Studer A800 audio noise from PSU

[Pat Maki]

Greetings good people at GrouDIY. Getting a ~100hz hum off the repro head when recording (electrical hum). It's on channels 17-24 only, so logic tells me it's caused by something that effects an entire row of channels. After testing with 24/15v cables in the VU section, I tried swapping audio power supplies (the 3 biggies at the back bottom) and that made it clear the issue is with the third audio power supply. I'd previously updated all the elcaps and rifas and double checked all solder joints, so it's not a cap, or poor connection issue. Wondering if it might be a bad rectifier. Have a scope, but not sure what I should measure to trace the issue. Any guidance by the experts here would be greatly appreciated.

 

[juanito2008]

Check with your oscilloscope the +/-15V rails.
And verify the Molex that come from the power supply.

 

[guy_4]

Probably a bad rectifier bridge.

 

[Pat Maki]

Check with your oscilloscope the +/-15V rails.
And verify the Molex that come from the power supply.

Thanks @juanito2008. Was able to get PSU #2 and #3 (GR 70/71) both plugged in fully and with lids taken off to run some comparison tests. Your idea on the +/-15V seems to be on to something. I found that the red wire that goes from the +15V stabilizer board to one of the 22,000uf elcaps is showing ~23V instead of 15V! Also the thin red wire next to it that goes to the power supply PCB is reading at 23V as well. No bueno. Next, I need to get in there to see if I have something wired incorrectly, or if it's a faulty component, or something else. Thanks for the tip!

 

[Brian Roth]

The schematic you posted shows 23V at the positive end of C2, a 22,000 cap (although it shows the wire as orange). That would be correct. The regulator needs a voltage input higher than the desired 15V in order to function.

However, the voltage between terminals 4 (or 6...same thing) and terminals 5 (or 2) should be 15. Another place to measure is across D2 since it connects to terminals 4/6 and 5/2.

If there is indeed 23V there, the regulator isn't doing it's job. A likely culprit would be a collector to emitter short on Q1 which can be checked with an Ohmmeter with the power off and maybe a minute wait after power down to allow the voltages to bleed off.

Bri