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Re: 200B 5 PA head hum/hiss issue [message #28116 is a reply to message #28094] Mon, 15 March 2021 18:02 Go to previous messageGo to previous message
Drjones1106 is currently offline  Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
Junior Member
So after some more investigation I think ChicagoBill's initial suggestion of the reverb board being the issue is correct. I desoldered the input wire from the reverb board to the power amp board and the noise was gone completely. I reconnected the power amp and disconnected all the preamp boards from the reverb board and the noise was back. I went ahead and measured voltages at all the transistors on the reverb board against the schematic and the only thing that was drastically different is the collector of Q801. The schematic has it as -0.9v and it measured at +3.3v I checked the surrounding components, and the 1k and 220k resistors were out of spec but not but too much. 1.1k and 240k. I replaced them regardless. the 10uf coupling cap was right on the money, so it stayed. the transistor itself seems to be fine, none of the junctures failed a diode test, though I might just throw in a 2n3904 to be safe.

In an effort to be thorough I'll go take voltages on the power amp board, and continue testing components on the reverb board too.

Any other recommendations on what to look at? you guys have been invaluable so far, I can't say thank you enough.

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