I've been building this site for several years now, and The Old Man, my father, appears in various places. He spent most of his professional life as an electronics repair technician, first for an independent repair shop and later for two major department stores. He worked on some of everything from wire recorders to DVD players, vacuum cleaners and microwave ovens; but he was most at home with audio and radio equipment.
He was an exceptionally good repairman. I'm obviously biased, but I saw enough examples over decades to feel confident that I am right. I often sought his advice and he set me straight many times when I was wrong and would never have figured out the problem on my own.
After he passed away, I found one the notebooks he had kept, and he wrote down some advice—the kind of things that used to be called "hints and kinks" pages in the old magazines. I've copied them here for my own reference, and for anyone else who may get something out of it.
Dead Radio (All-American Five)
- Are all the tubes lit? Check and replace bad tubes.
- Turn volume control up.
- If no audio
- Check voltages in audio amp circuit
- Check speaker
- If audio is heard, problem is behind the AF output tube
- Inject IF signal on the grid of last IF tube. if audio is heard, problem is behind IF tube
- Continue working back to antenna or control grid of the converter (mixer) tube
- If signal is heard injected at the antenna, the set is not oscillating. Check voltages and coil connectivity. Check coils for open
- If no audio
Weak Audio can be caused by open cathode bypass caps (which provide signal path to ground).
AVC filter caps can cause tuning problems and poor quality audio.
IF transformers can cause static in the audio. Noise goes up and down with volume setting. This is caused by the small caps within the IF cans leaking or shorting. When this happens, it puts a positive voltage on th control grid of the following tube. Check grids with an analog VTVM to see voltage fluctuate with static.
Check suspected filter caps by jumping them with a good one. Do this while radio is on and humming.
Be Careful. Radios with power transformers usually put out 300V or more. That will make your eyes light up and say TILT.
Old Car Radios (tube type)
Especially Delco brand put out 300-400 volts B+. If you change a vibrator tube, be sure and replace the buffer capacitor. This cap is located on the rectifier tube socket and goes from plate to plate. It's a high voltage cap, usually 1600V or so. Failure to replace this cap voids the vibrator warranty.
Delco radios using the 0Z4 tube hav a unique problem. The radio plays fine until the car comes to a stop sign. When the engine RPM drops to idle, the audio quits. When the car resumes speed the radio plays until the comes to another stop. This problem is caused by an old weak 0Z4 rectifier and the car's generator voltage naturally going down below battery voltage when the RPM drops. Replace the OZ4.
You can't really check a 0Z4 tube on a tube tester; if you suspect it is bad, replace it.
You can replace a 0Z4 with a 6X5 or 12Y5 depending on the supply voltage. If you do this, rewire the socket and run a filament lead to supply heater voltage.
When testing tube type AM car radio on the bench, use an external antenna about 3 feet long only. Using a long-wire antenna will overload the first stage.
The AVC in old car radios is very active. When you align a radio, turn the volume up 100%. Use only enough input signal to be heard comfortably. When aligning IF stages, I sometimes will kill the local oscillator to prevent misalignment and birdies. This is very easy to do on radios with a tuning capacitor: simply short the rotor plates to the stator plates on the smaller of th two ganged tuning capacitors. Inject signal via a small loop of wrie from the signal generator near the radio's antenna coil.
Automatic Voltage Control (AVC)
AVC is a strange thing. It can cause many small problems that appear to be unrelated to AVC.
Poor sound quality can be caused by a bad filter on the AVC line. Also check the resistors in the AVC circuit. Weak audio can also be caused by too much voltage. Check the high-value resistors which are the most likely to change value.
AVC can cause tuning problems too, but this is very rare.