The DC level at IC 502's pin 3 and 14 should be ZERO at min volume.
Measure the DC voltage at power at these two terminals (pin 3 and 4) IC 502.
You will get DC voltages at these two pins at power up but it should decay to 0V with a seconds time. If the DC value at either of these pins exceeds more than 50mV after say 1 or 2 secs then you have a blown STK (probably).
Another issue could be a missing supply rail to the STK.

 

IC502 pin 2,4,13 & 15 are the main supply rails to the STK. Check them to see they are with in stated values.

DO NOT CONNECT SPEAKER DURING REPAIR.

 

Tell us how the operation indicator LED's are working.

 

Thanks guys, will resume testing with new instructions from #12 and R!f@@ shortly. Help and patience is much appreciated.

 

Tested, there is a a DC level at IC601 Pin 8 and at both Q604, Q605. I can't tell you exactly what the level is as I am using an analogue tester and the needle is flying up the scale whilst I try not to fry anything else.
R!f@@ the level at 3 and 14 pins IC502 are very high continuously at 0 volume. The levels stay at 3 & 4 stay high....well above 50mv all the time.
Thanks guys It does seem to look like I need a new STK2038-IV?

 

R!f@@ the level at 3 and 14 pins IC502 are very high continuously at 0 volume.
Thanks guys It does seem to look like I need a new STK2038-IV?

yes

 

U did not state the supply voltage at STK.
I rarely come up with a bad Left and Right outputs. It is always one side that gives the problem.

If one of the supply voltage is missing it might be the cause. If one rail is below the rated values both output will show DC.

It's worth checking the supply before buying an STK with a good DMM.