The case of the crazy Sapphire amp. - diyAudio
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The case of the crazy Sapphire amp.

Posted 3rd April 2014 at 01:24 AM by rjm
Updated 3rd April 2014 at 11:07 AM by rjm

Case report:

A set of Sapphire boards gave the proper V+, V- voltages out of the Z-reg, providing about 10.5 and -10.5 to op amp power pins. The output offsets were unusually high however, apparently at about 2 V in one board, and somewhat less in the other. Typically the offsets are in the order of +/-15 mV.

Changing out transistors and op amps did not help, and to all inspection the passive components were installed correctly and working properly. The offset voltages were extremely temperature sensitive. Measurements for the various circuit voltages were just screwy enough to be inconclusive.

I could ask for no more tests, so requested the boards be sent back to me. I found the circuit basically worked as expected, but the offsets were indeed high on both boards, though I measured 0.6 V max rather than 2 V.

***** stop here and make a guess *****

Blowing on the board through a soda straw, the offset shot up when I blew on the op amp.

Washing the boards with hot water and soap, rinse and, after drying thoroughly overnight, the problem went away. Both boards have DC output offsets under 10 mV.

Lesson learned: assorted gunk on the board can provide electrolytes which may make the surfaces conductive, varying with the amount of moisture adsorbed.

Lesson learned: Stay clean (and dry) out there!
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