Q4 emitter is close to 0V anyway during use.? It is grounded by R11 0.27 Ohms, as the output current sense resistor.? Q4's base should never be +ve vs 0V by much more than a Volt or so, ever.? If it is, the device has probably failed.
That -12.6 rail regulator layout is somewhat confusing at first, as the pass transistor is in the raw DC +ve feed, from the top + end of C6 (5000uF think it's C6, the schematic I have is a bit fuzzy) and "drives" into "Ground" via R11.??? The raw DC -ve (from the other end of the above cap, is the "Output".
If you have Q10 conducting (more than +0.7 V on it's Base.)? And all downstream loads disconnected, I'd check C9 in case that has failed short.? The 3A fuse, would take some time to fail at 4A.
Q10 CE short/leaky could cause the -12.6 output to not come up to spec' too.
The error amp "output voltage sensing" input from the -12.6 output, is the emitter of Q11.? It's "Reference" (Q11 Base) is set by R47 via CR17.?? The drawing here shows the Base of Q11 at -12V, so that makes sense for an output of -12.6V.
If the -12.6V output reduces (moves closer to Zero Volts.)? Q11 tends to conduct less.
Then, R44 provides the drive to Q9, to in turn drive Q4 that tries to "pull up" the top of R11, in turn "Pulling down" the raw +ve, that in turn causes the -12.6 output to move more -ve, to where it starts to turn on Q11, that in turn starves the drive to Q9 to cause it all to regulate.
Q10 conducts if the current needed to get to -12.6V output correct, is too high (some 4A or more) sensed by the voltage across R11 via CR16.
Check also C8 (? Fuzzy image)? and R46, as they "Slug" the regulator loop, keeping it stable.
I suspect R49 is another stability enhancing component, in series with C9 (47uF) from output to 0V.
While your at it, check the raw DC across C6 after CR1 and CR2.?? I think it should be some 18V from the voltages marked on the schematic.
The drawing I'm looking at is at:-
Not the best quality, but if you zoom in...?? It has typical DC values marked for normal operating conditions.
Hope something helps.
Dave B.
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