Problem with one Aleph 2 monoblock

[Lucas_G]

Dear fellow Pass builders,

After several years of excellent performance, one of my DIY Aleph 2 mono-blocks got defective.

The strange thing is that it fires up well and it stays well at idle as long as it does not get an input signal or even a wire attached to one of its inputs (either RCA or XLR), because then the fuse (I already went to 4A slow,230 Volt) blows.

I already checked some voltages. On the G and the S of the output 240’s there is with all 12 of them a voltage of about 4,12 Volt. The PSU voltage on the rails is +48 and -48 volt. There is only 12 mV DC on the speaker terminals (without any load). So this seems to be ok.

I have to add that in the PSU there is a slow-start module, but that too seems to be working fine.

Before I start deassembling everything, does anyone have a clue into which direction I should look?

Any advice dearly appreciated!

Best wishes,

Lucas

[KatieandDad]

Try checking to see that the feedback components are good.

If something is amiss there it could go into wild oscillation with any input.

[Lucas_G]

Dear KatieandDad,

thanks for your suggestion.

What partnumbers should I start looking at? I have got the schematics here, but I don't know what parts might be the the most likely culprits.
Is it around Q3?

Best wishes,

Lucas

[Eric]

Hi Lucas,

Check out the circuit explanation that is posted on my Aleph-X page. The Aleph-X amp is essentially two Aleph amps running in parallel and the circuit and components are described on my page.

Hope this helps a little.

Eric

[KatieandDad]

Originally Posted by Lucas_G

Dear KatieandDad,

thanks for your suggestion.

What partnumbers should I start looking at? I have got the schematics here, but I don't know what parts might be the the most likely culprits.
Is it around Q3?

Best wishes,

Lucas

Can you post your schematic as everyones render of these amps is different.

In Erics schematic you would be looking at R30 and C4.

[Lucas_G]

Ok, thanks to you both.

In the original Pass service manual we are looking at R5 (100K) and C12 (10pF).

Shall I first exchange those two? Could the problem also be caused by the two Zeners closeby (who act as surge protection?)

[Eric]

I'm sorry that I don't think I'll be able to help much more beyond this. Replacing elements in the feedback chain seems like an easy enough place to start...

Are there any particular circumstances that preceded the failure? Prior to the failure, did you move anything? Wires? The amp itself? Open the chassis? Disconnect anything? Just looking for some clues that might suggest a course of action. Do you have a scope or any other measurement equipment?

[generg]

My first idea is to check the pair of 9610.........
Because I can imagine more a failure of the active components first, possibly followed by a failure of passive parts....

I do not know the current through the differential pair in the input, but let us say it is about 20mA so there should be about 4V over the 221 Ohm feeding the differential pair.

Am I correct?

But your idea with the Zener is also a good idea!

[generg]

And of course I would measure with a DMM if there are some Volts at the inputs....

No caps!

[Lucas_G]

Originally Posted by Eric

I'm sorry that I don't think I'll be able to help much more beyond this. Replacing elements in the feedback chain seems like an easy enough place to start...

Are there any particular circumstances that preceded the failure? Prior to the failure, did you move anything? Wires? The amp itself? Open the chassis? Disconnect anything? Just looking for some clues that might suggest a course of action. Do you have a scope or any other measurement equipment?

Yes. there is maybe one: a friend of mine has used them with a pair of 10m long unshielded! balenced cables. Maybe this has led to oscillation and damaged some parts in the input section?