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

[Rhod]

That’s fair enough, Helmholtz. Sorry!
C15 58.4 V wrt ground on +ve terminal
C25 -58.0 wrt ground on -ve terminal

although readings fluctuated up to a volt.

I noticed that when I lifted the board up vertically to access the underside the hum git worse significantly as the board approached vertical. This was with board earthed with flyleads btw.

To be fair, the hum is better than it was! Definitely an octave below the transformer hum by the way (it would improve things if I could do something about that!).

Appreciate your help!

[Helmholtz]

Two more ideas:
1) You mighthave a strong hum field source in the vicinity of the amp. Try to see if the hum changes when you move the amp to a different position.
2) Shielding might help. Tack some aluminum foil to a large piece of cardboard and cover the chassis with it. Make sure you have electrical contact between foil and chassis.

And one last thing: Check that you have 10k resistance between 0V points and chassis.

[Rhod]

Approx 10k between 0v points and chassis (approx 9.9-10.1k)

Rectifier measures the following mv drops out of circuit:
590
572
552
563
with OL in opposite direction in each case.

The rectifier DR1 is heat sink mounted. The underside looks as if its been melted and no evidence of thermal compound. Does that sound right?

[Helmholtz]

Approx 10k between 0v points and chassis (approx 9.9-10.1k)

Rectifier measures the following mv drops out of circuit:
590
572
552
563
with OL in opposite direction in each case.

All good.

The rectifier DR1 is heat sink mounted. The underside looks as if its been melted and no evidence of thermal compound. Does that sound right?

Can't tell without a picture.

What are the voltage drops across the output emitter resistors R28/44 (0R33)?

[Rhod]

Fair point.

[Helmholtz]

Surely not what I'm used to see. But doesn't mean it's bad, especially as measurements were ok. As there is a possibilty of failure at a higher temperature I would exchange it.

[Rhod]

I will order a new rectifier - looks like RS have a direct replacement.

On another matter, in the picture is what I’m guessing are capacitors designed to reduce switch thump. I don’t know how long that one has been disconnected, but I’m assuming this has no bearing on my issue...just checking!

[Rhod]

OK, not sure if I'm measuring wrong, but the voltage on R44 is -60.6V measured at both ends - I did notice that this looks like a replaced part with some heat damage underneath - and R28 measures 0V both ends. What's going on here, or what am I doing wrong?

[Helmholtz]

OK, not sure if I'm measuring wrong, but the voltage on R44 is -60.6V measured at both ends - I did notice that this looks like a replaced part with some heat damage underneath - and R28 measures 0V both ends. What's going on here, or what am I doing wrong?

I meant the voltages across the resistors (measured from end to end), not wrt to ground or 0V. Expected values are around 10mV or 0.01V or even lower. Can your meter read such low voltages?

[Rhod]

Noticed that R44 had some heat damage on the board near it - and when I saw this I thought the resistor had been replaced. Removed it to test and the resistor itself has some heat damage to it. It is nominally 0.33 ohms and measures approximately that on my multimeter (not very accurate: actually measures 0.8 but the meter measures 0.5 with the leads shorted).

EDIT: Just read your latest post - I did measure for this but my meter didn't register a drop. Rats. Maybe I need a better meter

[vintagekiki]

I've got an old Trace Elliot Commando 15 100W model 1001 bass combo ... 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).

https://www.mikrocontroller.net/attachment/250285/trace-elliot-commando-schematics.pdf

https://www.mikrocontroller.net/attachment/251295/trace-elliot-commando-mk-ii-schematics.pdf

https://www.mikrocontroller.net/topic/361023
_{Reparatur Trace Elliot Commando 15}

Hi, I got an old Trace Elliot Commando 15 for a cheap price last week
Bass combo bought with the description "Constantly loud hum".

1)
Question
Does a loud buzzy hum exist when RV1 (Volume) is at min (0)

Check that the cause of the hum is due the ground loop.
Check signal and chassis grounds.
Check C21, C28 (100n), R40 (10k)

[vintagekiki]

It is nominally 0.33 ohms and measures approximately that on my multimeter (not very accurate: actually measures 0.8 but the meter measures 0.5 with the leads shorted).
Maybe I need a better meter

Your Ohm meter measures correctly.
Measurement with the leads shorted, in fact measures resistance of measuring cables (0.5 Ohm)

[Helmholtz]

In the German thread linked by vintagekiki (thanks !) the OP could cure his hum problem by replacing the volume pot (RV1,1MA).
If the hum stops when you solder a short wire between vol pot lugs 1 and 2 (thereby shorting the wiper and the gate of TR3 to 0V), this could indicate a bad pot.
And make sure that lug 1 measures zero Ohm to 0V points.

[Rhod]

Thanks, vintagekiki,for your input and for tracking down the mk1 schematic.

Helmholtz: RV1 lug 1 measures zero ohms to 0v points.

1. When you short pins 1 and 2 of RV1 it removes the louder hum at full volume, but the quieter hum when pot turned to minimum is still there, so could be worth replacing... This also answers vintagekiki's question:

Question
Does a loud buzzy hum exist when RV1 (Volume) is at min (0)

2.

Check C21, C28 (100n), R40 (10k)

Apologies for my ignorance, but when you say check C21 and C28 (polyester box capacitors) and R40 can I ask what you mean? Remove from circuit and see if they're working (remembering that I have no test equipment other than a multimeter) or report the voltages in circuit?

Many thanks, all!

[Helmholtz]

[QUOTE=Rhod;n912726]
Helmholtz: RV1 lug 1 measures zero ohms to 0v points.

1. When you short pins 1 and 2 of RV1 it removes the louder hum at full volume, but the quieter hum when pot turned to minimum is still there, so could be worth replacing...
[QUOTE]

As shorting pins 1 and 2 disables the vol pot and corresponds to zero vol setting, there is no indication of the pot being the culprit.