If you are seeing distortion in CW/SSB and not AM, I would start with circuitry that is used in CW/SSB, and not AM. This includes the product detector, and the SSB/CW preamp. The SSB/CW preamp in particular has some high value resistors that drop 140V down to levels suitable for the 2N2222A preamp transistor. So, start with the 140V supply and work you way in from there. The number of parts to check in this area of circuitry is relatively low, so look for heat damage. The resistors are 10% carbon comp and those can age and change value over the years. There's also a electrolytic emitter bypass capacitor on the SSB/CW AF Preamp that should be checked, that and other electrolytic caps are probably starting to age. As pointed out earlier, from a signal point of view this can also be checked with a simple RF carrier injected at the antenna then tuned for the correct sideband reception. You should be able to see a sine wave all the way through the receiver chain which you can also check for where the distortion is introduced.
Also, the signal for AGC is sampled ahead of the last IF Amplifier, so there is no AGC voltages that are part of the product detector or SSB/CW preamp circuitry.
I hope this helps,
Jim N8CAH
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