We can probably get this done just with the multimeter.
The reason I wanted to see Q19 and Q20 was they are ultimately driven by the Y+/Y- plate signal (between Q13/15 and Q14/16) after amplification by the boosters, Q17/18. I figured oscillation in Q17/19 area could affect Q13/15.
And, I wanted to see the whole picture. That's why I wanted to get measurements on Q5-9.
3. Both Q13 and Q15 show ~22V for Vce when trace is wonky. We already know the Y+ signal is ac -- so we know Q15 Vc and Q13 Vc are varying. For Vce to vary, Ve must vary *with respect to* Vc. Pretty significantly, at that.
What are we looking for... the source of the AC. Something, somewhere, is oscillating. If it's Q13 or Q15 themselves, then why? What other components are making this happen? Transistors cannot themselves oscillate without external components like resistors and capacitors.
oscillator basics
So what I plan to do is go thru every transistor measurement and see what logical deductions I can come up with based on what I know of transistors and electronics. For example, if you have a transistor with emitter tied to ground, but Vbe is AC, you know that Vb is ac because Ve is stuck at 0V. Likewise if a collector is tied to a voltage source directly, but Vce is ac, you know Ve is AC because Vc cannot alter. Vb will probably equal Ve + 0.6V or so. It may vary a little bit if the transistor current varies so you might see a little AC on Vbe. And so on.
I'll be looking for stuff that either shouldn't be possible or is unlikely to be wanted, like Q13 having a tiny Vbe with trace up, but mostly I am going to focus on when the trace is wonky unless I can determine a relationship between the trace being up and what we are seeing with the trace wonky.
That last part said-- one thing that has been nagging me the last few days is... there's a balance adjustment for the circuit... we need to go back and look at the service manual to see *where* the balance potentiometers are my printout of the vertical amplifier description references VR1,3,4 and 7,9,10 but I am also curious as to the purpose of VR6.
I can't help but wonder if the circuit were severely unbalanced, if it might start to oscillate on only one side. And if the trace would seem to be oddly "offset" up or down, or some such. And if some devices that should be a little bit off (Vbe=0.5, say) are wildly off (Vbe=0.012, say) and other devices are seeing weird drive voltages (Vbe=2.68V? Or -2.68V?)
A severe unbalance could be due to defective potentiometers, cold/broken solder joints, incorrect adjustments, broken traces near the pots, broken components near the pots, etc.
In case I don't get a chance to look into all this later... could you do me a favor and investigate VR6?
If it were me, I would first eyeball its position -- is it cranked all the way to one side or the other? Then I'd measure ohms resistance between the middle leg and each outer leg. It's acting as a variable voltage divider. So there should be some resistance on each side. If it seemed to be offset significantly, I'd remember it's original position, but try varying it to see what if anything that does to the trace when it is wonky.
Michael