OMG what have I done?

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a 2 channel MosFET power amp using 2 pairs of K135/J50. On a parent board plus a further 3 pairs on a daughter board.

The actual work wasn't that much. The MOSFETs are on TO3 packages and the brass bushes that they are bolted to had cracked their soldered joints. Unbolt to break the heat path to the devices, remove old solder, clean up flux, resolder, clean boards. While its apart, replace the electrolytics and check orientation. All quite within my capabilities.

Result:- no output. All the devices make more heat 45deg C as opposed to 30deg C in the untouched chanel. The power rails are steady at 75V.
So my questions are:

What would make it run hard but give no output? There is no DC at the output either. At switch off, the resistance across the speaker terminals is identical on both chanels.

Can I disconnect the top board since its only a parallel board?

I don't have access to a scope :(

I have one of these as well (can be seen here:
A long time ago there were problems arising with the output relays. Maybe this is the case with yours as well.

Where did you measure for DC "at the output" ? On the backplane or on the PCB ? If there is no DC on the output sochet/clamps doesn't mean that there is no DC at the amp outout as such since there is the DC protection inbetween so watch out !

BTW: If the bias current is set to the recommended value this amp doesn't get warm - it gets HOT !


An externally hosted image should be here but it was not working when we last tested it.

This is mine. Never noticed it get hot but them I never hd cause to turn it up. Its older than the drawings that I have. I can hear the output relays working and I didn't have any problem before. So its logical to assume that it is something I did.

I have been building electronic equipment for the last 15 years but as to how it works - my knowledge is ....simplistic.

You say that know the "recomended value" of the bias. Do you have some other schematics?

First thing first though; If the top PCB is just more fets in parrallel, can I operate the amp with it completely removed? It would give me space to move.
Check to make sure the output relays aren't shorted. a common problem with Ampeg Bass amps is that the relays short out when they go bad. they switched the speaker load from ground to the output of the amp. when the relays go bad they still click, but the output of the amp is shorted to ground.

Also, the K135/J50 mosfets like to blow the gates if the amp is overdriven and there isn't sufficient gate overdrive protection. the output can and will oscillate like crazy.

<shrug> just ideas
My relays click also but they have bad contacts (like the mains switch that you'll have to replace on these amps every few years).

BTW: I have a copy of the whole service manual.

And I must say if it weren't for the age-related ailments this would still be a very fine amp. I used it for the hornloaded small-P.A. from the above link, as bass-amp (in conjunction with a DIY bass-preamp) and for listening at home (bass section of Klipschorn copy - actively driven :devilr: ).


I am an idiot,

Mistake 1/ I failed to test the amp BEFORE I worked on it.

Mistake 2/ I failed to feed the amp a good signal from a mixer. Instead I used a Jack plug wired to 2 pins of an XLR.

Mistake 3/ I measured the temperature difference at different gain settings.

ACTUAL amplifier fault:
the left side amplitude LEDs aren't working! The amp itself runs sweet as a nut. I could still use the manual though, since I have NO idea how the LEDs work.
I got your files Charles. A public thankyou. Between the 2 of us we have a document that makes sence. When I get a PDF editor I'll stich them together.

I just need to make the indicator lights work. Its interesing the in 1983 Studionmaster regarded pin 3 of an XLR as "in phase". Nowadays pin 2 is hot. Which means that I have always run it out of phase with the outer amp and not noticed. (there is a lesson there).

It also needs to be configured as EITHER balanced OR unbalanced input. Today most amps take either.

It does sound nice though.

Thanks again
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