Help needed with Analog PA

Thread Starter


Joined Feb 3, 2023

I'm trying to repair an analogue PA. The PA suddenly stopped working so I took it apart and saw that the main output transistors had blown. So I found the circuit diagram (attached, but that's all I have - no service manual or anything) and set about repairing it. The first thing I noticed was that the output transistors had obviously been replaced before, but not with the ones in the circuit diagram, but with an MJ3001 and a MJ2501. Fairly close to being equivalent but lower max voltage ratings I think. The circuit calls for MJ11016 and MJ11015.

Anyway I replaced like for like (I used MJ3001/MJ2501). I also replaced all the transistors associated with these drivers (Q101/2/3/4/5/6/7/8 in the circuit diagram).

All looked good when I fired it up, it all seemed to work fine. However I did hear some distortion, so I injected a 1khz signal and saw on the scope some crossover distortion at the output. I adjusted RV107:1 in the circuit and managed to adjust the distortion out. Happy days I thought. Until within a couple of minutes..... pop. Output transistors blown again.

So my question is why? I have ordered up a pair of MJ11015/MJ11016 output transistors now, so I am using the ones the circuit says I should use. Would this have been the problem? Or is there something else broken which is drawing too much current through the output transistors?

Any help or guidance would be greatly appreciated.