Request for help troubleshooting AA5 radio
 

Mid 60s GE radio, bone stock AA5, all on one horizontal PC board. Nothing sexy about it, but it's used daily and I want it to work properly. So far: replaced filter capacitors. Replaced 12BE6 and 50C5 due to excessive H-K leakage. Peaked the two screws on the tuning cap with an RF generator and scope . Did not adjust IF transformers due to not owning a plastic hex key.
Radio is now sensitive, hum mostly gone, IM hum completely gone with new 12BE6. Remaining problem is audible IM distortion on complex music. It's not in the audio section, and doesn't vary with volume. Talk radio or simple music sounds fine Any ideas? Alignment? Silver ,ica disease? Note this radio has all ceramic disc caps except the filters which are brand new

Also, I have confirmed avc is working, it makes -5.5 volts on a strong station. All plate voltages measure fine too, output tube is correctly biased. Stopped short of checking bias point of RF tubes

Also, I have confirmed avc is working, it makes -5.5 volts on a strong station. All plate voltages measure fine too, output tube is correctly biased. Stopped short of checking bias point of RF tubes

It might be the speaker. Try a different one.

I should have mentioned.. I already tried a different speaker, and even pulled the output transformer and did a frequency sweep on it. Aside from being the worst audio transformer I've ever measured there's nothing wrong with it.

It's likely a bias issue. Tap from the detector output to an external audio amplifier and see what you get. Feed in a modulated signal and look at the detector output, and each stage prior to see what damage is being done to the waveform.

OK update - did an alignment. waveforms looked nice and clean, so connected audio oscillator between ground and wiper of volume control.

WOW, the speaker connects to the cabinet with plastic tabs... the speaker has a little buzz near resonance due to its extreme cheapness, and the speaker frame vibrates against the cabinet very noisily because the loose little plastic clips GE used to hold it in place don't do their job very well. All the while the waveform going into the speaker looks nice and pretty. Going to make a gasket out of double sided tape and get this thing back together.

I guess the loose mounting is why the second speaker I tried also sounded terrible.

Got it back together. The gasket cured the distortion problem. Thank you Bob, your first guess was right!

The AA5 amazes me, talk about more than the sum of its parts.... there's a classical program on now, and I can't believe that the puny horrible output transformer, which rolls off at 150 Hz, the miserable distortion ridden 4" speaker with an invisibly small magnet - the cheap resonant plastic enclosure... all without so much as a screw to hold it together, and it sounds totally enjoyable. I think its owner will be happy with the improvement.

Congrads! You've earned yourself a dancing banana. :banana: