Hum/buzz on Trace Elliot Commando - not filter caps!

[Helmholtz]

The 0v points are within 0.2 Ohms

I think 0.2R is good enough.
Does the cabinet have some shielding inside? If so mounting the amp into the cab might help.

[Rhod]

No shielding that I can see, only hunks of classic green carpet covered plywood and hardboard, unless there’s some in the sealed driver cabinet - I’ve not looked inside! Might it be worth lining the amp cavity with something as I’ve done around guitar pickups in the past?

[g1]

If a clean & tighten on a couple pots' chassis connections made a difference, it may be worth doing it for all the pots and jacks regardless of their ohm readings. And screw type ground connections of pcb also (if it has any).

[Rhod]

Thanks, G1 - good to see you here!
Yes, when I cleaned the pots I did them all, but there is a screw ground connection that I've not touched, which I will return to this evening... From memory I'm pretty sure the jack sockets are insulated from the chassis at the point at which they are screwed in (plastic threads) but I will check those as well.

[Rhod]

Yep, sockets are all plastic. Tried putting it back in the cab - no difference, sadly. Still got that 50hz with the vol on zero, which changes as you turn the volume knob with the 50hz returning, louder, as full volume is reached. Will be taking it out again and scratching my head a bit!

[Helmholtz]

You might check if all 4 preamp rectifier diodes (D1,2,3,4) are good.

Do you have a scope?

[Rhod]

D1-4 all show a 575-585 mV drop and rising to OL when reversed.
No scope, sadly .

[Helmholtz]

D1-4 all show a 575-585 mV drop and rising to OL when reversed.
No scope, sadly .

Ok, no rectifier problem.
Earlier you reported that the hum changes if you touch certain points. What are these?

[Rhod]

I made that observation when the pots were not attached to the chassis, apologies, so not relevant I would think. Touching the pot shafts does not affect the sound now that they are firmly attached to the chassis.

I'm interested in the observation that shorting R10 cuts the hum. BTW when the preamp power supply PL1 is disconnected the hum is still there, something I found out by accident...

[Helmholtz]

I'm interested in the observation that shorting R10 cuts the hum. BTW when the preamp power supply PL1 is disconnected the hum is still there, something I found out by accident...

This however points to power amp. Please check for +/-15V supplies, especially across Z1 and Z2.

[Rhod]

15v supply:

At junction of R34/C18 = 14.97V
At R37 = -14.52V

At Z1 = 14.99V
Z2 = -14.51V

Voltage drop across Z1 = 15.11V
Voltage drop across Z2 = 14.66V

[Enzo]

15v each, close enough.

[Helmholtz]

The OP reported that the 50Hz (?) hum stops when shorting R10, but disconnecting the preamp PS doesn't make a difference. So I'm wondering if the hum could be introduced by the thermal mute circuit around TR1. Maybe disconnect pin 3 of TR1 to test.

[Rhod]

Still the hum at the speaker with pin 3 of TR1 disattached with vol at zero, but less hum when vol is turned up all the way.

[Helmholtz]

Problem is that without a scope and some output measurements I don't see a systematic way to trace your problem. Leaves trial and error.
I would check the power amp rectifier bridge OR1 (removed from circuit) and ecaps C1, C2 and maybe C18.
There is also a slight possibility that one of your new C15/C24 caps is not good. But that would cause 100Hz not 50Hz ripple hum.
Do you have equal voltages across C15 and C24?