Acoustic 150 Amp - Reverb driver?

[kdawg]

Hello - I have an Acoustic 150 amp that has a constant distorted signal when I plug a 1khz tone into Channel 2, regardless of any knob settings. Channel 1 is fine. I disconnected reverb tank, no change.

I finally dug in to schematic and lifted the C210 input to the reverb driver... and then bad signal stops and channel sounds fine. I've checked all the resistors, caps, and transistors around there - swapped Q203 with a good one. I only had one replacement for Q205/Q206 but all the transistors there tested OK out of circuit. Seems to be about 35v going across each Q205/Q206 and they get extremely hot.

I'm just not understanding how that signal is getting back into the mix. This is a 17-11E preamp board.

Any ideas? Thanks,
-kdawg

[Enzo]

Don't just throw transistors at it.

You lifted C210 and the channel now works. That makes me suspect C210 rather than the transistors past it.

This amp is 40 years old, the electrolytic caps in it are probably dried out. Have you replaced the little 1uf e-caps like C210? There are a whole bunch of them on this board. When they get leaky, they can upset your DC levels wherever they sit.

The top lead of Q205 is its collector. From there follow the line to the left, then up and over to the right where it is the +75v pin of the connector. +75 connects directly to Q205 collector. Q205 and Q206 are in series, more or less, and the emitter of Q206 connects to ground through a 180 ohm resistor. So we have then 75v across Q205 and 206, and that is roughly the 35v each that you found.

Heat is a matter of how much current is flowing through then, not just the voltage.

[kdawg]

I replaced the bigger electrolytic caps, I ran out of 1uf caps but did replace the C210. In the process, I found the 100k trim pot was cracked in half. I put in a new one, the distortion level does change as I adjust the pot but doesn't go away, but I don't know what it should be set to... it wasn't in the service manual.

Is Q213 just acting as a diode there? Not sure if it should be replaced if the pot was wide open.

Thanks,
-kdawg

[52 Bill]

...I found the 100k trim pot was cracked in half. I put in a new one, the distortion level does change as I adjust the pot but doesn't go away, but I don't know what it should be set to... it wasn't in the service manual.

The trimmer pot adjusts the bias to the fet that is used to modulate the signal for the tremolo. Set it to get the best tremolo effect.

If adjusting the trimmer changes the distortion you are getting, the problem could be in the final preamp output circuit.