Peavey XR600C 2 minute hum

[soundinvst]

There is something in the signal path of the preamp that is causing a buzz at cold start for a couple of minutes and then it fades away. The hum is only present when the 6-channel board is plugged up. Volume and EQ pots alter the the tone and volume of the noise and it's compounded, meaning each channel builds or adds to the previous channel leading me to believe the problem is at the beginning of the signal path.

Positive and negative low voltage rails are goods and I've swapped out each of the ICs just to eliminate them as the source. After a couple of minutes the noise will fade away and doesn't return until the unit is turned off and allowed to cool for a while. I've tried spraying components with cold spray to see if I can make the hum return but without success.


There is no abnormal DC voltage on the output when the noise is present. The reverb pan is disconnected. Other than the buzz at startup the mixer works fine. Any suggestions would be appreciated.

Thanks!

R

Peavey-XR600C-Schematic.pdf

[TomCarlos]

Just to make sure we isolated the problem, if you disconnect the 6 channel board and insert a signal into the Tape In inputs (use your iPhone, iPod, other device), the amp will work just fine?

Check the solder connections on the 6 channel board. Check to make sure the XLRs are not damaged. I would pull the 4558 ICs but leave channel 1 in place. Do you still have the hum? Test it with a mic or some other source. Then add ICs to get channel 2 working. Again, same thing. See if you can isolate which channel is the problem.

[The Dude]

When I hear that something has a symptom of hums until warms up, I think filter caps that are starting to dry up. If you have one, scope your power supplies at power up and see if one of them has excessive ripple. If not, check/substitute/replace/clip in filter caps and see if you can find the offender.

[soundinvst]

Thanks for your response!

Unplugging the jack board and putting signal through tape in works perfectly.

Removing all ICs from the jack board kills the noise, but when I add 3 or more in any location the noise starts, faint and briefly at first, and increasing in duration and intensity when more ICs are added.

I touched up the solder connections on all the XLR and 1/4" inputs early on, and ran plugs in them to see if it had any effect on the noise.

I'm looking for something at the beginning of the signal path of the jack board that runs through all 6 volume and EQ pots, amplified by all the ICs, and becomes stable after 2 or 3 minutes of operation. Phantom power?

[g1]

Like The Dude suggested, it sounds like the added load (3 or more IC's) on the low voltage supply is making it hum. Look at the filter caps for the +/-16V (or whatever those IC's are running off).