Superphon Revelation Dual Mono - buzz/hum one channel only

[kirk57]

My Superphon Revelation Dual Mono has developed a loud buzz in one channel, audible both line level inputs and louder on phono input.

Some background: I was testing some speakers and moved the speaker wires and interconnects during testing (preamp was muted at the time)
After this, the preamp immediately had a loud buzz in the right channel.
Signal is still present on the channel with the buzz.

This preamp is dual mono all the way to the (single) transformer. I've replaced the electrolytics last year (exept for the 4 largest ones in the power supply) and it's been working very well. I've checked for cold solder joints, but see none, the preamp was not moved/dropped etc.

I have an EE degree but have done little circuit level debugging since graduation 12 years ago (more of a software-related job). I do have a scope, DMM, L/C and ESR meters, etc.

So far I have tried isolating any bad components by tapping with a wooden stick but can detect no change in the sound when doing so. The parts count is very low in this unit, so I pretty much tried the tap test on everything. The large 1000 uF electrolytics in the PS are 25 years old but show no sign of leakage.

Can anybody think of anything that I can check? Where would you look first?

The schematic for the preamp is at http://www.diyaudio.com/forums/galle..._schematic.jpg.

I appreciate any suggestions you might have.

[PB2]

Measure the DC offset at the output between the diodes and before the output caps - what is it for each amp?

Measure the bias as the voltage across R6 and R7 - again for each of the amps.

Are you sure it is the pre amp, have you swapped channels to test or listened through headphones to rule out the power amp?

[kirk57]

OK, I don't know why I didn't think of this before, but I have 2 of these preamps. So I set them up side-by-side and started comparing voltages. Straight away I found a large voltage drop across one of the 1000 uF electrolytics in the PS, and I can confirm that this cap has failed. (reading 2uF). The power supply in not shown on the schematic.

I checked the voltages across R6 and R7, they are .41 volts across R6 and R7 for both units.

At this point I assume that the problem is the 1000uF cap. Seem reasonable?

[PB2]

If by "large voltage drop" you mean AC voltage then yes that is probably the issue.

[kirk57]

Originally Posted by PB2

If by "large voltage drop" you mean AC voltage then yes that is probably the issue.

Yes, I should have been more specific. Seeing a little over 9v AC across the bad cap, about 0.07 v AC across the other 3 good 1000 uF caps.

I'll replace all the 1000uF caps in both units for good measure.

thanks for the help!

[kirk57]

Update: got the new caps, installed them and all is well. woohoo!