PM1000 project gone weird

Hi,

I've been working on racking a pair of PM1000 master modules and something weird is going on.

I used the Monitor Amp section as the 1st stage and the main line amp as the 2nd stage. So wiring a input xfrmr to C15 then taking the output at c20 and connecting it to the level control R9 and then on through line amp, ouput xfrmr and out. I also added a  gain control to the 1st stage of the amp, by replacing trimmer R40 with a 100k log pot. I'm using input and output transformers from the original PM1000

Its powered by a power supply kit providing +44 and +48. The power xfrmr is in a seperate chassis, and the PS PCB is in the chassis with the audio boards.

I'm getting a horrible oscillation when I turn up the module. The frequency varies widely as I move the controls from independent clicks to high squealing. That seemed like parasitic oscillation to me. I've replaced all the signal runs with shielded cable. I've tried reversing the leads on the transformers. It does pass signal along with the oscillation but only at the gain level where the oscillation starts.

I'm missing something. Maybe I put an electrolytic cap in backwards? Ideas?

Sam

one thing I notice on the schematic is that the fader is grounded to the "Ground pin on module frame"

is this connection getting to the "common bus"? if not, then I would make this connection.

What are you trying to acheive? Line in? Mike in? Why didn't you just use it from the mix amp on?

Break the two stages apart and see which one it is. 

The trimpot you replaced is 4K7.  100K will give you a lot less feedback (except at the very beginning of travel) which might be making the stage unstable.  Put the trimpot back and see if it settles down.

Do you have a load R on the input transformer?