This document is intended to present what superheterodyne alignment accomplishes, and how to perform an alignment. Enough background is given to enable the reader to understand service literature, and to develop his own alignment procedure when none is available.
It does not give a single, detailed, cook book type step by step procedure to align every receiver. The details of receiver alignment are too varied in their specifics by individual receiver for that to be of use. Each receiver must be handled on an individual case by case basis.
The general presentation format for each adjustment performed is
The distinction between the purpose of an adjustment and the goal of an adjustment may seem obscure, but may perhaps be explained as the purpose is how the adjustment affects the circuitry being adjusted, whereas the goal is some performance objective of the receiver as a whole.
Although a general or usual procedure is presented, if the manufacturer has provided a detailed procedure, it should be followed. The background necessary to tailor the general procedure to a given receiver is given for when there there is little or no alignment information available.
This document covers the alignment of tube type single band AM broadcast band receivers, not multi band, communications, FM, TV, nor other receivers, though here and there other receiver types are mentioned briefly where they may differ. However, that does not mean that the information is inapplicable to these other receiver types. The principles and overall procedure both apply to all superheterodyne receivers. A thorough understanding of the information presented here will stand the reader in good stead when he moves on to these other receiver types.
The use and adjustment of voltmeters and signal generators and the like is not addressed. There are too many different ones available. They are like people. People are all alike, but people all have different personalities; no two are the same. Get the user manual for your meter and your signal generator, and familiarize yourself with their controls and operation. It is also advisable to check the signal generator for accurate tuning, or to use it in conjunction with a frequency counter, for best results.
Superheterodyne receiver alignment data and instructions are often so sparse as to seem mysterious, as in "How could anyone know what to do with these three numbers?". Even when the instructions are given in great detail, there is usually no motivation or insight provided as to why the adjustments are being made, nor what they accomplish. This document explains the goals of superheterodyne alignment, how each adjustment helps achieve them, and gives some practical advice on how to perform an alignment with examples.
The goals of superheterodyne receiver alignment are threefold:
Sensitivity is the ability to receive weak or very distant stations. Generally, five or more adjustments have for one of their goals maximizing sensitivity of the receiver.
Selectivity is the ability to select the signal of the desired station from among all those intercepted by the antenna, and to reject other undesired signals of whatever origin. There are four important kinds of selectivity in superheterodyne receivers:
The receiver must select for the signal from the desired station, and discriminate against undesired signals. There are usually five and sometimes more tuned circuits which are adjusted such that they respond most strongly to the desired signal, and discriminate against undesired signals. These are located in the antenna circuitry and IF amplifier circuitry, and sometimes in other RF amplifier stages.
Adjacent channel rejection is the ability of the receiver to reject signals from stations operating on a frequency near to that of a desired station. Stations are assigned to frequencies which differ by multiples of 10 KHz in the U.S.A. Such discrete frequency assignments are called channels (think of TV channels). Stations in a given area are never assigned to adjacent channels, as an aid to the designers of receivers in achieving adjacent channel rejection.
Adjacent channel rejection is achieved through proper IF transformer design, construction, and adjustment. Engineers and manufacturers do the former; the latter is performed during alignment.
The single most important advantage that superheterodyne receivers have over other receiver types is constancy of bandwidth. Other receiver types have the disadvantage of bandwidth that varies with tuning, which adversely affects adjacent channel rejection. The superheterodyne has a vastly superior performance in regards to adjacent channel rejection. This comes with an important disadvantage, reception of signals not intended to be received, or spurious reception. One type of spurious signal sensitivity is called Image Frequency Reception. All superheterodyne receivers include circuitry designed to suppress image frequencies.
The usual way image reception is suppressed is by having the antenna circuitry tuned to the frequency actually being received. The frequency the receiver is tuned to is set by the IF and the local oscillator. It is important that the antenna circuitry be tuned to the same frequency for good sensitivity. The difference between the frequency the receiver is tuned to and the frequency the antenna circuitry is tuned to is called the tracking error. All superheterodyne receivers have at least one adjustment intended to minimize tracking error.
Another form of spurious signal reception is a susceptibility to receiving signals at the intermediate frequency. The usual source of this signal is the IF amplifier in the receiver itself. Circuitry to suppress this spurious reception is usually only included in receivers with external antennas. Those receivers having IF suppression circuitry have an adjustment for it.
Dial tracking is having stations appear on the dial at the indicated frequency marker on the dial. If a station at 820 KHz appears on the dial at 850 KHz, we say there is a 30 KHz dial tracking error. All superheterodyne receivers have at least one adjustment whose primary goal is to achieve proper dial tracking.
Before proceeding into how to align a receiver, it is perhaps worthwhile to discuss when to align. Alignment is an adjustment intended for an otherwise perfectly performing receiver. It cannot compensate for circuit defects.
There is no point in attempting to align a receiver unless basic repairs have already been accomplished. That means verifying all transformers and coils for continuity, replacing all paper and electrolytic capacitors, and replacing all out of tolerance resistors. Alignment should be considered a "last resort". All other causes for poor performance must be explored and eliminated first.
Consider the goals of alignment. If the problems a receiver has are not the goals of alignment, then alignment is not likely to help. Alignment is a procedure which is intended to improve performance. Misalignment is not likely to result in no reception at all. Lack of sensitivity may be a symptom of misalignment, but may have other causes, like weak tubes. Unless another problem associated with misalignment, like overly broad tuning or dial tracking errors is present, alignment is not necessarily indicated solely by lack of sensitivity.
If at any time during an alignment it is found that one of the adjustments has little or no effect, or has an erratic effect, that is an indication of a problem with the component being adjusted or an associated one. Stop the alignment, and investigate the cause. It may be that a coil has shorted turns preventing it from being tuned properly, or there may be a corroded ground connection, etc. Adjustments should have smooth effects.
There are three stages to alignment: intermediate frequency adjustment, dial tracking adjustment, and tracking error adjustment. The IF adjustment affects the other two, and the dial tracking adjustment affects the tracking error adjustment. So, these adjustments must be performed in order.
First, the IF tuned circuits are all tuned to the designed intermediate frequency.
Next, the Local Oscillator circuitry is adjusted to provide proper dial tracking, usually by adjusting a trimmer to set the highest frequency the receiver can tune to.
Afterwards, the Antenna Circuitry is adjusted to minimize tracking error. Sometimes an additional adjustment to the local oscillator is made to further reduce tracking error.
During alignment, the signal generator is tuned to provide a signal simulating one present within the radio during normal operating conditions. One wants to exclude other signals, not under control, from the radio. Disconnect any external antenna from the receiver if it has an external antenna connection. During IF alignment, when the local oscillator operation is not needed, disable the local oscillator as well.
Usually, there are three and sometimes four signals which are used during alignment. The signal generator is adjusted to provide the requisite signal, and is attached to an appropriate point in the receiver circuitry. An indicator, usually an AC voltmeter (with a moving pointer is preferred), is connected to another point in the receiver, usually across the speaker terminals, to gauge the response of the receiver circuitry to the signal. Then various adjustments are made to components, usually so that the indication is maximum.
It is usually important during this procedure that the signal generator be adjusted to provide as weak a signal as possible while still getting a reasonable indication. Turn the radio's manual volume control to maximum volume, and reduce the signal strength provided by the signal generator as the receiver becomes more sensitive. There are two reasons for this.
First, most receivers have some sort of automatic sensitivity control circuitry, inappropriately called the Automatic Volume Control (AVC), which automatically reduces the sensitivity of the receiver with stronger signals. As the receiver's inherent sensitivity increases as the alignment proceeds, the AVC circuitry decreases the effective sensitivity, masking the effect, and producing a nearly constant output. This makes it difficult to ascertain when the adjustment is optimum. Alternatively, one may disable the AVC circuitry. This requires more knowledge of the specifics of the receiver construction, and especially if no alignment procedure is available in service literature, it is simpler just to use the weakest possible signal. There is another compelling reason to do so, anyway.
Second, a strong input signal over-drives the amplifiers, which then produce their maximum, fixed, outputs, again causing difficulty in ascertaining the optimum adjustment.
This section provides a General Procedure which in broad outline is applicable to all superheterodyne receivers. The procedure is presented in step by step manner, along with the purpose and goals of each step. If the manufacturer's alignment procedure is available, it should be followed rather than the general procedure. In other cases, a thorough understanding of the general procedure and the goals of the adjustments performed will enable the technician to tailor the general procedure as necessary.
The first adjustment necessary to the receiver, which is often not mentioned in the service literature, is the proper location of the tuning indicator. The tuning knob or tuning pointer must be set so that it points to an index mark on the dial. If the indicator is a knob, usually this means installing the knob on the control shaft so that it points to the lowest frequency indication on the dial, or to some other index mark on the dial, when the radio is tuned to as low a frequency as possible. If the indicator is a pointer carried on a dial string then it is moved on the string so that it points to an index mark on the dial or on a backing plate, again when the radio is tuned to as low as may be. Other tuning indicators may require some other adjustment, but it must be made before further adjustments are attempted.
The goal of this purely physical (not electronic) adjustment is proper dial tracking. It is always important to check for tuning indicator indexing before beginning electronic alignment, since if it is incorrect, the resulting dial tracking error may cause you to believe that electronic alignment is necessary, when it in fact is not. Actually, electronic alignment is not often required, and one should always look for other causes for a receiver to perform poorly before considering alignment.
Remove the receiver chassis from the case, and connect it to the power through an isolation transformer. Turn the receiver on, and allow it to warm up for perhaps fifteen minutes to allow for temperatures to stabilize. Most electronic components are sensitive to temperature, and they must be allowed to achieve normal operating temperature before attempting any adjustments. If the set is adjusted "cold", it will be out of alignment when in actual use.
The adjustments made at the IF are to the IF amplifier, and if the receiver has one, the wave trap.
The first circuitry to be adjusted is the IF amplifier transformers. The purpose is to set the tuned circuits in the IF amplifier all to be tuned exactly to the designed IF. The goals are all of sensitivity, selectivity, and dial tracking.
If all the IF amplifier tuned circuits are tuned to the same frequency, then they all respond maximally, and sensitivity is maximized. Even if they are all tuned to the same, but incorrect frequency, sensitivity is compromised, because tracking errors will result, and the antenna tuned circuits will discriminate against the signal the receiver is tuned to. Also, if the tuning of the IF amplifier is to an incorrect frequency, dial tracking will suffer.
Selectivity is enhanced in two ways. First, when all the tuned circuits are tuned to the same frequency, they respond most strongly to that one frequency, and select for the desired signal. Second, if the IF amplifier tuned circuits are not all tuned to the same frequency, but only to nearby ones, then the bandwidth of the amplifier is broadened, reducing adjacent channel rejection. If they are all tuned properly, then proper bandwidth is assured by the design of the transformers.
Note, however, that in some FM and TV receivers the IF amplifier bandwidth is intentionally broadened by tuning the IF transformers to slightly differing frequencies in order to accommodate the additional bandwidth these signals occupy. This technique is called "stagger tuning".
Since the converter circuitry is presumed out of adjustment, it cannot be relied upon to provide the IF signal. So, the signal generator is used to simulate a properly operating converter.
Set the signal generator to provide a modulated signal at the IF indicated in the service literature, and attach its ground lead to the chassis. Attach the output lead of the signal generator, via a blocking capacitor of perhaps 0.1 uF, to the signal grid of the converter tube. Connect an AC voltmeter to the speaker terminals. Disable the local oscillator by connecting a jumper (clip lead) across the local oscillator section of the main tuning condenser. Connect one end of the jumper to the terminal of the oscillator section, and the other end to the frame of the tuning condenser.
Adjust the IF transformer windings to produce maximum output voltage. These adjustments have slight interactions, so the order the adjustments are performed in is important. Adjust the IF transformer connected to the detector, secondary first and then primary, then adjust the IF transformer connected to the converter, again doing the secondary first, then the primary.
Do a second pass, which should require only a minor tweak, again from "back to front". If the adjustments are done properly, even 1/8 turn of the adjusting tool should make a difference in the response.
If the adjustment being made is to a compression type trimmer capacitor, then the last adjustment made should be to tighten the adjusting screw, not to loosen it. If the last adjustment is to loosen, then later vibrations from the speaker may cause one last little bit of relaxation of the moving plate, causing the component to go out of adjustment. If the last adjustment is to tighten, then this cannot happen. An indicator flag made from masking tape wrapped around the adjusting tool may help with noting and achieving the optimum adjustment via tightening.
Next, if a wave trap is present, adjust it. The purpose is to tune the wave trap to the identical frequency the IF amplifier is tuned to. The goal is to get better selectivity by suppressing a spurious response to signals at the IF. The likely source of such a signal is the IF amplifier, which the antenna might intercept and inject into the antenna circuitry. If this signal gets back to the IF amplifier, then the result could be that the IF amplifier oscillates, resulting in squeals, howls, and other undesired consequences.
Without disturbing the signal generator settings, remove the signal generator output lead from the converter and connect it to the antenna terminal, via a 200 pF capacitor. The capacitor is intended to simulate an antenna, which we have disconnected. The wave trap is then adjusted for minimum voltage on the speaker. As the sensitivity of the receiver is reduced, increase the signal generator output as necessary to get an indication.
There is an adjustment to the converter to set proper dial tracking, and one or two more adjustments to set tracking. Since the converter must actually be functional in order to perform these adjustments, don't forget to remove the jumper disabling the local oscillator before diving in!
The dial tracking adjustment done at or near the highest frequency the receiver is designed to receive. The purpose is to adjust the tuned circuit in the local oscillator to set the highest frequency the receiver can be tuned to. The goal is proper dial tracking. The dial tracking error is set to zero at or near the highest frequency.
This adjustment is done at the highest frequency because it has much more effect there, making the setting more critical. It is done before the primary tracking error adjustment because it affects the tracking error, but the primary tracking error adjustment does not affect the dial tracking. See the Appendix for more details.
Set the signal generator to provide a very weak modulated signal at the specified frequency. On sets requiring an external antenna, attach the output lead to the antenna terminal via a 200 pF capacitor. On sets with an internal antenna, simply place the output lead of the signal generator near to the antenna. Set the receiver main tuning to the indicated frequency (or simply as high as may be) and adjust the local oscillator circuitry so that the receiver's response is maximum. The adjustment is usually made to a compression trimmer part of the main tuning condenser oscillator section.
Remember to make the final adjustment be a tightening one!
The next adjustment is the primary (or only) tracking error adjustment. The antenna circuitry is adjusted for maximum response at a specific frequency near to but approximately 20% of the tuning range below the maximum frequency the receiver is designed to receive. The purpose is to adjust the antenna circuitry so that it is tuned to exactly the same frequency the receiver is tuned to, making the tracking error there zero. The goal is sensitivity.
If the tracking error were set to zero at the very highest frequency the receiver can receive, the resulting maximum tracking error would be greater than when it is set about 20% inside the total tuning range. The adjustment has more effect at the higher frequency end than it does at the low frequency end. See the Appendix for more details.
Leave the signal generator connected as it is, but tune it to the frequency specified in the literature. Carefully tune the receiver to the signal, and adjust the antenna circuit for maximum response. The adjustment is usually made to a compression trimmer part of the main tuning condenser antenna section.
Sometimes, there is an additional adjustment to the local oscillator circuitry performed near the low frequency end of the band, about 5-15% up from the lowest tunable frequency. If the tuning condenser has circular plates, this adjustment is necessary. Tuning condensers with snail shell shaped plates ("cut plates") do not need this adjustment. The local oscillator is adjusted so the radio is tuned to the same frequency as the antenna circuitry. This creates a second zero tracking error point to minimize the worst case tracking error. The goal is sensitivity, though dial tracking is also affected.
The reason that this adjustment is performed at the low end of the band is that the antenna trimmer has very little influence at the low end, and another adjustment must be provided to set the low frequency zero tracking error point. Tuning condensers with cut plates automatically track properly at the low end with just one adjustment. The adjustment is either to a padder condenser, or to the local oscillator coil. See the Appendix for more details.
Leave the signal generator connected as is, and tune it to the specified frequency. Tune the receiver to the signal. Next perform an adjustment called "rocking in". Proceed stepwise, turning the local oscillator adjustment a small amount thereby detuning the receiver, then "rock" the main tuning control back and forth to re tune the receiver to the signal. Tweak the local oscillator adjustment again, re adjust the main tuning control as described. With practice this becomes a more or less continuous process.
As this is done, you will note that when turning the local oscillator adjustment in one direction, the maximum response while rocking the main tuning decreases, whereas in the opposite direction it increases, then starts to decrease again. Stop where the maximum response is itself maximized, so the receiver has maximum sensitivity at the specified frequency.
This adjustment of the padder or inductor in the local oscillator interacts with the adjustment of the local oscillator trimmer setting the highest frequency tuned, which in turn affects the antenna trimmer optimum setting. So, usually, if "rocking in" is required, a second pass through all three adjustments is necessary. A third pass is rarely required, with just a slight tweak to the highest frequency adjustment sufficing.
This section gives descriptions of the detailed alignment procedures for three receivers, starting with one whose service literature has a comprehensive step by step description, and ending with one which has practically no alignment information whatever. The text explains how to interpret the instructions given, and how to develop the extra details in those cases where they are lacking.
The service literature for the RCA Victor model 9X561 has a detailed step by step alignment procedure. If you understand the General Procedure given above you should have no problems following it. There is also some excellent advice on proper lead dress which can be adapted to other receivers. This is about as good as it gets.
The Dial Calibration describes setting the tuning indicator to the index mark, as described in the General Procedure, and shows where to find the specified tuning settings when the chassis is out of the case and the dial is not available.
Note the comment on keeping the signal generator output as low as possible. I suggest that the warning to use an isolation transformer is not strong enough. An isolation transformer is absolutely required, no exceptions. No description of how to gauge optimality of the settings is given. I'd put a moving needle AC voltmeter across the speaker terminals using clip leads to attach it, and turn the volume control to maximum.
Steps 1 and 2 are the IF amplifier alignment steps. Note the order specified for performing the adjustments, second transformer first, secondary first. Some of the reasons to tune the receiver to a quiet spot at the high frequency end of the dial are to eliminate interference from received stations, and to move the local oscillator frequency far away from the IF and its harmonics. Disabling the local oscillator is even more effective, and is recommended, in addition to tuning to the high end of the band.
This receiver has no wave trap, so no adjustment for it is provided.
Step 3 is the local oscillator dial tracking adjustment. Tune the receiver to minimum capacitance (i.e., highest frequency physically possible). Note the line to C4 on the chassis drawing, with the associated frequency. Often, the service literature will only have these indications, that is, a line drawn to the adjustment point and an associated frequency. The technician is supposed to be able to figure out the order of adjustment and the tuning of the receiver on his own.
Step 4 is the antenna trimmer tracking error adjustment, as you see performed about 20% of the total reception band lower than the maximum receivable frequency. Note the instructions about positioning the loop antenna as it will be in the case. This is because the case contributes some additional capacitance to the loop antenna via proximity, and affects the adjustment. Do not use the dial to set the receiver, but rather tune to the signal from the generator. Then adjust the trimmer for maximum response.
Step 5 is the local oscillator tracking error adjustment, here the oscillator coil, done about 5% above the low end of the tuning range. Note that, as mentioned in the General Procedure, Steps 3, 4, and 5 must be repeated since the secondary tracking error adjustment is provided.
Here, the alignment data are presented, and are step by step, and complete, but the details are not given; they are up to the technician to divine. The presentation of the data is simplified, by putting the step number in a circle with a line showing the location of the adjustment to be performed, along with the frequency to set the signal generator to. Details of how to connect the generator and indicating meter are omitted.
The POINTER ADJ. SCREW is the place to do the physical pointer index adjustment, though no information is given on where the pointer should be when properly indexed. Presumably, one tunes the radio to the low end of the dial, and sets the pointer to some fairly obvious location, such as the end of the tuning scale line. Another physical adjustment is given, which is not mentioned in the General Procedure since it is specific to this receiver. After indexing the tuning pointer, one should tune the receiver so the pointer indicates 1300 KHz, and set the adjustment MAXIMUM TRAVEL FOR CARRIAGE so that 5/16 inch of space exists between the indicated points.
No information is given on how to attach the signal generator to the receiver. Connect the ground lead from the signal generator to the chassis. I'd put a clip lead on the ANT. RECEPT. terminal, and lay it on the work bench next to the output lead of the signal generator. As always, use the weakest signal which gives a good indication. There is no description of where to attach an indicator. Use a moving needle AC voltmeter connected to the speaker terminals.
Steps 1 through 4 are the IF amplifier alignment adjustments, with the signal generator set to 455 KHz. Connect the signal generator output lead via a 0.1 uF capacitor to the converter signal grid, pin 8. Note on the schematic that the arrows indicating that the IF transformer coils are adjustable also indicate which is the bottom and which is the top adjustment. As usual, the secondary is adjusted first, and the alignment proceeds from the detector towards the converter.
There is no wave trap in this receiver, so no adjustment for it is provided.
Step 5 is the dial tracking adjustment, and sets the highest frequency the local oscillator can tune to. Remove the connection from the converter, attach a clip lead to the ANT. RECEPT. terminal, and lay it on the work bench next to the output lead of the signal generator. Tune the generator to 1605 KHz, and adjust the receiver tuning control to the highest frequency the radio can tune to. Then adjust the trimmer for maximum reception. I wouldn't use a 200 pF capacitor and direct connection, because an automobile radio antenna does not look like a 200 pF capacitor to the receiver.
Step 6 is something not described in the General Procedure. This receiver has an RF amplifier stage, and the plate load is a tuned circuit, which needs to track the frequency the receiver is tuned to. The reasons to have a tuned RF amplifier are almost complete suppression of image frequency reception, somewhat better sensitivity, and less internal noise. The adjustment is usually at the same frequency as the antenna adjustment. The goal is sensitivity.
This step sets the RF amplifier tracking error to zero at 1605 KHz. Unlike most sets, the zero tracking error point is right at the edge of the tuning range. This is very unusual, and is an example of where one must deviate from the recommendations contained in the General Procedure and follow the service literature. The reason for this unusual adjustment is given below. Another way to interpret this adjustment is as setting the tuning range for the RF amplifier circuitry, and as a preliminary setting for tracking error. Don't disturb the signal generator or its connection for this step, and the next.
Step 7 is the tracking error adjustment for the antenna circuitry as described in the General Procedure. As with Step 6, the zero tracking error point is right at the edge of the tuning range, unlike with most radios. Again, follow the service literature rather than the General Procedure. This is also a limit type of adjustment, or preliminary one. The "real" tracking error is adjusted below.
Step 8 is the "real" tracking adjustment to the local oscillator circuitry. Step 7 was a "preliminary" adjustment at the edge of the band. This is the real deal, but the adjustment is being made to the inductor, not the capacitor. It's also made rather lower than usual. The usual high frequency tracking error adjustment is made in the 1400 KHz to 1500 KHz range, but especially with earlier sets 1300 KHz can be encountered.
It's a little unusual for the primary tracking error adjustment to be made in the local oscillator circuitry, but as always, follow the service literature instructions when they are available. Since this is a local oscillator adjustment, I suspect it's also a dial tracking setting. Leave the clip lead arrangement as is, but set the generator to 1300 KHz, set the dial on the receiver to 1300 KHz, and adjust the core for maximum reception.
Step 9 is likewise the "real" tracking adjustment to the RF amplifier circuitry, similar to Step 8. Leave the signal generator and receiver tuning as set in Step 8 and adjust for maximum reception.
Step 10 is the "real" tracking adjustment to the antenna circuitry. Again, do not re adjust the signal generator and receiver tuning. You might need to reduce the signal generator output, however. Tune for maximum again.
Since the adjustment performed in step 8 interacts with the adjustment performed in step 5, and step 9 interacts with step 6, and step 10 interacts with step 7, I would perform steps 5 through 10 a second time, expecting only small tweaks to be necessary.
The service literature for the GE model 136 is pretty sparing on alignment information. Basically all that is given is the IF. I've seen schematics with less, as in no information, but this is about as little as you can get while still having something. I have seen one which didn't have the IF given, but had a big hint; there was a wave trap (not marked as such, but obviously what it was) with just the frequency 482 KC by it. This hint provided the IF, which was not the usual 455 KHz.
One might wonder where to begin with this one. There are some clues to allow the intrepid technician to make an alignment which would provide if not ideal, at least acceptable performance. Let's start with the dial pointer indexing. There is not a clue nor a hint provided in the service literature. However, take a look at a photo of the radio.
If more work is needed, then follow the General Procedure for aligning the IF amplifier using 455 KHz as the IF. Afterward, tune the receiver to the highest frequency physically possible, and tune your signal generator around until you receive the signal. Record the frequency for later resetting of the local oscillator trimmer, if necessary.
At this point, a preliminary check to see if the set needs further adjustment should be done. Going beyond the information provided in the service literature involves some risk that the performance will get worse, not better. Recheck the dial tracking. If the IF amplifier was significantly out of adjustment, then you may want to try returning the dial pointer to its original location. If the set performs adequately, then stop.
Otherwise, there is a handy mark at 1600 KHz on the dial. Tune the receiver to that dot, set the signal generator to 1600 KHz, and adjust the trimmer on the local oscillator section of the main tuning condenser to receive the signal, setting the zero dial tracking error at this frequency. Probably the factory setting was higher, at the maximum frequency physically tunable, and at 1620 KHz or 1630 KHz. This setting should get us close, however. You might try tuning all the way up and setting at 1620 KHz. Do another check for dial tracking.
A quick scan across the dial listening to stations to verify reasonably equal sensitivity across the band is indicated. If there are problems, then a tracking error adjustment is needed. Make note of the antenna trimmer position, perhaps with a small dot next to the screw slot, in order to be able to return it to its original setting, if it later turns out to be necessary.
Using a "guesstimate" of where to set the zero tracking error high frequency of 1400 KHz, tune the signal generator to that frequency, carefully tune the radio for reception, and adjust the trimmer on the antenna section of the main tuning condenser for maximum reception. If another quick scan across the dial shows there are still problems, then trying 1500 KHz for setting the zero tracking error at the antenna trimmer might give slightly better results. A few experiments in the 1300 KHz to 1500 KHz range should give reasonable results.
If no reasonable setting can be found, return all adjustments (other than the IF transformers) to their original positions, and look for problems other than alignment as the source of the problem. It may be that one or more of the coils has shorted turns. Any time an attempted alignment results in unsatisfactory results, some other cause must be found.
In the case where no service literature is available, and not even the IF is given, it may still be possible to make some progress. The very early superheterodyne receivers used a variety of intermediate frequencies, often very low by later standards. However, by the mid 1930s intermediate frequencies were standardized around 455 KHz or so. Philco liked 460 KHz, and a few used 462 KHz, but most were 455 KHz. Very early on, lower frequencies around 262 KHz, 175 KHz, and even a few as low as 75 KHz were used. After World War II, 455 KHz was almost exclusively used, though I've got a few from the early 1960s which use 460 KHz.
Before beginning, mark the locations of all adjustments to be performed so they may be returned to their original settings.
One way to start is to connect up a signal generator to the signal grid of the converter or mixer through a blocking capacitor, set the output to maximum, tune around 400 KHz to 500 KHz, and see if the receiver responds. If someone has "tightened the loose screws", it is likely that the IF transformers will be tuned below the designed frequency, so be sure to tune lower in frequency than your guess.>
If an early set does not respond, then try around 262 KHz, and then gradually tune lower. It is important to start high and go to lower frequencies, since the tuned circuits may respond to a harmonic of the signal, and fool you into thinking the IF is 230 KHz, when it is 460 KHz. Once you have a general idea of the IF, some experimentation may get you close.
Suppose the IF seems near 455 KHz. You could try 455 KHz, and do a trial IF alignment and set the local oscillator trimmer at the highest marked frequency on the dial. If dial tracking errors are apparent, then try 460 KHz or 465 KHz. A few trials at 5 KHz intervals should give decent dial tracking. Set the signal generator to a few spot frequencies given on the dial, and see where it is actually received. Then change the IF, and do another IF alignment. If the dial tracking errors reverse from being high to low or vice versa, you have probably passed through the IF, and could try the average of the last two frequencies tried.
If the set has a padder or the local oscillator coil is adjustable, you may need to set the antenna trimmer at 1400 KHz, and rock in the local oscillator at 600 KHz before checking the dial tracking. The padder or coil adjustment affects the dial tracking.
After the dial tracking is acceptable, a couple of trials with different tracking error adjustments to the antenna trimmer (and padder or coil if necessary) should give acceptable tracking error and equal sensitivity across the band. Start with 1400 KHz for the high tracking error adjustment, and 600 KHz for the low (if present). Tune around, and see if the set appears to be "weak" at some spots on the band. That's a clue that you have tracking errors, and the antenna circuitry is not tuned to the frequency the set is tuned to. Be willing to go up to 1500 KHz, and possibly as low as 1300 KHz on the top end, and up to possibly 700 KHz when rocking in. Remember that the antenna trimmer has more effect at the high frequency end of the band. When reception seems equally sensitive across the band, then you're done.
Maybe not perfect, maybe not exactly what the manufacturer used, but probably acceptable performance, if the set really, truly, actually needs alignment.
The procedure suggested here is a sort of "desperate measures" approach, and should not be undertaken lightly. Be sure your set really does need alignment, or things may get worse. Try very hard to find service literature. I've used it only a couple of times. Once, I was repairing a radio for a neighbor of a friend. He purchased it in 1946, removed it from its case, and installed it in an old crank telephone he had gutted. The manufacturer and model were lost in the mists of time.
He brought it over, and announced that he had done me the favor of tightening all the loose screws. I smiled and thanked him, but inwardly groaned. After initial repairs were complete, I turned it on, and got the "all tubes light, no reception" symptom. This is the only time I've gotten no reception at all because of bad alignment. After a trial alignment using 455 KHz for the IF, 1600 KHz for the dial tracking setting, and 1400 KHz for the tracking error adjustment, the set received stations all across the dial.
One good way to get experience with superheterodyne receiver alignment, is to obtain an AC/DC receiver which has an intact chassis but with some cosmetic issues. Choose one with no case, or badly cracked case, which can be obtained at low cost, and which has good step by step alignment procedure information available. One can then deliberately mis-tune all the adjustments, and perform an alignment. Doing a few alignments will give the technician more confidence and knowledge than any amount of reading of documents.
Multi band radios, Communications Receivers, FM radios, Television receivers, all are likely to be superheterodyne receivers. As such, this document applies in its entirety as far as goals and general procedures. While details vary, the principles of superheterodyne receivers and their proper adjustment remain the same, regardless of the type of receiver. Other receivers simply have more associated adjustments, and likely some adjustments which are not covered here.
Multi band radios differ from single band radios in that they have a more or less complicated switching arrangement to switch in different local oscillator components and antenna circuitry components. One simply repeats the converter adjustments, usually in simplified form, for each band's top frequency.
Communications receivers are mostly just multi band receivers which sometimes have more than two IF transformers, and perhaps an RF amplifier stage which also need adjustment, and possibly a Beat Frequency Oscillator (BFO) which needs a tweak. They also usually have very detailed alignment instructions included with the service literature. One simply follows the service literature's instructions.
Except for the higher frequencies involved, FM receivers are the same as AM receivers up to the detector, which requires a specialized FM signal generator and procedure to adjust, and sometimes require stagger tuning of the IF transformers. This is no more difficult than AM alignment, just requires more steps of the same type.
TV receivers have several different types of circuits beyond those simply for reception, but the alignment is in principle not more difficult to understand. They do, however, require some specialized equipment which one must learn to use.
The technician armed with a full grasp of what the goals of superheterodyne alignment are, and how the adjustments achieve these goals, will have no problem adapting the General Procedure to each receiver he encounters. The author sincerely hopes he has provided the reader with this firm grasp, and the confidence to go forward.
In a few places in this document it is mentioned that trimmer capacitors used as adjustments are more effective at high frequencies, whereas padders are more effective at low frequencies. This appendix explains why that is so.
Trimmers are small adjustable capacitors in parallel with larger capacitors, used to compensate for unintended and unavoidable capacitance ("stray") in the circuit, and manufacturing limitations on tuning condensers or other components. The are usually in the form of a mica compression type, often manufactured as part of the main tuning condenser. They have a range of perhaps 2 pF to 40 pF, and are in parallel with the main tuning condenser.
In the antenna circuitry, they are in parallel with a variable capacitance of about 10 pF to 400 pF, in the local oscillator section from perhaps 8 pF to 200 pF, with greater capacitance associated with lower frequencies.
Consider the local oscillator circuitry. Since capacitors in parallel have their values added, at the low frequency end of the dial the trimmer can adjust the total capacitance from 202 pF or so to 240 pF or so, a 20% adjustment range. At the high frequency end of the dial, the adjustment range is perhaps 10 pF to 48 pF, or approximately 500%. Hence the necessity of adjustment at the high frequency end, as the adjustment is much more critical there. Since the antenna section of the tuning condenser is even larger, the trimmer in the antenna circuitry has correspondingly less effect at the low frequency end. This is why trimmer capacitor adjustments must be made at or near the highest frequency tunable.
A padder is a larger adjustable capacitor of perhaps 400 pF or more in series with the local oscillator section of the tuning condenser, and often of similar construction to a trimmer. Unlike capacitors in parallel, capacitors in series take on the value of the smaller capacitor. A 400 pF capacitor in series with a 40 pF capacitor has a total of about 40 pF capacitance. So, when the tuning capacitor is small (at the high end of the band) the padder has relatively little influence. At the low end of the band, the padder and the tuning condenser section are more nearly equal in value, so the padder has relatively more influence. This makes the setting more critical there.
Adjustments to coils have about equal influence over the entire band of operation. A small change in a coil's value results in a percentage frequency change which is about one half of the percentage change in the coil's value. That is, if the coil's value increases about 2%, the frequency decreases about 1%. This is independent of frequency. So, considered as a percentage change, the coil's effect is about equal over the band. Since the trimmers have very little effect at the low end of the band, any adjustment at the low end must be provided by some other means. That is either a padder, which is more effective at low frequencies, or the local oscillator coil, which is about equally effective. In either case of padder or coil adjustment to the local oscillator, the trimmer adjustment must be rechecked at the high end, since they interact.