behringer UB2442fx Pro mixer - clicking / popping noises on outputs

[jnsomerville]

Hi - i'm new to this forum - however reading through some of the threads I'm pretty sure there is a raft of information and likely one of you guys have expereinced something similar.

i'm looking at a Behringer UB2442FX pro mixer at the moment as it has developed a really interesting but annoying fault... not uncommon I know.. however I can't find the source.

Symptoms are: clicking and popping noises on main out and headphones, VU meters are responding to those, even with volumes turned down. Muting channels turnong all faders down etc have no effect. I initially considered that this was PSU derived however I cannot find any anomalies on the various voltage rails using a scope, I appear to have +/- 17V and 5v I think, but I can't see any glitches on those rails.

my gut feeling is that this is something common but head scrathing prevails! I admit i'm flying blind however as I don't have a schematic / service manual..

If anyone has any plausible ideas i'd be grateful for a steer or two!

Thanks

[km6xz]

If the master faders are down, and it is the same level it has to be something fairly high level and post fader. Are you monitoring the main out or control room out. Does the sound exist in Control Room out and sound the same as the mains out. Are all the returns at minimum? There are not many things between the faders and main output but there are inserts, what happens if you plug something into the main inserts, any change?

[jnsomerville]

Thanks for the reply, i've been away hence the delay in getting back to your post. I'm monitoring main out at present but is same in either mode. All returns are at minimum no change if I plug into the main inserts no change either, the pops, hissing and clicks are audible via headphone and noticeable on main outs when faders increased, ctrl out (same) main outs (but attenuated). plugging into inserts makes no effect.

[jnsomerville]

I also note that not on direct outs but is on sub outs all with faders down and muted etc

[diydidi]

unplug the fx board. See if it goes away...

[jnsomerville]

Done that and still there - currently scoping a Sub-out and headphones in - all faders down and channels muted clicking and popping still present :-(

[jnsomerville]

I note that it only appears on main outs if I ride the main fader, is on control room outs / sub outs in harmony with headphone out all of the time, with FX board removed and all faders up or down, with all channels muted

[diydidi]

Great! You are making progress.
Scope the last opamp before the main outs, and work your way back. Scope the main fader etc too.
The fact that it's only on the main outs, suggests the SMPS is fine.

[jnsomerville]

It not only on main outs, its on control out and sub outs as well, but not noticeable on the SMPS lines... strangely i've had unit on now for a few hours and the pops and cracks have become less frequent, i hazard a guess I have something warming up like a dry joint somewhere common ...

[diydidi]

You're gonna have to scope all opamps in the main section.
Sometimes using an amp as a signal tracer is quicker and more just than a scope, especially in these cases. BUT, its personal opinion I suppose.

[jnsomerville]

Ok thanks for the advice, i'm flying blind a bit as I dont have a schematic or layout but i'll work back from outputs

[Jazz P Bass]

See if the 1832 schematics are any help.

I would definitly prove that it is Not the power supply low voltage rails.

behringer ub1832fx.zip.

[jnsomerville]

Excellent thanks for the diagrams, i'll have a look this afternoon.

[jnsomerville]

this was really helpful, I think i've spotted itthe glitches coming from the 12 v line using your diagrams - much appreciated

[km6xz]

UB-SPSUREVD.pdfThe 12volt line only powers the external light socket so if you are seeing it there, you are probably going to find it on the +/- 15 rails that power the audio stages. Failing filter caps in the power supply is a common cause of that.

Here is the schematic.

Unless you are really familiar with switching power supplies, I would caution you to get into it if you do not have a fully isolated isolation transformer and know precisely which part of the PC board it mains hot. You can only be safe measuring on the secondary side of the power transformer.