Help repairing a Ecler XPA7000 power amp (schematics available)

[jmibk]

I have here an audio power amplifier that has its protect led on at channel 2.
When switched on, both protect led light up for a few seconds, eliminate and the one on CH2 goes back on again.

I checked all fuses (all OK), 4 voltage rails (all present) and checked the resistance of all power mosfets - all seems to be OK.
After that I checked the output voltage without any input signal. Channel 1 is 0,000V, Channel 2 is between 0,9V and 1,2V DC. The output protect led engages at around 1V DC.

I'm into electronics but I'm not a specialist in analog audio amps. Any Ideas or suggestions to get the thing back working? Thanks a lot.

Schematic is here: http://www.audiowerk.at/downloads/temp/amp.jpg
The whole schematics here: http://www.audiowerk.at/downloads/temp/Ecler_XPA_Service_Manual.pdf

[goaty]

I´d try twiddling with the pots a bit (carefully, while measuring if it affects the DC offset) . Maby its just imbalanced and needs adjustment ?

[jmibk]

Today we tried some careful tweaks at the pots.

VR4 is maybe for the switching of the higher two rails (class H), VR3 is for the adjustment of the bias, in german "working point?".
First of all we checked the voltage on R146 (between the two BJTs that are driving the final stage. The good channel has 6.5V, the bad one has 5.4V across that resistor. So I tried tweaking that with VR3, but nothing changed much when adjusting it. +-0.1V, thats it.

Now we analized the circuit from front to back....
Q3 and Q4 are the differential inputs, Q2 is a current source, that delivers 1.5mA. This current divides to the two ways of the diff amp input, so at idle state 4,5V are at the resistors R62 and R63 (below the diff amp). Measurements tell the same and are equal on both channels. Tweaking VR1 changed the offset a little bit but gets back to its 1V in <1 second (it seems there is some capacitors that charged up or something before returning to the faulty level.

Q21 and Q22 (right of the current source) seems to be a current mirror circuit if i remember correctly from school some...SOME... SOME years ago.
Now there are only Q26 and Q27 left, they seem to do some voltage amplification but sharing a pair of feedback resistors....? weird.

Now we stopped exploring and maybe there are some more hints to check...

[shakalnokturn]

Is power symmetrical at power MOSFET drains?
How do voltages compare across R9 and R10?
Have you tried disconnecting CN1/CON3 input?

[M4trix]

*bumpity bump

it's time to bring this topic back. It's about Ecler's XPA 11000 audio amplifier. Does someone have a schematic ? I wrote to Ecler but they
answered very politely that they don't give schematics (confidential stuff). There is nothing besides the specs on the web but looking at
some available pictures, it looks like its a class H, 3 tier power amplifier. Probably the output devices are Hexfets, IRF640/IRF9640. I'll
appreciate any information. Thanks.

[M4trix]

*bumpinator

Anything about this amp ? Schematics, service manuals ? 

[CaptDon]

Wow, the idea of "Let's start adjusting the pots" was a super bad idea!!! Sadly in a complex amplifier like that you WILL need a schematic and will have to go through each stage looking for the cause of the offset. Remember that because of the wrap-around feedback circuit any offset at the output will 'try' to be compensated by the input stages and the whole thing becomes a 'who came first, the chicken or the egg' troubleshooting experience. Class H amplifiers are not for the inexperienced to repair. Case in point, you may survive a 240vac shock where you can get away from it. The amps I use and work on have + and - 125vdc rails and 250vdc can be deadly, even more so than 240vac!!! Just ask an electrician on Iceland where we used D.C. for heating!!! It grabs your ass and won't let go!!!

[Audiorepair]

I would have to agree with the Captain.

Repairing this kind of stuff is highly advanced fault finding, and extremely risky and time consuming.

You need to set your sights a little lower, until you are confident you are an expert.
Nobody's going to fix this for you.


Edit:  I wouldn't touch it with a barge pole, I don't need that kind of stress and a week wasted.

[M4trix]

Guys, this thread is almost 2 years old. I presume that OP has already fixed it or dumped it.  Let's get back to more recent replies. So, does someone have the XPA11000 schematic ?