SX-890 Bias and Protection Issue

[N3w2Electron]

Trying to wrap up a refurb of an SX-890 and can't get one channel to bias (DC Balance) and it won't get out of protection. At one point it would get out of protection but wasn't stable on on either channel when trying to DC Balance

At this point, on the Right Channel I get ~ -35V at pin 12 and no response at all when changing VR6. I can set the Left Channel to zero. I can trace that negative voltage through Q7 to the base of Q20. I know that the Q20, Q7, and Q8 set is good because I swapped the Left Channel Q19, Q5, and Q6 ones into the Right Side and vice versa. They are original 2SC1775, 2SA979, and 2SC2291 configurations. The problem stayed on the Right channel regardless of which set of transistors was used. I've checked resistor values for everything touching that set and they all checked out.

I've read most of the SX-880 posts about protection issues. I've got the C418 oriented correctly. Seems like most of the other things noted don't seem to be the issue, but I haven't seen anyone mention this symptom.

I'd appreciate a tip for where to look for a most likely cause.

Thanks,

Craig

 

[markthefixer]

The left channel is Q5 (2sa979), Q7( 2sc2291), Q19 (2sc1775a)
The right channel is Q6 (2sa979), Q8( 2sc2291), Q20 (2sc1775a)

first check all transistors (q9, q11, q13, ) drivers (Q15, Q17) and outputs for 0.6v or so emitter to base, directly at the transistor.
This may weed out the bad one.

VR6 is your clue. Find where VR6 response starts (2.4v (1.2 + -1.2) plus VR6's millivolts up and down at the bases of drivers) -
there certainly will be the 7mA of current flowing through the 100 ohm resistor and thermal diode
then watch the emitters at 1.2v (0.6 + -0.6) with VR6 millivolts up and down and then the same 1.2v +- VR6 millivolts at the bases of the outputs - the emitters of the outputs you've already failed out.

Be sure all your outputs test good, and also that their emitter resistors (0.47 ohm) are intact.
be sure all your drivers test good, and also their 150 ohm emitter resistors.

The drivers will have +1.2v and -1.2v at their bases and 0.6v and -0.6v at their emitters.(referenced to output) if not, ***bzzzzttt***
This +1.2v and -1.2v comes from D3(4) stv-4h diodes and the 100 ohm pot which have 7ma being pumped through them
by Q9 (0.3w heat load)
The control comes from the feedback system manipulating Q11 & Q13 conductance to maintain 0.000v at the feedback (output) point
and that Q11 & Q13 effect cascading down the drivers and outputs.