Solid state amp noise / hum

[Ivan Funk]

I'm trying fix this 70's solid state Randall RG-90 amp. It works but there is noise and hum.
The noise and hum is slight at idle, increases with volume increase.

I replaced all the power supply capacitors and a few coupling caps etc...
Voltages are fine. The power transistor bias looks good.
Is there maybe some feedback loop oscillation happening?
I don't know where to look for that.

I had a 15" version of this amp years ago that I repaired and was able to get it dead quiet but my memory is shot and I didn't keep any record of it so I'm starting from scratch.
Any help is appreciated.

[Mike Auman]

If it were me, I'd probably do this:
1. Jacks without a plug are grounded by a switching contact in the jack. I'd verify that each jack is indeed grounding the tip when idle. An open idle jack can introduce noise.
2. Make a dummy plug with a 1k resistor across tip & sleeve of a 1/4 inch plug, and plug that into one input jack at a time (2 channels, hi-lo input for each channel.) Is the noise the same? If not, that can help you isolate one the part of the circuit.
2. Then I'd use an audio probe to check to the input and output of each FET/transistor to see if one of them was noisy. How to make one: https://www.diystompboxes.com/pedals/debug.html (make sure the cap is rated for 50V or more, you're running +/- 35V on your power busses.)
3. If you're still shotgunning, I'd check the protection Zeners Z1 thru Z6. One of those failing can cause noise.

[Ivan Funk]

Great info Mike thanks.

1. I have cleaned all the input grounding switch contacts but I will try your dummy plug also. Great idea.
2. I do have an oscilloscope and a signal generator but I like the audio probe idea. I will make one for testing the transistors.
I'm not familiar with how the JFET preamps work - trying to read up on that specific circuit.
3. I have checked Z5, Z6, D3, D4, but had overlooked Z1-Z4. I'll check those now.

I'm getting about 10mv ripple after the 1st filter cap in the power supply. Is that acceptable or is that not good?

Another clue - when I separate the preamps from the power amp (at C11) each side sounds good on it's own.

Thanks again,

[Dennis Detweiler]

Filter caps?

[Ivan Funk]

Thanks Dennis. Yes I replaced them all.

Oh my gosh Mike saved the day.
The LO input on channel 1 was the problem. Bad ground.
The inside lock washer had that white corrosion on it and wasn't making a good connection to the chassis.
That solved the main noise problem.

Now to tackle the slight hum at idle.