Speaker hum

[cigmas]

I have had my Logitech Z-2300 speakers, which I quite like, for around 15 years. I keep them on all the time. Unfortunately over the past several years they have increasingly developed a hum, which I am finding intolerable.

I contacted several repair shops and they won't take a look as they claim they can't get replacement parts, and instead suggest buying new. I see no reason for that and I would like to repair and keep using them if reasonably possible.

The facts:

- The hum/buzz is present without any signal input connected. (3.5mm jack disconnected)
- The hum comes mostly from the torroidal transformer in the subwoofer cabinet, (which also contains the electronics).
- The hum persist even if the speaker wires to the subwoofer and the satellite speakers are disconnected, i.e. it's really mostly from the transformer.
- There is a bit of the same hum coming from the speakers, but very little.
- The hum seems to be 120 Hz based on comparison to test tone clips on youtube.
- The two large 10000 uF capacitors, which are typically recommended to be replaced in the presence of such symptoms, measure at their rated capacitance by my DMM.
- None of the capacitors appear damaged to my eyes, though I did not test the others.
- The AC power plug for this speaker system has only two prongs.
- I tried plugging into different outlets: a UPS, surge protector, different outlets. The result is always the same.

In case it might help, this link has some information about the electronics of these speakers: https://www.electro-tech-online.com/threads/help-required-on-logitech-z-2300-toroidal-transformer.114427/page-6#post-941333

I'm not overly familiar with debugging electronics, but I have disassembled the board (so much glue!) and I have a DMM and soldering iron.

Here is my previous "work" on these speakers: https://www.avsforum.com/forum/89-speakers/3088462-diagnose-repair-buzz-hum-logitech-z2300-subwoofer.html I tried replacing the 10000 uF capacitors anyway, and not only did that not make any difference, but I lost the copper sleeving in the through-holes on the PCB and had to use wire to connect the two sides of the holes. Now I'm back to the original problem. Hopefully no more detours. 

Any recommended steps to diagnose and hopefully fix this hum would be most appreciated! It's driving me crazy.

[don.r]

Measure the AC voltage at the caps. They may measure fine for capacitance but you may need to check their ESR to be sure. Did you measure the caps in-circuit?

You could also try disconnecting the amp from the power supply, if possible, to see if its a loading issue (too much current draw). Could also be a fault in the transformer although usually happens in E-cores rather than toroids (core plates start to rattle).

Noise in the presence of no connections rules out ground loops.

Check that the toroid is secured tightly with rubber vibration dampers.

[m3vuv]

could it be the laminations on the transo come unbonded and loose?

[cigmas]

Thanks for the replies and suggestions.

I suppose I could take a look at the transformer, as that is one component I did not disassemble. Looking at online image searches for "Z-2300 transformer" it appears to have a plastic wrap, so it's unclear how much a visual inspection could achieve.

Do you mean measuring AC voltage across the capacitors? That would require access to the bottom side of the board and so far I've always reassembled things before powering the speakers.

I think the amp and power supply components are all on the same board, and I'm not familiar with the circuit diagram or where to measure. If you have clues, I wouldn't mind trying as long as it's reasonably safe.

As for measuring the 10000 uF capacitors, I measured them out of circuit, and also had them checked at an electronics shop with an ESR meter. Both measurements agreed: ~12000 uF for each cap. Their measurements were actually better than the replacement caps I bought/tested/returned.

[don.r]

Since the caps checkout the presence of AC on the power rail - you don't have to measure across the caps, anywhere on the main power rail will do - will indicate most likely excessive power draw further down into the amp circuit. If no or very little 120Hz AC is present say a few millivolts or less then your problem is likely the transformer. I wouldn't go tearing it apart but hold your thumb on it while its powered up - just don't touch any exposed wires.. haha - and see if it stops humming. If it does then it probably just needs to be secured better.

[cigmas]

When I get a chance, I will unbolt the toroidal transformer, which will require flexible wrench work and cutting some zip ties to free the wiring from the cabinet. For now I did (awkwardly) manage to press against it and the hum intensity changes, but it may just be due to better or worse contact with the cabinet and the resulting resonance. I could rotate the transformer though, so maybe it does need to be secured better. It will be frustratingly funny if the problem turns out to be purely mechanical.

I'm afraid I need the "for dummies" version of how to check for the presence of AC on the power rail. If I get a picture of the board, would that help with instructions?

[don.r]

If you can move the transformer its probably too loose. You can maybe unbolt the transformer and then add some rubber sheeting or other soft, non-conductive material to secure it and then bolt it down again. Might be able to work with a picture. You just need a ground point and an exposed point of the power rail to put your meter probes on, making sure the meter is in AC volts mode. Sounds more like a mechanical issue with the toroid, however.

[cigmas]

I finally got to the transformer. Reaching from the back was just too difficult so I relented and removed the grill and speaker front the front. Now I've disassembled everything!

The transformer hums even when not in contact with the cabinet.

Model: TOU433027F0
Input: AC 120V/60Hz (BLK-WHT)
Output: AC 19.9Vx2 (YEL-BLK-ORG)

I found a picture of the board from another forum (https://www.avsforum.com/forum/113-subwoofers-bass-transducers/2852082-trouble-logitech-z2300-sub-speakers-no-sound.html) that is clearer than what I could take:



I started by trying to measure AC voltage between the green ground (at middle left of image) connected by a screw to the heatsink and the orange (salmon looking) wire coming from the transformer (top right blade connected wire in the square at the lower left of the image). In that same area, the yellow and black (not connected at the square, that's just a crimp) also come from the transformer. The wall power is coming from the black and white wires at the lower right of the image, and passing through the fuse and switch, all surrounded by the yellow muck, and then going off to the transformer.

While taking this measurement (~20V) I got a bright and loud spark. Not sure what happened, but at least there was no magic smoke and the speakers still work, so I will wait for further instructions from the more experienced!

[don.r]

The black square is a bridge rectifier. It converts AC coming from the transformer to DC. You want your meter set to AC and to test NOT the wires coming from the transformer but the OTHER two wires connected to the rectifier. Your spark was probably you shorting something with the probes. Generally, place one probe and make sure its secure and then carefully place the second probe. Without the amp being plugged in, test in ohms mode on the meter if there is any resistance between the screw on the right near the writing on the board and one of the pins on the rectifier. If one of the pins registers 0 ohms or near enough, you have found the negative pin. The other pin (not connected to the transformer) is your positive and you can now use the screw as a ground reference to securely place your negative probe lead when doing voltage measurements.

Ideally, you should not see any AC. If you don't and the transformer still hums its likely the transformer laminations have become loose. Not really fixable without a new transformer and you would need an exact match.

One other outside possibility is that there is some DC present on your power lines. Not very likely though.

At least you know your transformer is spitting out 20V.

Somebody went to town on the hot snot. Are those screws at the top corroding or is it just the picture that makes them look like that?

[cigmas]

Thanks so much for the clear instructions, @don.r.

The negative pin (0 ohms to ground) is the green wire at the rectifier, and the positive pin (~10 megaohms to ground) is the pink wire. Both measured no AC voltage with the power on. I also tried the mV range on the meter.

Superficially, I don't see any obvious problem on the transformer. If that is the source of the problem, at least I know, and can look for cheap used Z-2300 sub or parts, or consider different speakers. For now I'll just power them off at night.

I really appreciate the diagnosis help!

And wow, yes those top screws in the image don't look great. Maybe corrosion or maybe glue to keep them in place. Mine are clean and had some bright red stuff on each, perhaps threadlocker. On the other hand, they went easy with the hot snot on that board. Mine got far more glue all over, and removing (some of) it was by far the worst part of removing the board the first time.

[cigmas]

I reassembled everything, and added foam sheets (from arts and crafts supplies) between the transformer and the cabinet and also between the transformer and the metal plate that secures it from above. A thin foam was already present, but has been crushed/flattened in some places. My main concerns are whether the new foam will withstand the temperatures since the transformer gets warm under load, and how long it will last before getting crushed and losing thickness. Since I could hear and feel the transformer vibrate even when held in the air, I'm not sure it can be made totally silent, but now it is significantly quieter...

... to the point where I now hear the hum through the speakers! i.e. while the main power switch on the sub is on, if I turn off/on the power at the control pod which toggles the amp, I can notice the difference in the hum. Plugging in headphones at the control pod similarly bypasses the amp and also stops the hum from the speakers. Changing the volume has no effect on the hum.

Is there a chance of eliminating this hum? How is what appears to be a mechanical hum making its way into the speakers electrically?

[m3vuv]

At a guess i imagine the laminations of the transfo moving will put modulate the output of the transfo,may be totaly wrong but my guess.,cant you try using a different psu cobbled into it?

[cigmas]

Unfortunately I don't have a spare transformer or PSU.