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previous: Tom Bob-From all you describe, ... -- 9/14/2000 5:50 PM View Thread

Re: HELP! Hummmmmmmmmm

9/14/2000 10:07 PM
Bob P
Re: HELP! Hummmmmmmmmm
It is 120 cycle. Looking on the scope, positive humps only of the sine wave at exactly twice the a/c line frequency. Looks like D/C ripple to me. Let me update the situation. Found and cleaned up some noise with the input jacks which mostly eliminated the motorboating. Now with the volume down, there is still noticible hum. Same thing if I pull V1. This is not as I noticed before. Now if I pull V2 the hum stops. If I inject a signal anywhere before V2 it hums, hum. Hmmm?  
ALL tubes have been replaced with known good ones.  
Ran a ground line from the input jacks to each pot and then to the same ground where the center tap is.  
Tried to ground out both 68K junctions. Takes the massive hum out, but there is still that low level hum. I don't think that it is the input jacks because, like I said, if I inject a signal, even with the input jacks pulled from the circuit, it hums.  
There is also a VERY loud electrical hit when throwing the standby switch to its on position.  
Wouldn't 120 cycle hum indicate something from the power supply?  
Bob P

Tom Bob-OK, it sounds l... -- 9/15/2000 3:46 PM