studer a779 / revox mr8 intermittent buzz problem

Hello,
it's been a while since I posted a subject.. but now a problem gets me back to you guys!
here 's the matter:
my newly acquired mid 90's Studer mixer has an intermittent problem in the psu.
almost everytime i power it up, after an hour of use aprox, a buzz arises suddenly. I tracked it down and found some 50Hz hamonics on the -15V power rail .
when the buzz is present (especially with a channel and preamp cranked), the rail is still at its  -15 v DC  value and  the mixer's current draw raises about 25% above normal. the voltage at the 2200MF coupling cap before the regulator then gets down from -21v aprox to -18volts. I can clearly see some sine spikes on the oscilloscope.
the temporary quick fix is to switch off and on the mixer. and the buzz disappears for another long period of time... but comes back a couple hours later.

I suspected some heat-related problem but my quick fix countersays  this as a quick restart doesn't change the temperature of the internal components.
I remelted every solder joint in the PSU card, without success. I also reset the regulators heatsink screws and inspected connections but everything looks fine... and the heatsink doesn't get too warm.

here's a schematic with my record of voltages of the psu when no buzz is to be heard.

maybe that some of you  have more experience on such PSU symptoms and faults?

I would love to get it back in shape as the sound/ format of this device  is top notch!

thanks in advance for your ideas !

My guess would be that one of the silicon devices that passes a lot of current is burned up inside. So the rectifier or LM337. It's such a simple board. You might as well just "shotgun" it. Meaning judiciously replace anything that is vaguely related and see if it fixes it. So in this case, I would replace the bridge rectifier and both LM317 / LM337 and larger electrolytics. It's $10 USD in parts.

thanks for your input squarewave,
today the board worked for 4 hours without a problem .. it's not the first tile and the problem keeps coming back.
I'll try replacing caps and regulators.
it won't  do no arm on this 20 y old board...

And rectifier. If I had to pick one part, that would be it.

sure !

How hot does the diode bridge get? But you say the current consumption goes up? Pull parts of the mixer till you can identify the part that draws the high current.

I 'll try touching the diode bridge and feel if that's warm.
the current is a little higher when the buzz is there. (+20% aprox)

I'll try unplugging the modules one by one.
but the noise is intermittent.
most of the time it's working perfectly. . so unplugging modules doesn't tell me which is problematic.
as soon as I power off / on, it takes 1-2hours for the buzz to appear...  multiply that per 8 tracks .. and it will be 2 days to know which is wrong, if the problem lies in that place.
However,  I already ruled out 3 channel strips as the problem appeared with those disconnected.

And also try spraying cool air over suspect components, to see if it fixes the problem.

thanks

Hello !

Squarewave, your first guess was the right one.

I sprayed cool air on the bridge rectifier and the buzz was gone. and came back later on.

I 'll swap this component.

thanks a lot to you all

btw, these little studer / revox are very nice sounding  little boards !