Re: Older Pre-Amp Volume Pot Not Working....


"Guitarmakermark"

I have a pre-amp out of an old Yamaha electric piano, the CP-70B.


** I know that horrible little POS - but never seen one for 20 years.

The
piggyback volume pots do not attenuate the signal at all, the signal
is always loud.

This pre-amp is circa late 70-'s mid 80's, and all components are thru-
hole soldered to a single layer of traces on the underside of the
circuit board. All the solder joints look good, there are no burned
or over-heated traces, nor are there any burned resistors or blown
caps.

The two volume pots seem to function okay, when tested with an
ohmmeter across the three terminal lugs.....the resistance rises and
falls as expected, when rotating the wiper shaft.


** Must be a crack in the PCB and a broken ground trace from the pot leg
grounds.

Do a ground continuity test and scratch the green resist around the relevant
traces till you can see the crack.


..... Phil



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