| Re: 200B 5 PA head hum/hiss issue [message #28155 is a reply to message #28094] |
Sun, 28 March 2021 08:11   |
stevem
Messages: 4746
Registered: June 2004
Location: NY
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Senior Member |
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Hi!
First off the only place in the amps signal chain where 60 hz hum can get in is my means of the power transformer magnetically coupling into the output side coil / transducer of the reverb pan.
Any in the amp, circuit wise after the rectifiers can only be 120 hz ripple voltage making for hum issues getting into the audio stream.
There was a factory fix for 803 board noise and feedback problems in amps made before April 4th 1969.
The fix is this.
R836 is changed to 82k.
R837 is changed to 82k.
R830 is changed to 47 ohms.
R848 is changed to 15k.
The reverb pan should test out ohms wise at about 175 on both the input and output side.
If you have the correct reverb pan for the amp then only the output side of the pan should be grounded to the main case of the pan and to the output RCA jack on the pan, and of course on back thru that RCA cable to the 803 board.
The input side of the pan should not be grounded to the main case of the pan.
If the output side of the pan is hooked up and mounted or sitting near the PT and the reverb is turned up you will get a lot of 60 hz heard thru the output.
Pops and clicks in the audio stream sound like the case of a noisy transistor to me!
Q703 is a 38735 which crosses to a NTE108.
Maybe that broken off lead has caused other components around q703 to go bad if the amp was powered up like that.
For continued trouble shooting I would disconnect and unpower all but the one preamp board that seems to work normal!
[Updated on: Sun, 28 March 2021 08:22] Report message to a moderator |
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