Strata mini comp. AV123. Please help!!!

One of my speakers Strata mini company AV123 started to create strange woofer noise without input signal. This “scratch” sounds with small time interruption and different level like sparks somewhere inside I hear it from up to 5-7 fit from woofer. It’s not a constant sound of 60 Hz. I disconnect inputs, only power is “ON” and noise doesn’t disappear.

I am Industrial electronics engineer and capable to fix it but without schematic diagram it almost impassible to locate witch components create this problem. The PC board double layer and all capacitors hard glued to the board, so It’s even more difficult to locate a problem. The output power transistors 2SA1943 and 2SC5200 blown out. 

I checked all components with Oscilloscope which have components tester without unsoldering them. And did not find any wrong component (I compare test with PC board on the good speaker, No power “ON”).  I changed power transistors, but it did not eliminate  the problem. 

This transistors were overheated too fast and I could not keep power on for a long time. I have all necessary equipment, but  don’t know what else to do.

It seams to me, my last hope is to buy working PC board but Don’t know were…. Company AV123 out of business.

I appreciate any advice.

Val.