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

[vintagekiki]

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?

C21 and C28 present the coupling between signal and chassis grounds. Both capacitors, if measured with an Ohm meter should show infinite resistance (range 200k or more) and if your multi meter has a capacity meter the capacity should be 100n +/- 10% The voltage at both points is 0V dc

[Rhod]

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.

Of course, that makes sense.

C21 and C28 present the coupling between signal and chassis grounds. Both capacitors, if measured with an Ohm meter should show infinite resistance (range 200k or more) and if your multi meter has a capacity meter the capacity should be 100n +/- 10% The voltage at both points is 0V dc

Will check this out tonight - measurements in circuit?
No capacity meter sadly.

[Helmholtz]

Did you try to shield the chassis?

[nickb]

Can you do a couple of things for me? With the volume at zero report the ACV across the speaker terminals. The other is can you post a clear picture of the guts?

Ta!

[Rhod]

Apologies for radio silence...

Vintagekiki:
C21 and C28 both measure 10k ohms or thereabouts in circuit - so lower than suggested.

Helmholtz: Have tried aluminium foil-on-cardboard connected to chassis, place.. Doesn't affect the hum (I wondered if it would, as the hum gets worse as the circuit board is lifted to the vertical, and I've also tried turning off nearby fluorescent lights and transformers with no change)

nickb - hello! - AC voltage at speaker terminals = 0.3v BUT while I was there I also measured the DC offset, which I hadn't done before, and it was 0.4v, which sounds high to me....and when I went back to check the AC again it measured 0v (speaker out of circuit btw). Picture coming up...

Thanks, folks.

[Rhod]

[nickb]

Thanks for the pic and the speaker AC info. I could not recall if this model had a separate preamp with a long wire going to the power amp. They can be a source of hum. This is an all-in-one so it's not that.

If you are measuring 0VAC hum then the hum would be quite low level. I'm suspicious that you might be trying to improve on the original design . Do you have a mV AC range? If you do, try that. BTW it sounds like your meter might be an inexpensive one that doesn't do RMS nor blocka the DC. When you measure add a 100nF in series with the meter lead. Also if you see 0.4V DC again that is on the high side.

[vintagekiki]

From the PCB is not seen best.
Check that it is installed correctly
R30 = 3k3; R31 = 22 Ohm

[Rhod]

If you are measuring 0VAC hum they the hum would be quite low level. I'm suspicious that you might be trying to improve on the original design

You may well be right! If you've had a look back at this thread you will have noticed that the hum improved after I cleaned up the ground connections from the pots to the chassis and in addition I realised that a lot of the noise was the old transformer, rather than anything coming through the speaker! But I did then feel a bit on a mission to get rid of the hum that remained at the speaker which I sort of felt shouldn't be there at all! (None of my other amps hum like this!). So apologies if I've led us on a bit of a wild goose chase...

No mV AC setting, I'm afraid - if nothing else through this thread I've learnt about the limitations of my meter...

Measured 0.7V DC on the speaker terminals when switched on, but this gradually reduces. After about 5 mins it goes down to between 6-20 mV (constantly changing).

[Rhod]

Has anyone had any experience of repairing noisy transformers like this guy?

https://www.youtube.com/watch?v=dtOwewfqucI

[vintagekiki]

The quickest way to determine where hum/buzz comes from is to connect the amplifier to an external speaker.
The transistor amplifier is free to power on without a speaker. If hum / buzz is still heard without the speaker from the amplifier, there are 2 causes.
Either the transformer is not evenly tightened to the chassis, or the hum/buzz originates from the transformer because the sheets inside the transformer are not tightened to each other. The solution is to "drown" the transformer with a special varnish.

[Rhod]

(Tried to post over the last couple of days but unable to connect to the website - don't know if that was a more widespread problem! Anyway, we're now back in the room!)

Thanks, vintagekiki. I'd established that I had both issues: 100hz buzz from the transformer AND 50Hz at the speaker. Both were annoying: this thread has focussed on the speaker hum but I have news on the transformer!

Well noticed regarding R30 and R31 - this area has been repaired in the past and their positions on the board have been reversed (i.e. R30 is next to the printed label on the PCB for R31 and vice versa - not sure why). Having checked the circuit, I'm pretty sure that they are wired up correctly, however.

So I'm still left with the speaker hum, and maybe nickb is right and I'm chasing something unobtainable. I do think it's worse than it should be, though, and I haven't completely given up!

BUT I was looking at the transformer and considering a repair as above.The transformer is riveted to the chassis, on a rubber mat, and all seemed solid (and not easy to remove as I would have to drill it out if I was thinking of glueing the laminations). I had previously tightened up two bolts running through the core (top and bottom) and noticed 4 "spare" holes at the corners. I happened to have some bolts of the right size with wing nuts that lodged into the corners (difficult to get a spanner or socket in between the chassis) and tightened them up. Bingo! Transformer buzz almost disappeared! I can hardly hear it running now...until I connect the speaker of course... I wonder if it was meant to ship with those bolts installed. Anyway, this is progress.

[Smudger075]

Hi Rhod did you get anywhere with the speaker hum. I've got a similar situation. I did quiten the transformer using the extra bolts.

[Rhod]

Yes - although it’s not super quiet - it’s an old amp after all, it reduced the noise to an acceptable level for gigging although I’m not sure I’d record with it. Tbh I tend to record bass straight into the desk anyway!

Although I was miffed at replacing all those caps in the quest for a quieter amp I was happy to get something eventually that was usable. And I like the sound of those amps. They’re dirt cheap but sound really nice to my ears!

Apologies for not picking up your message before now. How’s your Trace sounding?!