Behringer GMX110 fried power amplifier

[spsimmons]

So, I used the effects loop to determine that the preamp is good and the speaker is good, but the power amp is not.

I pulled the power amp board and IC1 (TDA2050) is burnt. Three of the leads are scorched and the case is cracked open. The board, where IC3 (7915A) attaches, looks blackened. The board at IC2 (7815A) is a little dark too.

OK, IC1 is done and needs to be replaced. IC2 and IC3 have clearly been very hot at some point. They both need to be pulled and tested, but the parts are cheap so I'll probably just replace them too.

Question: Why would these components burn up? Is there something else I need to be looking at?

Question 2: I don't find any transistors on the Mouser site that are just numbered 7815A or 7915A. They are more like: KA7815ATU or L7815ABV. The examples I named both have different specs. How do I know what to get?

Thanks, Shannon

[jrfrond]

Those power amp IC's really do not need am excuse to torch themselves. Just replace it. It's a routine fix.

7815/7915 regulators are commonly prefixed KA (Samsung), LM (National Semi), NJM (New Japan Radio), AN (Panasonic). Try LM7x15 at Mouser. I usually purchase the ST Microelectronics brand from them (511-LM7x15).

[Enzo]

As John said, the prefix is just the manufacturer. The letters after 7815 tell us something of the package it comes in. But all of the 7815s will be a 1A ( or more) voltage regulator with 15v output.

[spsimmons]

ARRGH!

So, I finally got a chance to replace IC1, IC2, and IC3. That fixed something, because I can now LOUDLY amplify the horrible noise that this amp makes! I don't really know how to describe it. It's not quite static, its too regular and staccato for that. Its got a pulsing quality to it. Terrible. There is no guitar sound... except for a split second when I switch channels. This is true for all outputs: speaker, external speaker, headphone and tape out.

The preamp still sounds fine if I run it through another power amp. If I run the pre from the other amp through the Behringer's power section I get the noise.

When running the preamp through the other power amp, all of the controls in the "modelling channel" seem to function, but the "24 bit FX Processor" has no effect on the sound. The exception is, when I move the "mode" switch to "clean", the amp goes silent.

I don't know where to go next. The schematic seems to be unavailable and the traces on the board are so small that I'm having no luck following them.

I'm not quite ready to give up on this amp because the preamp through an SWR power amp sounds pretty good and the split second of guitar sound I get when switching channels gives me hope.

Any ideas?

[Enzo]

Look on the DSP card. If anything is in sockets, reseat the IC in its socket.

Set the FX for that silent state. Now plug a signal into the tape in jacks, or at least one of them. Does that come out the speaker?

The DSP is after the FX loop, so that is why the FX send does not include its effect.

If you are getting raucous noise, I tend to suspect the DSP card is at fault. Just opinion.

[spsimmons]

Is the DSP card the one with the midi plug attached to it? This is the only card with an IC in a socket. This IC has about 20 legs on it. Anyway, I pulled and re-seated it. No change.

I could not re-create the silent state. I'm thinking that maybe I had a switch between settings or something. I don't know. This only happened when I had the Behringer's pre-amp running through another power amp. There is no setting that is even close to quiet when running through the Behringer power amp.

When I plug an Ipod into the "tape in", it plays through, and I can adjust the input level to make the Ipod signal as loud as the horrible noise. I still get no guitar signal on any setting.

I also found that perhaps I was wrong about the pre-amp working properly. When I was testing it this time, it was not switching to the "clean" channel. The indicator light goes on and off, and the "fx processor" display changes, but only the controls for the "modeling" channel have any effect. The normal channel gain knob has no effect. The modeling channel seems engaged no matter what the indicator light says.

So, is there any way to bypass the DSP in order to see if that is the problem? And if it is, are replacement ICs or whole cards availiable? I looked at Dbmproaudio's parts list and didn't see either.

I'm not sure if I'm beating a dead horse yet or not. I already replaced the output ICs, so it would be great to get some level of functionality from this thing.