SB-102 problem

[Brian Wood]

Hi everyone,

I have had several responses, all indicating that it's probably a bad solder joint or a corroded tube pin. I poked around some more and here's more data:

I continued to find it interesting that the voltage on the screen grid of the audio power amp was way low. I tried to measure the 22K 1/2W resistor that goes from the 312V B+ to that grid and found that I couldn't measure it, so I guessed it might be open. The power dissipation in that part, if it was working right, would be (312-200)*(312-200)/22K, or .6W, and it's a 1/2W resistor! So I pulled it and sure enough, it was open (although it looked perfectly good). I happened to have a 22K 1W resistor in my junkbox, so I replaced it. The voltages are now correct. It's sort of amazing that you can get any audio at all when running the product detector and audio amp at about 1/3 of their normal voltage, all from "induced" voltage coming from who knows where.

However, the AVC action is still pretty poor. I guess I don't understand how, with the RFG pot generating about -1 to -25V for an S1 - S9+60 signal, you can expect to get much of an AVC voltage from the circuit they used -- the 2nd IF amp output is fed to a voltage doubler made out of tube rectifiers followed by an RC circuit, and diode OR'd with the RFG voltage which becomes the AVC. It just can't generate that much voltage on a typical signal. I can only see about -1.1V on really strong voice peaks. But I guess my meter may not be able to respond fast enough to really see anything.

If I assume that the AVC detector is working, the problem must be in either the metering circuit, which is basically just sampled cathode current from the first IF amp, or perhaps the RC network that forms the AVC attack time constant (10M ohm, .02uF). Or maybe the meter resistance. Any further ideas?

Brian

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