Help with tracing a Solid State Marshall Hum Please?

[Helmholtz]

Actual hum is only around 3mVpp (maybe 3.5mVpp). The rest is the HF interference on top.

[Barnsleyboy]

Ok, this is no 100Hz hum as period is about 20ms, so we can exclude power supply ripple.

Looks like 50Hz line frequency.

Now short R12 using a short wire. Any change in output hum signal?

Shorting R12 stops the hum completely!

[Barnsleyboy]

Agree with Helmholtz, 5mV p-p hum is very little.
Try disconnecting one of the speaker leads and turn the amp on. How much mechanical hum do you hear?

Disconnected one, then both speaker leads, can hear mechanical hum of transformer, but obviously no hum through speaker!

[Barnsleyboy]

Disconnected one, then both speaker leads, can hear mechanical hum of transformer, but obviously no hum through speaker!

Sorry, just done it again, disconnecting the -ve speaker cable stops the hum.

[Barnsleyboy]

..... but obviously no output from the speaker!

[Helmholtz]

Shorting R12 stops the hum completely!

Means the hum comes entirely from the preamp.
I tend to suspect a shielding problem.

Does the amp cabinet have some metal shielding foit inside?

(Of course disconnecting any speaker lead stops speaker hum and signal output).

[Barnsleyboy]

No, it's all original. Is it likely to be a cold joint? or defective component do you think?

[Helmholtz]

No, it's all original

Referring to which post?

[Barnsleyboy]

Sorry, I meant to say there is no shielding in the amp!

[Barnsleyboy]

On another forum somebody asked me if I could remove the op-amp (it's soldered in), or alternatively "Try grounding pins 2+3 - is it quiet if you do that?"
Could it be an issue with the op amp?

[Helmholtz]

On another forum somebody asked me if I could remove the op-amp (it's soldered in), or alternatively "Try grounding pins 2+3 - is it quiet if you do that?"
Could it be an issue with the op amp?

Rather not. No component in the amp can produce a 50Hz hum by itself - except the power transformer and its leads to the rectifier.

Try some shielding: Cover the entire open side of the amp chassis with a grounded metal plate or (aluminum) foil.
I have to say that I never saw one of these amps. But physics is the same everywhere.

[Barnsleyboy]

I've just tried laying a sheet of aluminium foil over the top of the chassis. Crocodile clip back to the -ve speaker cable, no change in hum. clip to the chassis, no change and clipped to the amps internal earth terminal, no change. The hum is still there!

[Helmholtz]

I've just tried laying a sheet of aluminium foil over the top of the chassis. Crocodile clip back to the -ve speaker cable, no change in hum. clip to the chassis, no change and clipped to the amps internal earth terminal, no change. The hum is still there!

Ok, now scope the rails using AC coupling.

[Barnsleyboy]

Ok, If I knew what that meant I'd do it :-)
Could you talk me through the basics?

.... like I said, total newbie!

[Helmholtz]

Rails means the +/-19V power supplies.
AC/DC coupling is selected via the push button to the right of the input channel BNC connector.

For all the tests you should have (had) nothing connected to the amp input. Rather the input should be shorted to ground. Hopefully the amp has shorting input jacks.