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

[Rhod]

Dear all,

I've got an old Trace Elliot Commando 15 100W model 1001 bass combo. It sounds good, and is perfectly adequate for my modest bass playing needs. Schematic attached (schematic is from the mk 2, the only schematic I could find - I think mine may be the mk 1 but very similar circuits). It had been making a loud buzzy hum for some time, and I thought it was time to sort it out. Since it was a 100 Hz kind of a noise, I thought it must be filter caps, so ordered replacements for C15 and C24, and, noting that the pre amp had its own supply, ordered replacements for C3 and C5 as well for good measure. Popped the new ones in, and they made not a jot of difference! With the amp open, I noticed that the bulk of the noise was coming from the transformer itself, but with quite a bit still making it to the speaker. The transformer is mounted tightly with rubber insulation. The screws that run through the transformer are as tight as anything.

Anyone got any wisdom? Do I seek out a replacement transformer? What else could be the culprit that's allowing the noise to get through to the speaker?

Many thanks, folks!

[Enzo]

You have two things. One is the hum out the speaker. You need to solve that. As to mechanical noise/hum from the transformer, that isn't making anything come out the speaker. It makes the sound you can acoustically hear. I don't think a new transformer will silence the speaker noise.

100Hz means the hum comes from ripple currents, but not necessarily because caps are bad. Grounding is important. I mean in the circuits, not whether we are properly earthed.

[Rhod]

Thank you, Enzo. Is the grounding issue likely to be something inherent in the circuit or
something broken i.e. am I looking at modification or repair do you reckon?

[Jon Snell]

Ignore the transformer mechanical buzz, that is loose laminations and normal on these older amplifiers.
Check the 0v line between C15 & C24, make sure it goes to the transformer ground first and that the connectors are clean.
They are normally quite quiet.
100HZ or 50HZ hum?
Affected by any of the controls?

[Rhod]

Thanks, Jon - You may have to walk me through that a bit - apologies. The 0v line doesn't seem to connect directly to any ground from the transformer - looking at the schematic and the board itself. Are you able to point me towards what I should be looking for?

The hum is 50Hz at the speaker with the gain and volume turned all the way down (what I was hearing before was mostly the 100 Hz from the transformer). The gain knob makes it louder. The volume knob also makes it louder, and the tone changes to about half way inbetween a D and a D#, so I'm guessing 75Hz, before reverting to 50 Hz as you get to full volume.

Many thanks!

[Jon Snell]

This is not the easiest for most to understand.
This amplifier uses a floating ground output stage.
The +ve connection on the loudspeaker is virtual ground and the -ve is the signal. Very odd for most.
If the hum is 50HZ it will be a ground issue.
Check the 0v points and ensure no body has shorted either the input ground or output ground to chassis. Make sure the capacitors in the drawings are not short circuit. C26 and C21

[Mick Bailey]

I call that a flying rail amp.

[Dave H]

The 0v line doesn't seem to connect directly to any ground from the transformer - looking at the schematic and the board itself.

It's a “flying rail” design (as Mick said) The transformer doesn't connect to 0V. The output of the power amp is connected to 0V making the junction of C15 and C24 the output. The C15/24 junction and the output stage power rails move along with the signal.

[Rhod]

Thanks, folks. All the 0v points seem to be connected to each-other and there doesn't seem to be any continuity with chassis ground; C21 and C26 aren't shorted.
In a correction to my earlier observation, the gain knob doesn't increase the loudness of the hum in the way the volume knob does, but it does change in character as it (gain) is turned - introducing that 75Hz hum.
I guess this point the finger at the power amp section rather than the preamp?
(Just in case it's relevant, touching the vol pot shaft with a finger increases the volume of the 75Hz hum, touching the gain shaft removes it...)
Appreciate your help on this.

[Helmholtz]

Thanks, folks. All the 0v points seem to be connected to each-other and there doesn't seem to be any continuity with chassis ground; C21 and C26 aren't shorted.
In a correction to my earlier observation, the gain knob doesn't increase the loudness of the hum in the way the volume knob does, but it does change in character as it (gain) is turned - introducing that 75Hz hum.
I guess this point the finger at the power amp section rather than the preamp?
(Just in case it's relevant, touching the vol pot shaft with a finger increases the volume of the 75Hz hum, touching the gain shaft removes it...)
Appreciate your help on this.

I don't think your amp can generate 75Hz. Most likely it's actually 150Hz (octave above 75Hz), the third harmonic of 50Hz.

Does the hum stop if you short R10 (100k)? If so the problem is in the preamp.

[Rhod]

Yes! Shorting R10 stops the hum - so problem’s in the preamp, then, thank you Helmholtz. Where should I be looking to address this do we reckon?

[Helmholtz]

Yes! Shorting R10 stops the hum - so problem’s in the preamp, then, thank you Helmholtz. Where should I be looking to address this do we reckon?

Do you get 33V across the 33V zener (Z5) or C5?

Any change if you short terminals 3 and 1 of the gain pot (RV5)? ( actually should be the same as turning the gain down).

[Rhod]

33.6 wrt 0v across Z5/C5
No or minimal change when shorting Gain pot pins 1 to 3.

[Helmholtz]

Thanks, folks. All the 0v points seem to be connected to each-other and there doesn't seem to be any continuity with chassis ground; C21 and C26 aren't shorted.
In a correction to my earlier observation, the gain knob doesn't increase the loudness of the hum in the way the volume knob does, but it does change in character as it (gain) is turned - introducing that 75Hz hum.
I guess this point the finger at the power amp section rather than the preamp?
(Just in case it's relevant, touching the vol pot shaft with a finger increases the volume of the 75Hz hum, touching the gain shaft removes it...)
Appreciate your help on this.

Did you check for zero DCR between all 0V points?
Also tighten all pot nuts and make sure you have 0 DCR between all pot shells and chassis.
If you have shielded wires, make sure the shields are well grounded at one end.

[Rhod]

The 0v points are within 0.2 Ohms - is that as good as it gets or should I be looking for why it isn't 0 ohms?

I checked the pot chassis and noticed that two of the pots were about 0.5 ohms wrt chassis. Gave them a clean, retightened, and the hum level is much improved: there is still some 50 Hz at the speaker, but unaffected by the gain and volume controls. Definite progress, thanks!