Behringer GM108 problem

[undesiredshoe]

Hey guys, I have a Behringer GM108 that i bought a while ago as a cheap amp to practice with. Well I decided to record on my computer with it and connected up the headphone output to my computer and I turned up the volume on the amp kind of high and all of the sudden the amp stopped working. It just made a buzzing sound.

I have taken the thing apart and was looking to see if a blew a fuse or diode or something but i dont see anything visibly wrong with it. I am pretty good at soldering/desoldering and im alright with my electronics. I just need help trouble shooting what could be wrong with it and if it is fixable.

Thanks in advance.

[oc disorder]

Hi and welcome to the forum.
Firstly I noticed(I'm in Australia) you can buy one in Walmart for $59.99 !
Doubt if Behringer would fix it .
I'm guessing as I havn't ever seen one apart , that inside the main power module is a chip amp like a TDA 2030. Its usually mounted at the back of the circuit board on a heat sink.
I think you must have shorted out the output somehow (maybe a switch in the headphone jack that attenuates through the ground connection allowing the hot lead of the output out of the chassis) and this has fried the output chip.
The chip has 5 legs although there are others (chip amps) that do a similar job.Usually they are mounted with a thin mica washer or silicon rubber pad
on to the heatsink insulating them electrically also with a small insulating plastic grommet on the screw/bolt holding it to the heat sink.
When removing the old one take care not to damage these.
Eaisest way is to chop the legs off and clean out the debris carefully with a soldering iron and needle nosed pliers or preferably a solder sucker.
Then simply install a new one mounting it carefully first before soldering the legs so no strain is on the solder connections.
Hope that is useful of course it may be nothing like that and have discrete components for the output. Also need to check the power supply...
floundering in the dark here but possibly a+15volt and a - 15volt supply with a possibility of a 5volt rail for the modeling section.
9 times out of ten this type of amplifier has a fried output chip..!
If all else fails its a trip to Walmart !

[undesiredshoe]

Ahh yes i see that chip. Looks like i can get one on mouser.com for about $3....sweet. Is there anything else that could possibly be wrong so I could order it?

[Enzo]

Well any part in it COULD be bad, but really, that is the most likely failure in any small amp like that.

[undesiredshoe]

Well any part in it COULD be bad, but really, that is the most likely failure in any small amp like that.

Yeah I was just asking what other part could likely be blown....I was looking at the two diodes and they seem to look fine.

[oc disorder]

I presume you checked the number on the chip and I'll reiterate that I'm fumbling in the dark here.
You need to be able to solder neatly and not create any shorts and especially removing the old one you have to be careful you don't wreck any of the pcb copper tracks.
I'm assuming that the output is similar to the attached "H-phone skt.jpg"
where the full output would be on the tip and ring with the resistor to ground
(270 ohms in the example)shorted out by the computer ground putting the full output into the line in on your sound card. Its a wonder the sound card is still ok with 15watts going in.
If you used a mono jack plug this would have shorted the ring to ground
270 ohms being the load across the output until you plugged it into the computer where the respective chassis grounded together through the power
plugs would have bridged (shorted) out the 270 ohms effectively shorting the output blowing the chip!
Its impossible to tell what else could be faulty but for the cost replacing the chip is a good starting point.
Now for the future if you want to still record from the amp some suggestions:-
1.Mic it up
2.Buy a DI box with ground lift and attenuation.
3.Fit a line out to the amp
4. Make up a special lead

From a quick look on the web a D.I. box could cost more than the amp !

A line out from the amp could be achieved by connecting a 1uF 25v capacitor
to pin 1 (one on left chip facing you pins down) maybe a 10k resistor in series going to a jack mounted on the chassis (have to drill a hole).

This would tap into the signal going into the power amp.

Another alternative is to buy a 3.5 stereo (tip,ring and sleeve) mini jack some twin core shielded cable(gotto fit through the 3.5 jack cover)and a 6.5 stereo jack (with room inside to fit some resistors) and 2 x 5.6k 1/8 watt and 10 k 1/8 watt resistors 1/4 watt will be fine but harder to fit inside the plug.. this is just a rough guess but should work. (See Atten Jack.jpg)
Have to make sure nothing touches in the plug - its a bit fiddley but can be done.
I suggest you contact someone who's into these things to help ,may need to adjust the values before the final assembly.
but as Enzo said:-

Well any part in it COULD be bad

however you may be lucky...

[undesiredshoe]

I have checked the chip and it is a ST TDA2030A...there are five legs. I have desoldered and solder in many 28 pin chips into automotive ecu's so that part doesnt worry me. Ill try replacing the 2030A and see if that fixes it.

I presume you checked the number on the chip and I'll reiterate that I'm fumbling in the dark here.
You need to be able to solder neatly and not create any shorts and especially removing the old one you have to be careful you don't wreck any of the pcb copper tracks.
I'm assuming that the output is similar to the attached "H-phone skt.jpg"
where the full output would be on the tip and ring with the resistor to ground
(270 ohms in the example)shorted out by the computer ground putting the full output into the line in on your sound card. Its a wonder the sound card is still ok with 15watts going in.
If you used a mono jack plug this would have shorted the ring to ground
270 ohms being the load across the output until you plugged it into the computer where the respective chassis grounded together through the power
plugs would have bridged (shorted) out the 270 ohms effectively shorting the output blowing the chip!
Its impossible to tell what else could be faulty but for the cost replacing the chip is a good starting point.
Now for the future if you want to still record from the amp some suggestions:-
1.Mic it up
2.Buy a DI box with ground lift and attenuation.
3.Fit a line out to the amp
4. Make up a special lead

From a quick look on the web a D.I. box could cost more than the amp !

A line out from the amp could be achieved by connecting a 1uF 25v capacitor
to pin 1 (one on left chip facing you pins down) maybe a 10k resistor in series going to a jack mounted on the chassis (have to drill a hole).

This would tap into the signal going into the power amp.

Another alternative is to buy a 3.5 stereo (tip,ring and sleeve) mini jack some twin core shielded cable(gotto fit through the 3.5 jack cover)and a 6.5 stereo jack (with room inside to fit some resistors) and 2 x 5.6k 1/8 watt and 10 k 1/8 watt resistors 1/4 watt will be fine but harder to fit inside the plug.. this is just a rough guess but should work. (See Atten Jack.jpg)
Have to make sure nothing touches in the plug - its a bit fiddley but can be done.
I suggest you contact someone who's into these things to help ,may need to adjust the values before the final assembly.
but as Enzo said:-


however you may be lucky...