Marshall G80RCD Input Jack Replacement and Noise

[misterc57]

The existing input jack had all pads and traces broken or lifted. Jack tested good. Re-wired it, not a proud soldering moment.

Amp now has a buzz noise. Present on both preamp channels, but if those volume are dialed down to zero there is no buzz on the master volume turned up. Also no buzz if nothing is plugged into the input jack.

I am also seeing 35 to 40 mV DC on the output, even with all volume pots at zero and nothing plugged into the input jack.

Help! Thank you..

Images show before to after...

[misterc57]

Schematic

Marshall+G80RCD-G100RCD.pdf

[misterc57]

The sleeve of the input plug should be at ground, right? It is not.

[The Dude]

Yes, sleeve should be at ground potential.

[Jon Snell]

Usually when a socket comes in to my workshop in that condition, we must ask why.
The reason is usually due to a faulty contact in the socket itself and wobbling the jack around fixes it for a while but breaks the solder/track in time.
Doing all that work and leaving the faulty jack in there is a bit pointless.
Either you will have to do it again later on or you will lose a customer as he/she thinks you have done a crappy, unreliable job.

What would you rather do?

There is a link on the socket that switches ground to mute the final preamp stage. That is why you have no hum without a jack inserted.
Even 80mV DC on the output pin to the speaker is acceptable for this sort of amplifier.
(The schematic has a few errors, so don't believe all you read without checking because it is not the final version of the schematic).

[misterc57]

I hear you Jon. But first I'd like to confirm that it was not a mistake in my re-wire as it seemed like the socket was okay.

Perhaps I missed a ground connection or the socket is faulty.

If I attach a wire from the guitar bridge to the amp chassis, the buzz is gone on the clean channel and greatly reduced (but still present) on the gain channel.

Does anyone have a correct schematic?

Thank you

[g1]

You can see the ground pin of the jack goes to a trace that links to a much larger trace. Is it a wire link there or some kind of low value resistor?
Also circled is a pin on the jack that seems to be connecting to the pins with the diagonal wire but can't really tell from the picture. I don't think that pin should be connecting to the others.

[misterc57]

You can see the ground pin of the jack goes to a trace that links to a much larger trace. Is it a wire link there or some kind of low value resistor?
Also circled is a pin on the jack that seems to be connecting to the pins with the diagonal wire but can't really tell from the picture. I don't think that pin should be connecting to the others.

Is it a wire link there or some kind of low value resistor? It is a jumper wire.

I don't think that pin should be connecting to the others. There is no connection (2nd pic gray arrow).

After zooming in on the pic I see more trace cracking in the adjacent traces that I missed. Center of 2nd pic, red arrow and it looks like some need jumper wire repairs.

Have to work on another amp so I have to put this one aside for a few days.

3rd pic is for reference, another G80 I worked on last year.

[g1]

The sleeve of the input plug should be at ground, right? It is not.

Which ground? Not sure if you mean circuit ground, or chassis metal, which may or may not be connected. Especially if unit is disassembled.

[misterc57]

Which ground? Not sure if you mean circuit ground, or chassis metal, which may or may not be connected. Especially if unit is disassembled.

Yeah. Chassis ground. But I did find a jumper on the board with continuity to chassis ground.

[g1]

Here is a corrected version of input jack schematic.

[misterc57]

Fixed. Had to put two more jumpers in there. Shown by the white lines I drew (forgot to take a pic). The long one was the missing to ground connection needed to get rid of the noise.

Thank you all.