Behringer GMX212 problems

[zvir]

Hi everyone.
I'm having problems with Behringer's GMX212 amplifier.

A few days ago it started to make noises. I checked the power amplifier section with an oscilloscope and found out that one of the LM3886 burned out. I then changed both of them, just to be sure. However, now the amplifier doesn't put out any sound at all.
Oscilloscope traces show that both channels are working perfectly, I can see the signal at the presence pot, but I lose the signal at the end of the preamp section, on what looks to me as a TL072 (I can't read the markings...)
When i put the signal in the aux input, it plays fine, so the power amp is not in question. All voltages are good and where they should be...

Any suggestions?
Thanks.

[Enzo]

Isolate the problem. Aux in works, OK. COnnect a signal out front to the input. Is ther signal at the FX send? Does putting signal into the FX return come out the speaker? Does patching a cord from FX send to FX return restore the sound? Maybe they call the FX jacks Mono Insert on this amp.

The presence pot feeds directly to the IC14b, which is a 4580. A TL072 would be fine there as well. That IC drives the mono send.

[zvir]

OK, I put 1kHz @ 200mVAC RMS to these points.

Signal fed into input - output at 0V, FX Send at -6VDC. This is not good, right?
Besides, no use patching a cord from FX Send to FX Return now?

Signal fed into FX Return - Output at cca 1VAC RMS. And now i have signal at the master volume pot.

Correct me if I'm wrong, but it seems to me that this amplifier has some sort of a buffer between the preamp channels and the FX loop, and one between the FX loop and the power amp section. And there is something wrong with the first one.

One more thing that bugs me. IC15 looks like a TL074 to me. if it is, all itsoutputs are at 0V, two amplifiers have 1VDC at their inputs (both inverting and non-inverting), and the other two have inputs at 0V... This is not good either, right?

[Enzo]

OK, so the power amp is OK, and the amp works from the FX return onwards, right?

I mentioned in my previouos post that IC14b drives the FX send, so there is your buffer.


I don't know how you can get -6v on the FX send. IC14b is pin 7, but it drives the send through C47, a 10uf/25v. Even if IC14-7 is railed the cap should block it. C46 blocks DC on the return path, and the jacks are simple wired together. But easy enough to find out if that is the case.

ANy chance the meter was on the footswitch jack?

-6v or not, is there signal present at that point? And would it do any good to patch a cord?

Advice I always give my trainees is "never think up reasons not to check something."

The IC15 is an 074. It sends signal to the power amp at 14 and 1, while 7 and 8 drive the line out jacks. You already know the first pair works, and since feeding the FX return sends signal on through, stick a signal there and see if the line outs have signal. If I find no DC on the corners of a 074, I usually stop trying to find something wrong with it if it passes signal.

[zvir]

OK, double checked the FX Send. No mistake, -6VDC, no AC component. That's why i said there's no point in patching a cord there. Is there any other component in the FX Send chain that might have burned out to pass DC into the circuit?

Checked IC14b: pin 5 has signal, cca 200mVAC RMS with 200mVAC input and all controls set fully clockwise, no DC component. Pin 6 is at -1.3VDC, no AC component. Pin 7 at -14.3VDC. No AC component. Faulty op-amp?

When I put signal in the FX Return jack, all four outputs of IC15 work properly. I have signal on line outs and power amp.

So, something is definitely wrong with the IC14 and/or components around it?
One more thing. IC14a seems unused. Is that right?

[zvir]

I did some experimenting on my own. I completely disconnected IC14b (pins 5, 6, and 7 of IC14) from the rest of the circuit.Pins 5 and 6 are inputs, one inverting and the other non-inverting, which, when shorted together and pulled to ground, should yield 0V at pin 7, right? Well, I did exactly that, and pin 7 reained at -14.3VDC. Faulty op-amp definitively. And after I disconnected the IC, I found no DC on the FX Send jack.

So, I need to replace that IC. Enzo, you said that TL072 would work here too. Since I am unable to find 4580 in my country, and also an SMD version of the 072, I will try to improvise something to connect an ordinary 072 here and report back with results.

[zvir]

I changed the IC 14, an the amplifier works prefectly! It is a brutal improvisation, it looks like a giant spider on the PCB, since I have eight wires that leave the board and connect to the TL072, but I don't care, it works! I'm just worried that vibrations of the amplifier will weaken the connections and that it will fail someday...

[Enzo]

ANY 8-leg dual op amp IC should work in there.

TL072, 4560, 4580, 2068, 5532, etc,

[Adambrannon]

Would this issue be the same as a crackle and poping after the amp warms up?? I am having that issue. It seems to be only out of one speaker when it starts, as far as I can tell.