FX Loop Buffer Circuit Hum (Not a Ground Loop) (1989 Laney AOR 50 Series II)

[J M Fahey]

The schematic only shows the power supply for the IC on IC3B for some reason

which is: both Opamps are on same silicon wafer and share supplies, feeding one feeds both.

[neptoess]

Jon Snell
Reading 1.2-1.3 mVDC tip to sleeve on a short cable inserted into the return jack

Helmholtz
Hum is still present with a shorted plug inserted

[Helmholtz]

Hum is still present with a shorted plug inserted

Ok, that shows that the hum is produced in the amp (and not by an external ground loop) after the return input and that hum level depends on the return termination resistance.

I suspect 60Hz ripple on the -30V supply. Check all related filter ecaps.

[neptoess]

Helmholtz
All electrolytics were replaced 5 years ago. I don’t have an ESR meter, but I can certainly measure DC resistance to test them for dead shorts

[Jon Snell]

Jon Snell
Reading 1.2-1.3 mVDC tip to sleeve on a short cable inserted into the return jack

Helmholtz
Hum is still present with a shorted plug inserted

If hum is present when a link cable is used, it must be present when no link cable is used or do you mean just one plug inserted to break the link if so which socket are you plugging into.

[pdf64]

...I suspect 60Hz ripple on the -30V supply. Check all related filter ecaps.

If the multimeter is able, Vac at either end of R50 could be checked.

[nickb]

Helmholtz
All electrolytics were replaced 5 years ago. I don’t have an ESR meter, but I can certainly measure DC resistance to test them for dead shorts

Things can fail at any time. More than once I've fitted brand new components that were bad from day 1.

My money is on C28 or associated solder joints. Hum on that node would precisely explain with the variation in hum you get with the guitar volume. When the source impedance is about 25K the hum will null due to the differential input action of the opamp.IC3B

[neptoess]

If hum is present when a link cable is used, it must be present when no link cable is used or do you mean just one plug inserted to break the link if so which socket are you plugging into.

No hum when I actually jumper send to return. The hum is present when I simply insert a shorted plug into the return jack though.

[neptoess]

So, whenever I do get a chance to open this up and run it, I'll turn on the knobs on the front of the amp to 0, insert the shorting plug into the return jack, and check:

1. VDC and VAC between ground and pin 8 of IC3 (should be -30 VDC and 0 VAC, if AC present, likely that filter cap(s) C34 and/or C35 are bad). This should cover Helmholtz and pdf64 's suspicion of ripple on the -30 VDC supply
2. VAC between ground and the right side of R43 (should be 0 VAC, if AC present, likely that filter cap C28 is bad). This should cover nickb 's concern about C28 (I've got a good feeling about this one, since it's the only one that lines up with the weird behavior with the guitar volume pot)
   

[Helmholtz]

My money is on C28 or associated solder joints. Hum on that node would precisely explain with the variation in hum you get with the guitar volume. When the source impedance is about 25K the hum will null due to the differential input action of the opamp.IC3B

That's what I meant in post #18.
I seems that the output impedance of the send stage of around 1k provides good hum cancelling.

All electrolytics were replaced 5 years ago. I don’t have an ESR meter, but I can certainly measure DC resistance to test them for dead shorts

If a cap was shorted, opamp supply voltages would be off. There should be roughly -15V between R42 and R43 (as well as at the output pin of IC3B), meaning at the negative terminal of C28.

But check if the ecaps (especially C28) in the -30V / - 15V supply are wired with correct polarity, meaning positive end towards ground.

[neptoess]

But check if the ecaps (especially C28) in the -30V / - 15V supply are wired with correct polarity, meaning positive end towards ground.

C33, C34, and C35 are on the right side of the PCB. They have a + symbol printed to show proper polarity of the cap. They all look correct

[Jon Snell]

No hum when I actually jumper send to return. The hum is present when I simply insert a shorted plug into the return jack though.

Inserting a shorting link into the return socket means the hum is coming from the ground connection on the socket as it is connected directly to C30 and AC coupled to pin5 of the IC. There is an issue with C28 and the virtual ground to actual ground.

[Helmholtz]

Maybe increase the cap value of C28. Though actually 47µ should be large enough considering the low currents.

[neptoess]

If hum is present when a link cable is used, it must be present when no link cable is used or do you mean just one plug inserted to break the link if so which socket are you plugging into.

This was referring to a test I did where I inserted an instrument cable into the Return jack, and measured AC voltage tip-to-sleeve, and got a little over 1 mV

[Jon Snell]

If you measured with your DVM, you may well have been mislead by an inaccurate reading. DVMs are notoriously bad at low value AC. The inductance in the leads will give a few millivolts on their own just by shorting the probes together.
What have you got on your oscilloscope?
There will need to be more than that sort of ripple on the recovery amplifier as it only has a gain of around 4 even if there was 5mV of ripple, that equates to just about audible against the noisy valves.
I would go back to basics and start a fresh with the diagnosis.