Peavey Alphabass round two-troubleshooting

[catalin gramada]

Output capture with PI in and off

[catalin gramada]

All board and chassis contacts in power section was reflowed and cleaned. The hum is still there, Swapping couple of novals was same. Think the noise is part of circuit supplied at such high voltages. The tubes are very high supplied 450v for buffer,425 for PI and 400 for fx recovery. The heaters are well supplied at 6.5-6,6v aso. May be possible to get a bit of noise in this circuit , is minimal, but still annoying.

[catalin gramada]

Output capture with PI in and off

I don't know what is all this junk. Looks like a summation of signals with a periodically timing..May some enlighten me please ?

High frequency over 1 khz components was filtered by scope to get a clearly image about modulation

[Helmholtz]

I don't know what is all this junk. Looks like a summation of signals with a periodically timing..May some enlighten me please ?

Hum with PI tube in is mainly 50Hz fundamental with some harmonics. Pulling the PI removes the 50Hz content.
Doesn't look like PS ripple.

[catalin gramada]

Hum with PI tube in is mainly 50Hz fundamental with some harmonics. Pulling the PI removes the 50Hz content.
Doesn't look like PS ripple.

Thank you. There is a chance to get rid of it please ?

[Helmholtz]

I dunno.

Maybe try a humdinger with elevated wiper.

[catalin gramada]

I dunno.

Maybe try a humdinger with elevated wiper.

Allready did. Lifted original 4.7k and did 45v from plate voltage with a 220k/22k divider generously filtered by 220u cap right to whiper. Did not do any difference.

[Helmholtz]

Does the hum stop when you ground C27 at J62?

If yes, the power amp is fine.

You might try to find hum sensitive circuit point by approaching (without touching!) components or wires with your finger (other hand in pocket!).
Maybe wear an insulating glove for safety.

[catalin gramada]

Does the hum stop when you ground C27 at J62?

If yes, the power amp is fine.

You might try to find hum sensitive circuit point by approaching (without touching!) components or wires with your finger (other hand in pocket!).

Yes, shorted PI (at C27) input to ground make the hum to stop.

[catalin gramada]

So I have this scenarios: v2b input shorted- hum, v2 pulled off -hum, PI input shorted-no hum.

[catalin gramada]

Scooping the PI input with v2 inserted or not did not show any difference

[Enzo]

We were discussing this area of circuit 20 posts ago. Did you break the circuit at the 5-pin connector? That is where the signal goes off board to the FX loop jacks and back.

[catalin gramada]

Yes I did. Due to the fact those conector carries also the nfb and local grounds to chassis I broked the circuit to the other end of the cable who connect the DI board ,Then with couple of "U" shape jumpers wire I tried all combinations out of DI board (eg. local ground conected and not conected to chassis with pre conected and not connected to PI) etc. I.ll do a drawing...

[Enzo]

What if you simply use a piece of wire and ground the signal path at that connector? The ground pin is right next to the sigma pins in the connector.

[catalin gramada]

This is what I did. The DI board was left out of circuit. I did some jumpers to keep nfb conected and local ground of PI tried to link to chassis in different points