Looking for refurbished power stage, Trace Elliot GP12 SMX bass amp

[MenderMoose]

This amp was presented with a solid +27v DC output to the speaker, and 2 of the 4 MOSFETs running hot.

I replaced one clearly o/c resistor and then all four MOSFETs, but the problem remains and I fear I've killed some or all of the MOSFETs again as two of them don't quite switch off when I bias the gate on and off with a meter. I think I was too hasty with applying full power, before thoroughly understanding and faultfinding the biasing circuitry. Should have ramped things up slowly with a variac and tweaked the bias current, agh.

Wondering if anyone in the UK is adept at refurbishing this particular output stage? Although I design electronics for a living and can probably hack this eventually, I need a result sooner rather than later. All suggestions appreciated.

Regards,
Matt

[MenderMoose]

Update: output transistors look ok on bench, at least at 10V 1A range; no shorts or leaks, and gate controls the current ok... must be the preceding circuitry...

[gbono]

Do you have a schematic to post here? Assuming the power supply voltages are all correct first wouldn't hurt (i.e. disconnect secondary from load). You might try bringing mains voltage up with a Variac and the output devices removed.

[Fran]

Hi, just wondering how you got on with repairing this. I've got pretty much the same problem and am really stuck! Thanks

[Jazz P Bass]

Seeing that this is an old post, how about if we start anew?

What is the problem?

Vdc on the outputs can only be caused by:
Shorted output devices or there driver transistors.
If the driver transistor & bias circuit is faulty, that will turn on the mosfets.

Note: Do Not conect a load (speaker) until you get things sorted out.

GP12 600Watt Power Circuit.pdf

GP12 SMX.zip

[olddawg]

Seeing that this is an old post, how about if we start anew?

What is the problem?

Vdc on the outputs can only be caused by:
Shorted output devices or there driver transistors.
If the driver transistor & bias circuit is faulty, that will turn on the mosfets.

Note: Do Not conect a load (speaker) until you get things sorted out.

[ATTACH]34081[/ATTACH]

[ATTACH]34082[/ATTACH]

An unbalanced diff amp on the input can cause a DC offset as well. Not as common as it once was, but I see still see it occasionally.

[Fran]

Yep. DC at output. -ve, ie, the tip of the jack skt is minus x volts. I say x because I have managed to alter this voltage accidentally. When the fault occurred at a gig (I'm a bass player, not an electronics expert - but I dabble, and soldering skills are good..) I was getting a low volume hum through my speaker. I unplugged the speaker, switched to full range; not bi amp mode, turned the volume to zero on the faulty Chanel, and carried on with the gig using the remains good power amp stage....had to, needed the money...after gig, at home, I measured the output with digital multimeter, and fond just over -3v at output of faulty channel. Good channel was zero volts. I figured this would be mosfets, and as the amp was built in 1993, I thought I'd replace the full set. As I had stripped the pcb, I though it woul make sense to replace the other transistors in the driver stage, as they were cheap, and accessible. I had previously done a check with my meter in circuit, and as far as I could tell they were ok...but I fitted new as i didn't want to have to strip the amp down again.

With new mosfets and drivers fitted, I now got over -60v at out put. Unable to test mosfets, I started swapping old for new, and found a combination where I could get voltage to about 275mV. Safe to plug in speaker. Tried it with signal from my bass, and got very distorted output, at very low volume....other channel still fine. Psu gives +-75V, but one of the smoothing caps in the psu (the one on negative rail) does discharge much quicker than the one on positive rail. It does this whether or not the faulty power amp is connected....I wonder if I've got a faulty set of mosfets? I got them from RS on line. So, I'm still confused and frustrated, and am not quite sure which way to go now....

[Fran]

Please explain unbalanced dif amp, presumable this is a stage before the driver stage? But I'm guessing....I've kept looking at output and driver stage...I don't have the technology or know-how to check the mosfets, but transistors and diodes I can check easily...thanks

[Jazz P Bass]

I would like to see you post some voltages.
The output Mosfet grid's cannot be turned on hard at idle.
Example:
TR3 is a PNP transistor.
Its Base lead will be high. So the Emitter will be off (This is the mosfet grid voltage)
Lowering that voltage will turn the transistor On.(the positive half of the signal will do just that)

The differential pair is TR1 & TR2.

TR1 will turn on TR4 & TR2 will turn on TR5.

[Fran]

Ok. Will get measuring tomorrow. Thanks

[J M Fahey]

Yep. DC at output. -ve, ie, the tip of the jack skt is minus x volts. ... I measured the output with digital multimeter, and fond just over -3v at output of faulty channel. ... I figured this would be mosfets, ... I thought I'd replace the full set. ... I though it woul make sense to replace the other transistors in the driver stage, as they were cheap, and accessible. ... as far as I could tell they were ok...but I fitted new as i didn't want to have to strip the amp down again. ...With new mosfets and drivers fitted, I now got over -60v at out put. Unable to test mosfets, I started swapping old for new, and found a combination where I could get voltage to about 275mV. ... got very distorted output, at very low volume....other channel still fine. .... I wonder if I've got a faulty set of mosfets? ... still confused and frustrated, and am not quite sure which way to go now....

Sorry Fran, replacing parts at random is the worst way to "repair" an amp, you have probably introduced more problems than the original ones.
Looks like this is over your head, I suggest getting in touch with an experienced UK Tech.
I guess there's a couple here which could do that job; just out of the top of my head I'd contact Mick Bailey http://music-electronics-forum.com/users/19611/ , there's probably others.