Help fixing a Trace Elliot GP12 Series 6 [SOLVED]

[zayance]

Hi There,

I have this bass head (had it as an exchange) for some time now, the guy told me "it powers up but it's not working" but that's that, and because i didn't needed it, i never looked to repair , until a friend of mine became interested on it, and so i figured lets sell this thing and get rid of it, good one or not, he will use it more than me....
So first i changed all caps, since this one was made in 1991, so a fresh recapp won't do no harm....
But after powering on this one exhibits a weird issue. You get a loud HUM, and not a Mains hum, louder coming out of it, wether bass plugged in or not.
So i first checked to see if the PSU section is doing ok, and it is, the front panel board is feeded according to this schematic, and Power board section seem to be feeded good as well, all parts on board seem to be doing ok on DMM check, transistors, diodes etc.....
So just for curiosity i checked voltage coming out of the Speaker OUT, and i read more than 40VDC! when speaker is plugged IN, So i tought the Power MOSFET's must be fried, but after checking them on DMM they are doing fine also (these are 2SJ50 and 2SJK135), and all the other transistors on Amp board section are doing fine too on DMM, but maybe they aren't doing fine in situation while DMM makes them look ok?
Problem is that i cant check on board while having this HUM coming out of the speakers, as this will surely fry it pretty quick so....
Anyway, i'm not sure of what can cause this, so just wanted to gather some toughts and suggestions, i'll look deeper into it later today....


Thanks a lot.

 

[zayance]

Forget my post, found the prob, TR1 was dead huh?? Changed to something equivalent, and it's ok now.

EDIT: Will change them all on that Output Stage, so to get them by pair and in same specs anyhow, this one was for testing

Thanks anyway.

 

[mjrippe]

Glad to hear that you solved it!  In the future you might consider using a "dummy load" instead of a speaker.  It is just a high wattage resistor like 100 watts at 8 ohms.

 

[12afael]

for a solidstate amp would be a better idea a lightbulb of 100W.

 

[PRR]

Transistor amps don't need a dummy load, particularly for DC checking.

I always looked for <<1V DC *before* connecting a speaker. (Back in the old days, 40V DC was fatal to any speaker.)

 

[zayance]

Dummy load is on its way as we speak, it's something i had to have in the toolbox anyhow.
And yes PRR, it was a quick and stupid move on my side, gotta be sure before any harm.
Lesson learned, and fortunately the speaker is OK, well I guess...
Speaker was unplugged when checking voltages, when I said checking was for spotting the point where it was going weird in situation, but fortunately it wasn't necessary.

Thanks for your suggestions.

PS: what got me to miss that transistor when checking was my tips, I have those tiny flukes tip
that are great for hard to reach places, but after all this time it seems the probes are going dead, not the first time it misses things now that I think about it. Anyway....

 

[s2udio]

Replace all the output devices in these with exicon mosfets 10n-10p types, work a treat and much more rugged than
The originals
I have the SMX version and absolutely love these bass amps
The schematics for these are available

 

[mjrippe]

Transistor amps don't need a dummy load, particularly for DC checking.

Good point, but for power and distortion measurements...