EDITED a couple times.
As for the o-scope... the
only way we're going to get this done is to be thorough and methodical, testing components patiently until we find and fix the problem for good. There may be multiple bad components.
If it makes you feel better, I spent about a month or two chasing down the intermittent horizontal amp problems on my Hitachi V1050F. It takes time and patience. I've been chasing down problems on this HK 680i for quite awhile too, and had to take a break from it for awhile. But in both cases, I found the problem. It can be done.
Btw, I have learned more about BJTs and their associated circuits in the last few months so hopefully that will help us. I've also thought about this a lot and had some light bulbs go off in my head.
We found that Q11 Vbe was wrong when the trace was in the "up" position and so it is replaced, now. Where we left off -- we thought that Q13 was not getting the correct drive current into its base with the trace up.
Vbe for Q13 was 0V in the "up" position.
But our wonky trace problem is occurring with the trace in the
middle and lower positions, not the up position!
We WERE seeing both Q13
AND Q15 (looking at Vce) either fully on or fully off, instead of being partly on with the trace in the middle. What should happen is that their Vce values should linearly vary (inversely proportional to each other) with the trace position knob. We were also seeing AC voltage across Vce for Q13/15 which should NOT be happening when displaying a DC (or 0V) signal trace! More on this in a minute.
It's quite possible Q11 had nothing to do with the wonky trace. We need to make sure swapping Q11 actually fixed or changed something. Either way we'll gain
valuable information about what is (still) going wrong in the circuit. But if the behavior changed and we don't know about it, we'll be chasing ghosts so we have to know how things are behaving now. You can use this spreadsheet on google docs to record your measurements:
https://spreadsheets.google.com/ccc?key=0AjFa-cL8YjH_dHpVaWM1MHhZQnBwNWxSUjFYSHNKNkE&hl=en
1. Measure Vbe for Q11 with trace up, wonky, and down. Hopefully it is normal now.
2. Measure Vbe for Q13, Q15 with trace Up, Wonky, and Down.
3. Measure Vce DC Volts for Q13, Q15 with trace Up, Wonky, and Down.
4. Measure Vce AC volts for Q13, Q15 with trace Up, Wonky, and Down.
Since you are still getting the wonky trace, I suspect that whatever was wrong is still wrong.
If you think about it, something was (is?) causing AC voltage
and incorrect DC voltage across Q13/15, but that same something wasn't affecting Q14/16 despite the fact that they appear to me to share the same source for base drive current (despite errors on the schematic)
It seems logical that whatever is causing the problem is related to Q13/15 but unrelated to Q14/16. Like, Q10, Q17, Q19, maybe even Q5. There are other possibilities like bad resistors or capacitors, but we'll get to that.
Let's not get too distracted. Let's focus on doing the following (use the spreadsheet above)
1. in-circuit BJT diode test on Q9 (PNP) Q10 (PNP) Q17 (PNP) and Q18 (PNP) using spreadsheet (numbers may not exactly match due to being in-circuit) -- use the "diode test" tab on the worksheet
2. Test Vbe DC voltage on Q9, Q10, Q17 and Q18 - up, wonky, down.
3. Test Vbe AC voltage on Q9, Q10, Q17 and Q18 - up, wonky, down.
Let's see if that turns up more suspect components. We're slowly tracing our way through the circuit until we find the root cause of all this.