Aleph P - where F!@?& does my signal go?

[Mfly]

Me and a friend are in the final stages of finishing up 2 Aleph P 1.7's. One is already singing very happily (and making me very happy) . Other then enclosures, they're both just about finished.

However! The other one WAS working fine, untill i did some soldering on the volume board (moved the output resistors from the XLR connectors to the board itself). After this or maybe some other action, one channel stopped working .

My immediate reaction was of course: I did something to the volume control. But after extensive testing, both channels of the volume control proved to be working as they should (and exactly the same and the same as the other amp). Hereafter I tested the connections between the pre-amp board and the volume control (measured from the output caps to the first resistors and from the output caps to the output of the volume control and the output of the amp (also with different volume settings): still everything as it should be. After also checking the input selector, I started to suspect the pre-amp board itself. So I checked:

-DC voltage input & output caps: ok (around 14,5v and 30v)
-DC voltage over input resistor: ok (around 0,178 or something)
-DC voltage over 22,1 and 33,1 resistors: ok (0,656v)
-Input resistance vs. ground: ok (33k)
-Output resistance vs. ground: ok (don't remember the values but was the same for the bad vs. good channel vs. other amp)
-DC voltage of the different transistors: ok (same for both channels and for other amp)

So power supply is working, current source is working, no leak to ground from the input or output. So then I applied a sinus signal and started taking AC measurements. Result: both channels and other amp measure exactly the same, but for 1 difference: the channel with no sound has a voltage swing BEFORE the output capacitors (both + and - side) and all working channels have the voltage swing after the output capacitors (as it should be). So then I tested the capacitance of the output capacitors, which was correct. And I extensively tested if there weren't any busted connections, leaks to ground on the output side. None of those...

So can anybody give a suggestion on what is going on? Cause I'm completely lost here (and getting a bit frustrated ). Are the output caps bad (all fr*ckin six om them on both the + as the - side)?? What am I missing here?

Thank you for your input.

Maarten

[generg]

Hi Maarten,

sounds really complicated....

I would recheck these red surrounded parts...

[Mfly]

Originally Posted by generg

I would recheck these red surrounded parts...

Thanks for the input. Checked those. All seem to be fine. They measure correctly, no shorts or anything.

Also replaced all the 610 fets, to no avail...

I'll disconnect the board from the volume control and attach a dummy load to once and for all rule out the volume control. If that doesn't result in anything; replace the ZTX's as well. And if that doesn't help, I'll have to have a look at the output caps....

And if that doesn't help...wel..............

[Mfly]

Problem solved!

Probably compounding problems. When replacing the 610's I probably damaged the trace from the drain of one of the 610's to the gain resistor network. When measuring resistance, the pressure of the probe resulted in a connection so it looked like nothing was wrong..

Conclusion; probably first a busted mosfet and after that a busted trace. Am I glad the search is over.

Thanks for your input.

[generg]

Much pleasure with wonderful Aleph P 1.7!