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Notes from the June 20, 2009 Meeting
Program Director Mike Grimes reminded us of our upcoming July 18th Repair Session (starts at 9:00 am) and our August swap meet (starts around 7:30) as well as our September meeting, where Randy James will give a presentation on Crosley Radio (Bring your Crosleys!). Our program subject was the alignment process for a superheterodyne radio. Mike introduced Cleo Cherryholmes to present his well-prepared program. He had set up a very effective array of test equipment and illustrations.
Cleo touched on the subject of Resonance, mentioning that Sir Oliver Lodge (a Scot) studied it and defined it in mathematical terms, using parallel conductors. He explained that resonance is responsible for selecting and passing the signals that we want, while rejecting those that we don't. The alignment process is simply one of adjusting the radio's resonant circuits to pass the radio frequency signals that are being amplified by the tubes. Cleo used a breadboard setup and test equipment to illustrate the properties of an L/C (inductive-capacitive) circuit. Using a signal generator and oscilloscope, he showed how the signal voltage across a parallel inductor/capacitor rises to a peak value at a certain signal generator frequency and falls off on either side as the frequency is adjusted. The effect of loading was also demonstrated by connecting an oscilloscope test probe to the circuit and noting that the resonant frequency moved from 130 KHz to 121 KHz simply due to the approximately 10 Pico farads capacitance of the probe. Cleo pointed out the number of tuned circuits in the radio that he had chosen for his demonstration - 6 total (two IF's, with two coils in each; oscillator, and RF). The loop antenna is actually part of the tuned RF circuit, in combination with one section of the tuning condenser. The other section of the tuning condenser is connected to the oscillator coil. Stations are converted to the IF frequency by the difference between the station frequency and oscillator frequency. By tracking the tuning of both the oscillator frequency and the antenna across the dial, optimum performance is achieved. Once the 6 tuned circuits are adjusted, alignment is complete. Cleo proceeded to align the example set, emphasizing that the correct tools are essential. He showed several types of adjustment - adjusting screws, slotted ferrite slugs and hollow slugs with hexagonal holes through them. It is critical that proper tools are used for the ferrite slugs, which are sometimes difficult to turn or are stuck in place. (Special alignment tools can often be found at our swap meets and flea markets). A steel screwdriver or Allen wrench will de-tune the circuit while you are trying to adjust it. A slotted screwdriver is more likely to break a slotted ferrite slug (the corners are rounded-off on the proper tool). The hollow slugs let you reach through the one slug to the other one, letting you do the IF alignment from above the chassis. You must be very careful with the slugs! If a slug is frozen, it is best not to try too hard to move it, especially if the radio plays OK otherwise.
When do you align a radio? - Only if it's already playing to some extent. Mark the locations of the adjustments before you move them and make a reference mark on you alignment tools so you can turn an adjustment back to the original setting if there is no change in output. Also, mark each device - L1, L2, T1, T2 etc. for quick reference and to avoid accidentally adjusting the wrong thing or undoing one you've already made.
The radio that Cleo used is an RCA training school kit. He showed the result of doing a quick manual adjustment of the tuned circuits without any test equipment. After peaking it all up, it played just fine on the station that was being used as a signal, but the dial ended up pointing to 990 instead of the 1080 frequency of the station, and a station at the low end (570) could not even be tuned-in! This illustrated that the quick way is not a good way. The dial will be incorrect, and at the very least the sensitivity of the radio will vary across the dial because the tracking has been upset.
Cleo showed the procedure for coupling the test signal generator into the radio with a home-made gadget consisting of 5 turns of wire supported on about a 3 inch diameter slotted piece of cardboard. This loop was connected to the signal generator output and placed near the radios' loop antenna (or loop stick, or RF coil). To measure the output of the radio a voltmeter was attached to the speaker terminals and set up for a low-range AC measurement. The procedure started with setting the signal generator up to produce a signal at the IF frequency, modulated with a tone (usually about 400 Hz). Then, all four IF transformer adjustments were made in order to peak the output. (If there is no signal, the signal generator setting may be moved around until you get an output, then moved back toward the correct frequency until the signal is very weak, then making an adjustment to bring the output back up. This brings the adjustments closer in a step-wise fashion until you get them set to the right frequency.) Once the IF's were adjusted, the service instructions were followed, setting the signal generator and radio dial to 1200 and then peaking the RF and oscillator adjustments on the tuning condenser. This completed the alignment process, as demonstrated by the radio putting the 570 station "back on the air".
Some sets will have an RF amplifier stage that will need adjustment at the RF frequency used for the tuning condenser adjustment step. The advantage of using the test loop to inject the test signal is that for most radios you will not need access to the underneath side of the chassis. You can use this method, even though the procedure called for in the service information may call for a direct connection through a capacitor from the signal generator. Cleo pointed out a simple test to see whether the RF alignment is OK on a radio with a loop antenna. If you bring your hand near it and the radio plays louder, it is not correct. Your hand capacitance is actually correcting the tuning.
Cleo's final bit of advice - don't start an alignment job if it's midnight!
Authors notes:
Sometimes a slug can be freed by taking the correct-size Allen wrench, placing it into the slug and transferring heat from a soldering gun down the shank of the wrench while putting some torque on it. Don't try it too long, as you may burn the transformer cardboard core. As Cleo said - leave well enough alone!
Cleo touched on the subject of Resonance, mentioning that Sir Oliver Lodge (a Scot) studied it and defined it in mathematical terms, using parallel conductors. He explained that resonance is responsible for selecting and passing the signals that we want, while rejecting those that we don't. The alignment process is simply one of adjusting the radio's resonant circuits to pass the radio frequency signals that are being amplified by the tubes. Cleo used a breadboard setup and test equipment to illustrate the properties of an L/C (inductive-capacitive) circuit. Using a signal generator and oscilloscope, he showed how the signal voltage across a parallel inductor/capacitor rises to a peak value at a certain signal generator frequency and falls off on either side as the frequency is adjusted. The effect of loading was also demonstrated by connecting an oscilloscope test probe to the circuit and noting that the resonant frequency moved from 130 KHz to 121 KHz simply due to the approximately 10 Pico farads capacitance of the probe. Cleo pointed out the number of tuned circuits in the radio that he had chosen for his demonstration - 6 total (two IF's, with two coils in each; oscillator, and RF). The loop antenna is actually part of the tuned RF circuit, in combination with one section of the tuning condenser. The other section of the tuning condenser is connected to the oscillator coil. Stations are converted to the IF frequency by the difference between the station frequency and oscillator frequency. By tracking the tuning of both the oscillator frequency and the antenna across the dial, optimum performance is achieved. Once the 6 tuned circuits are adjusted, alignment is complete. Cleo proceeded to align the example set, emphasizing that the correct tools are essential. He showed several types of adjustment - adjusting screws, slotted ferrite slugs and hollow slugs with hexagonal holes through them. It is critical that proper tools are used for the ferrite slugs, which are sometimes difficult to turn or are stuck in place. (Special alignment tools can often be found at our swap meets and flea markets). A steel screwdriver or Allen wrench will de-tune the circuit while you are trying to adjust it. A slotted screwdriver is more likely to break a slotted ferrite slug (the corners are rounded-off on the proper tool). The hollow slugs let you reach through the one slug to the other one, letting you do the IF alignment from above the chassis. You must be very careful with the slugs! If a slug is frozen, it is best not to try too hard to move it, especially if the radio plays OK otherwise.
When do you align a radio? - Only if it's already playing to some extent. Mark the locations of the adjustments before you move them and make a reference mark on you alignment tools so you can turn an adjustment back to the original setting if there is no change in output. Also, mark each device - L1, L2, T1, T2 etc. for quick reference and to avoid accidentally adjusting the wrong thing or undoing one you've already made.
The radio that Cleo used is an RCA training school kit. He showed the result of doing a quick manual adjustment of the tuned circuits without any test equipment. After peaking it all up, it played just fine on the station that was being used as a signal, but the dial ended up pointing to 990 instead of the 1080 frequency of the station, and a station at the low end (570) could not even be tuned-in! This illustrated that the quick way is not a good way. The dial will be incorrect, and at the very least the sensitivity of the radio will vary across the dial because the tracking has been upset.
Cleo showed the procedure for coupling the test signal generator into the radio with a home-made gadget consisting of 5 turns of wire supported on about a 3 inch diameter slotted piece of cardboard. This loop was connected to the signal generator output and placed near the radios' loop antenna (or loop stick, or RF coil). To measure the output of the radio a voltmeter was attached to the speaker terminals and set up for a low-range AC measurement. The procedure started with setting the signal generator up to produce a signal at the IF frequency, modulated with a tone (usually about 400 Hz). Then, all four IF transformer adjustments were made in order to peak the output. (If there is no signal, the signal generator setting may be moved around until you get an output, then moved back toward the correct frequency until the signal is very weak, then making an adjustment to bring the output back up. This brings the adjustments closer in a step-wise fashion until you get them set to the right frequency.) Once the IF's were adjusted, the service instructions were followed, setting the signal generator and radio dial to 1200 and then peaking the RF and oscillator adjustments on the tuning condenser. This completed the alignment process, as demonstrated by the radio putting the 570 station "back on the air".
Some sets will have an RF amplifier stage that will need adjustment at the RF frequency used for the tuning condenser adjustment step. The advantage of using the test loop to inject the test signal is that for most radios you will not need access to the underneath side of the chassis. You can use this method, even though the procedure called for in the service information may call for a direct connection through a capacitor from the signal generator. Cleo pointed out a simple test to see whether the RF alignment is OK on a radio with a loop antenna. If you bring your hand near it and the radio plays louder, it is not correct. Your hand capacitance is actually correcting the tuning.
Cleo's final bit of advice - don't start an alignment job if it's midnight!
Authors notes:
Sometimes a slug can be freed by taking the correct-size Allen wrench, placing it into the slug and transferring heat from a soldering gun down the shank of the wrench while putting some torque on it. Don't try it too long, as you may burn the transformer cardboard core. As Cleo said - leave well enough alone!
Beware that someone has deliberately moved the pointer on a dial cord or misplaced it when renewing the cord. Sometimes there a paint dots or marks on a slide-rule dial to indicate the proper setting for the pointer with the condenser fully closed. An alternate method of monitoring the output for alignment is to measure the negative voltage on the AVC buss and to maximize the amount of negative voltage while making the adjustments. The volume control can be left turned down. Another advantage to this method is that it helps keep from adjusting the set with nearly saturated conditions. As you go along, you reduce the signal generator output to a level just adequate to get a good reading, something you should do even when measuring the output at the speaker connections. If someone has totally misadjusted the radio (meddled with it), it may be necessary to check the output stage-by-stage, using an oscilloscope to monitor the signal.