Support with identifying and repairing a Trace Elliot GP7 AH150 Bass Amp

[Banane]

Hi everyone,

I have registered in this forum specifically because there is expertise here in the repair of bass and guitar amps .

I was given a bass amp by a fellow musician in the rehearsal room and asked if I could have a look at why it no longer works. One of the fuses would blow as soon as it was switched on.

I'm not a great expert in electronics repair, but I do have access to things like an isolating transformer, multimeter, LCR meter, oscilloscope, transistor tester, etc. Even without much experience, I am mainly interested in troubleshooting.

The fuse doesn't blow for me. I have carefully switched on / turned up the device to 220V at the isolating transformer without input and speaker load at the output (I hope this is OK for mosfets / transistors). I have the gain all the way down.

What I notice: The overload lamp is permanently switched on.

I have two questions for now:

1) What kind of amp is this exactly? What year of manufacture? The circuit board looks like something from the 70s / 80s, almost hand-etched. The model number was already faded, I could just about make out an AH150. It says Trace Elliot GP7 on the front, on the circuit board you can see an MKII. Here are a few photos.




2) Does anyone have any tips on the best place to continue with the fault analysis? What does it mean if the overload LED is permanently on?
I hope these are the right schematics: https://elektrotanya.com/trace-ellio.../download.html

I look forward to any feedback. Thank you

Bests
Christian

[mikeydee77]

Hi and welcome... What skill level are you Christian? Can you take measurements? Do you have tools? CAn you solder parts in and out without damage? Do you know that electricity can ruin your weekend?

Trace Elliot Amp from late 80s or aearly 90s. The TE range has quite a lot of variations over the years so finding an accurate schematic may be tricky and may require looking at a few to get a general idea. The one you have posted is not correct as it mentions a three led level indicator but you don't have that. It might be correct in other respects though and enough to check some basic stuff. Each board has a part number and you may have to look carefully to match them up with schems.

From what you said so far I would say that if you connect a speaker then the amp then it may output a large signal and that could cause a power fuse to blow with the excess current. Please be preceise as to which fuse you think might blow as there are a few on these amps. The overload lamp might be indicating this....

So first thing I would suggest is a few basic measurements of voltage:
Check to see if there is any DC voltage on the output spekaer connector. There should be nothing but a few mV. When a power amp goes wrong it often will have 30-50v from the supply rail on the output which is not kind to the speaker
Check the power supply voltages on the big caps they will be in the range of 35-55v plus and minus voltes
Check the low voltage rails +15v and - 15v they should be about give or take a volt.

[Jon Snell]

The overload lamp is nothing to do with the power amplifier section. Don't get confused!
The overload light is illuminated when there is too much input signal so ... look in the preamp side for something very obvious, like a blown ground track from incorrect use.

A clue to which fuse had blown would help with diag nosis.
This amplifier is the AH150, (Gp11) they are all very simple and similar.

[Banane]

Hi and welcome... What skill level are you Christian? Can you take measurements? Do you have tools? CAn you solder parts in and out without damage? Do you know that electricity can ruin your weekend?

Trace Elliot Amp from late 80s or aearly 90s. The TE range has quite a lot of variations over the years so finding an accurate schematic may be tricky and may require looking at a few to get a general idea. The one you have posted is not correct as it mentions a three led level indicator but you don't have that. It might be correct in other respects though and enough to check some basic stuff. Each board has a part number and you may have to look carefully to match them up with schems.

From what you said so far I would say that if you connect a speaker then the amp then it may output a large signal and that could cause a power fuse to blow with the excess current. Please be preceise as to which fuse you think might blow as there are a few on these amps. The overload lamp might be indicating this....

Hi and many thanks for your quick reply.

I'm not an expert, but following the principle of "monkey see, monkey do," I have carried out a few repairs under guidance. This means I have access to all the necessary tools, am aware of the dangers of high voltages, can measure components, and solder things in and out. However, when it comes to consciously interpreting circuits or even understanding schematics as a whole, that's where my knowledge unfortunately ends. I am highly motivated and confident that, with a few nudges in the right direction, I can find and fix the issue.

Unfortunately, no fuse is blowing here. Do you think that could be because there's no load/speaker connected? Maybe I was told nonsense about the fuse.

Here are photos of the two PCBs I have:

I guess the one the close to the speaker outputs is the power amp.

The other one with the inputs contains the preamp section, right? I see op-amps and various transistors there.

So first thing I would suggest is a few basic measurements of voltage:
Check to see if there is any DC voltage on the output spekaer connector. There should be nothing but a few mV. When a power amp goes wrong it often will have 30-50v from the supply rail on the output which is not kind to the speaker
Check the power supply voltages on the big caps they will be in the range of 35-55v plus and minus voltes
Check the low voltage rails +15v and - 15v they should be about give or take a volt.

Everything as expected, only a few VDC on the outputs, 55VDC on the large smoothing capacitors, low voltage rails okay.

The overload lamp is nothing to do with the power amplifier section. The overload light is illuminated when there is too much input signal so ... look in the preamp side for something very obvious, like a blown ground track from incorrect use.
A clue to which fuse had blown would help with diag nosis.

Isn't it rather this, i.e. what Jon says ? :-)

As written, I do not detect a blowing fuse. Only that the overload LED is on.
I can't see any damage, coking, interrupted traces, short circuits or anything like that. You can see the photo of the board above. What can I check next?

http://hpbimg.someinfos.de/diy/allge...ot-GP7-MK2.pdf

[The Dude]

"Everything as expected, only a few VDC on the outputs".

There should not be more than a few millivolts on the output. A few volts indicates a problem.

From your description, it seems you either have more than one problem in the amp, or a power supply problem common to multiple circuits. Measure both DC and AC on your power rails (low and high voltage). Poor filtering (bad cap) may cause a problem. Are positive and negative rails symmetrical? Since preamp stages are AC coupled, I doubt a shorted op amp would cause this problem, BUT if an op amp is shorted, it might be loading a supply. Since they are socketed and it's an easy test, I'd pull the op amps and see if the OL LED is still on. Just be sure to reinstall them oriented correctly after testing.

EDIT: If you actually do have DC voltage on the output, do not hook the amp to a speaker or load until you get it fixed. You could do more damage to the amp and speaker.

[mikeydee77]

Sounds good and you have motivation which is really good.

Power amp is not outputting DC which is really good news. Most amps are made of three parts Power amp, Power supply and pre amp. Based on your information it is all pointing to the pre amp section as per
Jon's note.

you are right about power amp is next to speaker cab socket with four big transistors.
The preamp has input jacks and 7 slider potentiometers.

I would like to know what is coming out of the amp. But because we suspect the pre amp is overloaded and generating a large signal there are two ways to check the output safely

1. Way is to measure AC on the output with the meter. If it is low mV then you should be able to safely connect speaker and listen.
2. Is to check the pre amp output and you can do that by taking a signal from the pre amp out connector on the front panel to another amp with the volume set low.

Recommended you take option 2 if possible.

[mikeydee77]

Ah thanks duderinio I did not spot that about the few volts on output. Misread it as few mV doh

[g1]

attaching schematic package that has J50/K135 Mosfet outputs.

[Banane]

Thank you all for the great answers, it really motivates me to get this done .

Forgive me for the mistake with the “volts” on the output, it was really only a few milivolts. Everything is okay here for now, I can take another look later. I'm actually a computer scientist by profession, even though I may have several problems later on, possibly also in the power amp, I'd like to take analytically care of the preamp and the overload light first, following the “divide and conquer” principle.

I removed all the transistors in the preamp section yesterday in parallel to the thread and tested them externally with a transistor tester. I noticed that these color-coded capacitors were already starting to crumble apart on the surface just by touching them. I'm going to replace them all for now, as I had to order them I won't be able to continue for a few days. I'll report back here immediately

Thanks @g1 for the additional schematics. I have also added these blurred “GP7-MK2” schematics above. Somehow we are piecing this together. Isn't there a single, sharp, easily recognizable version that fits this amp exactly?

I had to guess the capacitors to be replaced with the LCR meter, so an exact specification in the circuit diagram would be very helpful. There were deviations of 20% in some cases. Is this due to age or simply the quality of the components?