In EHT's board the cap does not appear to be discharging.
How did you test that the capacitor C8271 is not charging/discharging? Please explain.
I don't suppose you actually measured the voltage from the poor 150pf capacitor?
Well, I stuck a 'scope probe on it looking for the sawtooth. It was just showing about 8VDC (need to repeat that to re-check the exact voltage but definitely no sawtooth).
I have confirmed that, on a working board, the sawtooth is visible without too much difficulty (on a Lecroy 9354).
Can you confirm that pulse train is coming into base of V8126 and then if N8062 Pin 4 [DTRDYN] has a pulse train out, C8271 is charging and discharging has been confirmed. Can you confirm this?
Well, I think I can confirm that with a trace displayed (so the 'scope is triggering) I
can't see a sawtooth on C8271.
My next thought was to look at the bases of V8126 and V8124, I managed to get a probe hook on the base of V8124 but could not see a pulse there. However because of access difficulties I had long leads, especialy the ground clip which was attached "miles" away. So a 20ns pulse might be difficult to see. Also it could be quite a small swing that I am looking for - the 8/8.5V label suggests that it might only swing through half a volt.
Don't fix your thinking into the DC opamp theory too much, because this circuit is pulsed On/Off very rapidly, hence the dynamic transient response, capacitive effect on FET gate comes into play, e.g. to isolate the important key capacitor C8271 from drain-gate capacitance of V8134 [60uA source] so as not to affect the precision current source and glitch free.
Let us know your findings.
My current thinking is this:
I can't demonstrate that the RUNDT/RUNDTN pulses are good. If they are not then the board is scrap as they originate in one of the large custom chips and they connect almost directly to the bases of V8124/6. But let's assume for a moment that they are OK.
At some point I will swap back in a known good card and see if I can see those pulses on a 'scope - I'll know what I am looking for at that point and know that I can probe it so I can see if it is present on the faulty card. In the mean time I was trying to find a 20ns pulse of unknown amplitude with a 15cm unscreened lead and about 20cm of ground loop. I'm not terribly surprised that it elluded me!
I want to limit board swaps though - EHT's card is pretty much "1st generation" and has a lot of (factory) re-work on the card so it is a bit mechanically fragile, some of the RF connectors on the board need quite a bit of pulling to get off and I don't want to keep doing that repeatedly.
So, given that the charge pulse is short, the charge source runs all the time and is steered to and from the integrating capacitor and the discharge source is connected all the time then there
are a few things that I can look at with DC levels.
I can look at V
BE on V8124/6 and make sure that V8124 looks as though it is not conducting and V8126 looks as though it is.
I can look at the voltage on R8496 and see if the discharge current is flowing - if not (which I suspect/hope) I can check the voltages around V8074 (which is definitely just operating at DC) and see if they look OK (but for that I need to know what they should be as it isn't given in the manual - hence this thread).
If the voltages around N8074 and the reference input to N8066 look OK but there is no current flowing (which there should be if the cap is charged to 8V) then I'll probably replace N8066/V8134/V8136 and check their associated resistors and see where that gets me
Sound reasonable?
Not sure when I'll get any time on the workbench though. "Real" work is a bit busy at the moment.