low voltage), attach ONE lead of the meter with a clip lead to the reference point (usually B- ground), and with ONE HAND BEHIND YOUR BACK, use the other hand to put the meter probe on the points you want to measure. It is a good idea to wrap electrical tape around all but the last 1/8 inch (3 mm) of the probe to avoid inadvertent short circuits.

It is a good idea to be insulated from ground when making high voltage measurements. An isolation transformer helps in this regard. Other helps are insulating workbenches, flooring, and seating.

Another point to ponder is that modern tools differ from vintage tools. There are certain special adjustment tools sometimes needed for various alignments. MODERN VERSIONS OF THESE TOOLS MAY BE MADE FROM CONDUCTIVE PLASTIC, since high voltages are no longer much present in modern equipment, and small sparks may damage modern devices. Bear this in mind.

I began dealing with high voltage when I was 14 or so years old. I received my first shock when repairing high voltage equipment a few years ago, when working on a microwave oven. I had discharged the high voltage (multiple KV) capacitor, and installed a clip lead across its terminals for several seconds. I then removed the short, and verified with my meter that there was no high voltage present. I then began investigating various points of interest to me, and a few minutes later my right hand came into inadvertent contact with that capacitor's terminal while my left was holding the chassis. I received a shock, through my heart, which made my body tingle and jump for an instant. The capacitor had "recovered charge". That was my first moment of fear when dealing with high voltage. I had shown that capacitor inadequate respect, and like the Mafia Godfather who felt slighted, it gave me a friendly warning: "I forgive you this time, but don't let this happen again."


On another note, high voltage is NOT THE ONLY DANGER present on the work bench. Soldering irons get HOT, and need to be treated with respect. Do not stare at the underside of your chassis and grope for the soldering iron. Look at the iron while you pick it up, and while you move it to the equipment. As you replace it in its holder, do the same. If you should drop your iron, LET IT FALL. Do not try to catch it. It is a good idea to wear eye protection when soldering. Rosin sometimes suddenly boils, splattering small droplets of solder. Molten solder in the eyes is not a good thing.

When cutting leads or wires, put your finger on the end to be cut off, preventing the shock of severing from causing it to fly off. The ends are sharp, and it is not good to have sharpened missiles flying around the work bench.

Keep the top of your work bench clean and free of debris, especially conductive debris, like solder splashes and bits of cut off wire, etc. USE those little trash cans. Additionally, concerning conductive debris, NEVER use steel wool or emery (either emery sticks or paper) on or near electronic chassis, nor on or about your workbench. Both are conductive, and may cause short circuits. Both are also magnetic, and will work their way into speaker magnets, where they'll play havoc with the speaker. If you want to use steel wool during cabinet refinishing, do it in another room, and clean up before approaching the work bench. Use non conductive ceramic scrub pads, sand paper, or brass bristle brushes for cleaning rust from chassis.

Some tools are sharp. If you have an assistant, then when transferring tools, always transfer them handle first. Keep sharp tools in their sheaths, or closed, or in some other safe condition. Put sharp tweezers' points in a wine cork, for example. Always keep your soldering iron in its stand.

DO NOT work on equipment when you are tired or ill. Many mistakes are made late at night when one decides to do "one last thing". NEVER do the "last thing". Always put it off until later. DO NOT work on equipment when you are under the influence of drugs. If you are having a bad allergy day, take your antihistamine, and put off working on the equipment to another day. It may prevent a bad burn from your soldering iron, or a shock. Don't work on equipment after having a glass of wine.

In general, use common sense, don't touch anything you don't have to, even the chassis, and there won't be a problem. Get careless, and it may be the last time.

Preliminary Tests

The chassis is now on the work bench ready for restoration to begin. Where do we start?

The first step should be to obtain the service literature if possible. Make a copy of the original and retain it in your files, and make service annotations on the copy, as needed. If no service literature is available, then make some careful drawings for your service notes as you go, and as needed. If necessary, make your own partial schematics as you go. Ideally, the search for service literature was started before disassembly, so the literature is available when the chassis hits the work bench.

Spend some time studying the general type and layout of the equipment. Is it a radio receiver? Then is it a Tuned Radio Frequency (TRF), Regenerative, or Superheterodyne? Where are the various stages located? If a Superhet, then is there an additional TRF stage? Are multiple bands present? What are they? Start getting a feel for the "lay of the land" so to speak.

Before embarking on a lengthy equipment restoration, it is important to check for any serious impediment to a successful restoration, i.e. defective components which are expensive or difficult or slow to obtain. If one is intent on restoration, regardless of cost, then it is important to start the search for difficult to obtain parts early on, so they will be available when the time comes.

Even if no serious impediment was noted during feasibility checks, those checks were only cursory. Now, a serious search for defective expensive components needs to be made.

Power Transformer

The single most expensive component is the power transformer, if one is present. That is our first "show stopper". Measure the resistance of the primary of the power transformer. For 120 VAC transformers, the resistance should be in the range of 5 ohms for a higher power one to 15 ohms or so for a lower power transformer. Verify continuity of each of the low voltage windings. They'll be less than up to perhaps one ohm, generally. Verify continuity of the high voltage winding, which may be several hundred or perhaps up to 2000 ohms. If you have service literature which indicates the resistances, verify them. Any serious discrepancy means a likely transformer replacement. If any winding is "open", then you may try to peel back the cover of the winding, and perhaps find a break near where the external connection is made. If so, it's often possible to repair a broken winding. If not, the transformer must be replaced. Defer the check of filament windings until after the set has been detubed and all dial lamps removed, to prevent interference.

When all windings show continuity, detube the equipment, and remove all incandescent lamps, like dial lights. Mark each tube as you remove it, noting its type and location on the chassis. Use little stickers or other means. These tubes go into a box or bag marked with the equipment identifier. If the rectifier is solid state, then desolder the connections to it and to the filter capacitors. We want there to be no load on the power transformer. If necessary, replace the power cord with a new one. Be sure to put the LINE wire to the switch side. Check continuity from prong to prong, and that the resistance is not materially greater than that of the primary winding. Verify that the power switch on the equipment to be tested functions properly by switching it off and on while measuring the resistance.

When you have good continuity through the primary power circuit, connect a variac (if you have one), isolation transformer, and dim bulb in series, but do not plug these into a source of power. The exact order in which they occur is not important, but I like the dim bulb to be the last in the chain so it registers only the power consumed in the equipment to be tested, and not the variac and isolation transformer. Turn off all switches on the variac, transformer, and dim bulb, if they have such. Set the dim bulb to limit if it has that provision. Insert a 40 W bulb into the dim bulb receptacle, and plug the equipment to be tested into the dim bulb. Turn the variac, if you have one, to minimum voltage. Turn on the equipment to be tested. Plug in the variac or dim bulb to a source of power.

If you have a variac, then turn it on, and ramp the voltage up from minimum to 120 VAC over a period of about five seconds or so, while watching the dim bulb. If the bulb starts to glow, then immediately turn the variac back to zero. If you do not have a variac, then simply engage power with the dim bulb, or by turning on a power strip, etc. If the bulb starts to glow, immediately switch off power. Otherwise, the 40 W lamp should not glow more than perhaps an almost imperceptible very dull red. Any real illumination indicates a problem.

If there is a problem, then unsolder the secondaries from their loads. There may be a short in some filament wiring, or other short circuit loading the transformer. Also check the primary wiring for a short circuit somewhere. If the primary circuitry is good, and the secondaries are all disconnected, but the lamp still glows brightly, then the power transformer is defective.

When the lamp does not indicate a problem, it is time to verify voltages. Turn down the variac to 0V if you have one, turn off the variac and/or dim bulb, and unplug the variac or isolation transformer. Select a winding to test. Set your voltmeter to measure VAC, and set the meter to its highest range if it is not auto ranging. Connect one meter lead to one end of the winding to be tested. If the winding is center tapped, connect to the center tap, otherwise just select one end of the winding. With the other lead of the meter disconnected from the equipment to test, plug in the variac or isolation transformer, turn on the dim bulb set to limit, and ramp the voltage up again, while watching the dim bulb. If the bulb remains dim, then with one hand behind your back, use the other to put the probe on the other ends of the winding in turn, recording the voltages present in your service notes, along with which winding is being measured. As necessary, reduce the range setting on the meter, but only after making a measurement with it set to the highest voltage range. Do not risk damage to your meter by inadvertently connecting it to a high voltage winding when it is set to a low voltage range. Ramp the voltage back down, or turn off the dim bulb, and unplug. DO NOT TRUST YOUR LIFE to the action of a $2 switch. Unplug the variac or isolation transformer from the line power.

Do not attach both leads of the meter to a winding, and then switch power on or off. This can result in kilovolt level voltage spikes being applied to your meter, possibly resulting in damage. Only attach one lead, and after the power is on, attach the probe to make a measurement, then remove the probe, and then power down. After each measurement, be sure to reset the volt meter to its highest AC range.

Repeat the measurement for each secondary, and compare the voltage readings obtained with those expected for the transformer. Expect the voltages to be perhaps ten per cent high. A 5 VAC rectifier winding may measure 5.5 V or perhaps a little higher. A 6.3 VAC winding may be expected to measure 7 VAC or so, etc. For windings which are center tapped, add the two half voltages to get the full winding voltage. The two sides of the winding should have voltages which are within a per cent or two of each other. For example, a winding shown as 150-0-150 (or 300 VCT) may measure 177.3 V, and 178.2 V when not under load. Any center tapped winding which is not "balanced" is defective.

If it is center tapped, do not measure the high voltage winding across its full secondary. When not under load it may have a full voltage of 900 VAC or more, which may exceed the rating of your meter. Besides, it is unnecessary, and one should not expose his meter and its insulation to unnecessary stress.

If all windings have reasonable voltages, the center tapped ones show good balance between the halves, and the bulb remains dim, then the transformer can be presumed to be good. One other check to make, if one can, is the leakage between the windings and each other, and to the chassis. One can do this with a condenser checker. Anyone who knows how to use a condenser checker to check for leakage knows how to perform this test, so details are not given. Check each pair of windings, and from each winding to the chassis. This test is recommended for those who can do it, but it requires special equipment.

If the transformer is found defective by the above tests, then it is time to evaluate whether to proceed. A replacement power transformer can be expected to cost $50 to $100, or perhaps a bit more. When selecting a replacement power transformer, always consider issues related to physical mounting of the replacement.

Output Transformer

The output transformer is the next possible stopper. Measure the primary resistance and compare it with the service literature, if available. If no resistance value is available, then look for a few hundred ohms to perhaps 1000 ohms. Disconnect one end of the secondary, and check the secondary for continuity. A few ohms maximum is expected here, perhaps less than one ohm. Check the speaker voice coil for continuity while it is disconnected from the transformer. It should measure from 3 or so ohms to perhaps 10 ohms, except in intercom systems where perhaps 40 ohms to 50 ohms may be found. If a winding is found open, there is a possibility of repair, as with power transformers. Try peeling back some of the insulating wrap, and possibly find a break near the terminal or wire connection.

As a quick check, use a 9 V battery and a couple of clip leads to check the output transformer and the speaker voice coil. Attach one end of a clip lead to one side of the 9 V battery, and the other end to one side of the primary of the output transformer. Connect one end of another clip lead to the other pole of the 9 V battery. Making sure not to come into contact with any conductive part of either clip lead, hold the free end of the second clip lead by its insulation, and tap it very briefly to the unattached end of the output transformer primary a few times. If the output transformer and speaker voice coil are good, you should hear some pops from the speaker. Even an electrodynamic speaker with an unenergized field coil will make weak popping sounds during this test. If no pops are heard, then there is a problem with either the speaker voice coil, or the output transformer.

For a more stringent test, and if you have the equipment and time, use a filament transformer to inject about 12 VAC into the primary of the output transformer. Look for a hum to be heard in the speaker. Measure the voltage on the secondary of the output transformer. If you know how to do the calculations, use the voltage ratio to verify the impedance ratio, using the tube's recommended plate load and the voice coil impedance. If the transformer has a center tapped primary, measure each half winding's voltage, and compare for balance.

If the output transformer needs replacement, expect to spend $20 to $50 or perhaps more for high powered equipment, and make a decision about whether to proceed.


If the voice coil is open, then the speaker must be replaced. If the cone is warped, it must be re coned or replaced. Expect to spend $10 per inch of diameter of the cone. If the speaker has a field coil, check its resistance per the service literature, or according to the suggested values given in the Feasibility Evaluation section above. If the field is open, then it may be possible to repair it by peeling back some insulation and finding a break. If not, then the speaker must be replaced. There is a possibility of getting the field rewound, or of substituting power resistors and doing a redesign of the power supply, but this document does not address that.

Other Coils

Other coils and transformers may be cursorily checked simply by looking for continuity; sometimes actual resistance values are given in the service literature. IF transformers often have coil resistances in the 10 ohms to 20 ohms range. A bad IF transformer can usually be replaced without too much trouble, but the set loses some originality. However, there are some workarounds for IF transformers with either primary or secondary open. Some other coils are more difficult to deal with, especially with multi band radios.

If there are any parts noted during the preliminary checks which one needs to get repaired or to replace, now is the time to get that in progress. Send off speakers for re coning, or start locating a suitable replacement, etc. Make entries in your service notes showing the date and what components were ordered, costs, etc. If there are any frozen controls now is the time to try to free them up, along with dragging switches, etc. If you need any special drive belts for the tuning mechanism, or more dial cord, get started putting together an order.

Basic Repairs

At this point, there only remains the identification of inexpensive parts to replace, and the actual restoration work can commence. If any tube sockets need to be replaced, then make a careful drawing of the components connected to them, before removing them. Only unsolder one end of the components. Drill out the rivets, and replace the sockets. Now is also the time to replace volume and tone controls which are hopelessly frozen and cannot be freed.

If you have service literature, go through the parts list, and for each paper (tubular) capacitor, write down its designation, value, and the selected replacement you intend to use in the service notes. If you don't have service literature, then simply make up your own designation, including where in the circuit it is mounted, or its function. Do the same in another list for the electrolytic capacitors. If you don't have a complete set of necessary replacements, add them to the order sheet.

Make a similar list of all the resistors, but leave room for values. Then measure each resistor. It may be necessary to unsolder one end of a resistor if there are other components in parallel with it, in order to get an accurate reading. Record the measured values in your service notes. For each one which is out of tolerance, select a suitable modern replacement, and annotate your service notes appropriately.

If you have a schematic, circle in red each component needing replacement.

At this point, you should be able to put together a fairly comprehensive list of components which need replacing. Put together your order of components which you do not have in stock, or which this repair is going to deplete, and send off an order. The total cost of replacement parts (other than the expensive items noted above) is likely to be less than $20.

Now begin the actual repair work. As you replace components, check them off in your service notes, and on the circles you made on the schematic. Make an entry for each day you do work in your service notes indicating the date, and what components were replaced, or other actions or decisions were made. Write down any questions you may have, or useful observations. If some component ends up with only one end connected, the other end awaiting some replacement part, make a drawing of where it goes. Remember, someone else is going to finish this work, and he didn't see what things looked like before. Photographs may also be useful here, both "before" and "after".

Replace all paper capacitors, wholesale. Replace the electrolytic capacitors wholesale. If an electrolytic is in a can which fills a hole in the chassis, leave the can in place, but remove all electrical connections. Replace all out of tolerance resistors. Sometimes some ingenuity must be used to come up with a suitable mount for the replacements. Adding a terminal strip is sometimes helpful. Try not to make "flying connections" with components simply floating in space.

As you replace components, try to position the replacement as nearly as possible in the same position and orientation as the original. Use good lead dress, using the original as a guide. Keep leads no longer than necessary. Estimate how long to leave the leads, and trim them leaving a little more than you think you'll need before installing the component. Then install and crimp the leads through or on their mounting lugs, and after soldering trim again. Re measure any carbon composition resistor which is attached to a lug which had solder work done on it, to verify that it has not changed value.

If you need to replace a volume control or tone control, etc., leave all the components connected. Unscrew the nut holding the control, and pull it back from its mount. Insert the replacement and install its nut. Then, lug by lug, unsolder the components from the lugs on the control and the switch, if any, and transfer them to the corresponding lug on the replacement. Most likely, one of the components is a paper capacitor. This is a good time to replace that capacitor. This technique can also be useful for tube socket replacement.

If the rectifier is a vintage selenium (Se) one, then the recommendation is to replace it with modern components. A single Se rectifier has the appearance of several stacked plates with a screw through them. A bridge type may look simply like a flat plate with four terminals, and in some european equipments may look like a black plastic multi section electrolytic capacitor. This shows a Se rectifier I removed from a TransOceanic radio.

[selenium rectifier]

The suggested replacement is one or more 1N4007 silicon (Si) diodes. The 1N4007 can withstand 1000 V, and can pass 1 A of current. These ratings are adequate for all but the most demanding equipments. If B+ is 500 VDC or more, the 1N4007 may not be adequate, but Se rectifiers were rarely used for such high voltages.

If a single Se rectifier is used, then a single Si one may replace it. The replacement diode has a band around one end. This is the cathode, and goes to the B+ connection, the other end without the band goes to the source of AC. The Se rectifier may be left in place, if desired, and one of its lugs used as a mounting point for the replacement. The other lug should not remain connected to the circuit. If the Se rectifier is a bridge type, then it should be completely removed from the circuit, and replaced with four Si diodes. Sometimes as with replaced electrolytic capacitors one must use some ingenuity in devising a mount for the replacement. This image shows the TransOceanic the Se rectifier came from after the replacement. The long screw originally went through the Se rectifier. It now holds a terminal strip, which is also used to hold the replacements for the electrolytic filter capacitors. If you look closely, you'll see the replacement 1N4007.

[replacement for Se rectifier]

If your equipment has a power transformer, put a fuse in the primary circuit of the power transformer. Select a slow blow fuse rated perhaps 25-50% higher in current than you expect the set to draw, a fast blow fuse at perhaps 300% of normal current. Don't omit this fuse. Though your equipment is now in a usable condition, it is still vintage. Unless you replaced the power transformer, it is still old, and the insulation inside it is still old, and may fail. This does sometimes happen. The purpose of the fuse is not to protect your equipment, but your house. Even if you replaced the transformer, put in a fuse. It's just good sense, and it's not expensive. Put the fuse in the switched side of the primary circuit, preferably between the switch and the primary. That way, the fuse won't be "hot" when the switch is off.

At this point, there may be a few additional repairs necessary. Perhaps a re coned speaker has arrived and needs to be installed. Perhaps the dial cord needs re stringing. Some can be deferred, others cannot wait. Perform any additional repairs which cannot be deferred, and then we'll be ready for the First Power On.

First Power On

It's time for the moment of truth. Some call it the smoke test. I call it First Power On (FPO). First, ensure that you can exit the room the test bench is in without having to pass by the equipment under test, and can cut off power to the equipment without reaching over or near it. Reinstall all the tubes and lamps. Connect a variac (if you have it) isolation transformer, and dim bulb in series. Turn the variac OFF, set it to 0 V, and set the dim bulb to OFF and LIMIT if it has that capability, and install a 40 W lamp. Turn the equipment to be tested ON and plug it into the dim bulb. Plug in the variac, and turn it ON. Turn ON the dim bulb. Ramp up the voltage on the variac over a period of 5 seconds or so, while watching the dim bulb, or just observe the dim bulb when you turn it on. The bulb should brighten up to perhaps yellow brightness, then dim down fairly quickly. If the lamp glows brightly, then there is a problem. Immediately remove power, and ascertain what the problem is.

Otherwise, allow the equipment to warm up. It should be able to try to operate somewhat normally. Expect the dial lamps to appear slightly dim, but otherwise normally lit and especially not extra bright. Allow it to run this way for perhaps five or ten minutes, then turn off the dim bulb, and turn the variac down to 0 V. Unplug the variac or isolation transformer. Replace the 40 W lamp with a 100 W bulb, and reapply power as before. This time, the bulb should glow no more than a dull red.

Allow the equipment to run this way for several minutes, perhaps half an hour. The dial lamps should appear practically fully lit. If all seems well, and the power transformer (if present) isn't getting hot, then switch the dim bulb to FULL POWER. The equipment should warm up completely in a few moments, and normal operation should commence. If all looks well, allow it to play for a couple of hours while observing it closely. If any unusual smells, smoke, dial lamp dimming and brightening, or other unusual behaviors manifest, then immediately remove power, and make necessary corrections. After each correction, restart with another FPO using a 40 W lamp. Once everything looks good, it's time to reinstall the chassis in the case.

After reinstalling the equipment in its case, repeat the First Power On procedure with a 40 W lamp, to ensure that nothing untoward happened during re installation. If all looks well, then congratulations! Enjoy your new old electronic equipment!


While it is the sincerest wish of the author that this document has helped someone more fully enjoy the hobby of vintage electronic equipment restoration, it is no more than the barest of introductions. Be sure to draw upon other resources, especially the experience of others who have been active in the hobby. You'll be surprised at how willing to help others are.

Appendix: Constructing a Dim Bulb

A dim bulb is a necessary but impossible to purchase test equipment; you'll have to construct one. This means that you'll get to decide for yourself just how fancy to make it. It may be as simple as an extension cord which has a lamp socket inserted in the LINE or HOT side. The one I made is fairly elaborate, with separate ON/OFF and LIMIT/FULL POWER switches and sockets for the lamp and load, allowing me to install limit lamps as small as 4 W or as large as 700 W. In any case, the cost for even an elaborate dim bulb should not exceed $20.

Feel free to make your dim bulb the way you want. Don't feel constrained to use my design or construction techniques. Some people put fuses in their dim bulbs. My only real recommendations are to be sure that it is adequately insulated, the power line has adequate strain relief, that the controls (if any) are properly labelled as to use, that the bulbs be easily changeable, and that the case not be metallic and especially not grounded. That's it.

To construct an elaborate one like mine, you'll need these supplies, all of which may be obtained from any large chain hardware store:

Estimated Cost to Build a Dim Bulb
Item Cost
dual outlet
dual switch
double wide outlet box
cover plate
polarized extension cord
outlet to lamp socket adapter
lamps (each)

The dual outlet has a link on each side. These links make the dual outlet powered by a single set of wires, and "daisy chain" capable. We need to separate these two outlets from each other. Simply use a screwdriver to break the two links, making two separate outlets. Cut the socket end off of the extension cord and discard it. Cut off about a foot of the extension cord for use as internal wiring. Strip the ends of the wires, and insert them into the outlet box. Secure the wire by tying a knot around the entry to the box, to act as a strain relief.

Connect the LINE wire to the switch to be marked POWER or ON/OFF. This is the wire connected to the narrow prong on the extension cord. Connect a piece of the cut off wire from the other terminal of this switch to the HOT side of the outlet marked LAMP, the one with the narrow slot. In this way, when the dim bulb is OFF, the HOT connection is completely removed. Also connect a wire from the LINE side of the LAMP outlet to the LIMIT/FULL POWER switch.

Connect another piece of the cut off wire from the NEUTRAL side of the lamp outlet to the HOT side of the LOAD outlet. This places the lamp in series with the LINE connection. Yet another piece of wire goes from the NEUTRAL side of the lamp to the as yet unused terminal of the LIMIT/FULL POWER switch. In this way, when the switch is in parallel with the lamp, and when it is in the OFF position, the lamp is in series with the LINE connection, but when the switch is turned ON, the lamp is shorted, and full power is available to the load.

Next, connect the NEUTRAL wire of the extension cord to the NEUTRAL side of the LOAD socket.

[dim bulb] [circuit diagram]

You'll also need a few lamps. I have 4 W lamps (night lights), 7 W lamps, 40 W lamps, 100 W lamps, and 150 W lamps. I like to use the ones with clear bulbs, not frosted, so I may directly view the filaments in the lamps. Incandescent lamps are getting a little hard to find.