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Re: Peavey Classis 30 on the bench , PCB arc'ed!

8/5/2005 12:17 PM
Re: Peavey Classis 30 on the bench , PCB arc'ed!
I had a Crate solid state amp with enough conductivity to the PCB for one of the raw low-voltage power rails to bias off an FET-input opamp from about an inch away.  
It was a mystery for a little while even after I put a socket in there, with the chip removed, all test points were as expected, and junction opamps functioned well when inserted.  
But a proven good TL072 biased itself to cutoff when it was in the socket, and you could measure the discrepancy on the offending input pin.  
Then once I got away from the test points and poked the probe into the board itself, I could also see the problem stray voltage when the FET input device was there.  
The same voltage was absent when a junction opamp was present, apparently the naturally high gate resistance of the FET was much higher than the PCB itself, but the junction opamp sunk more than enough current to make any stray voltage unmeasurable, by me anyway.  
The answer was not to use a TL072 and put in the 4558 instead.