Tek 556 A side won't trigger properly (essentially same trigger circuit as 547)

[ezalys]

Hey all,

I'm struggling to find the reason why my 556's A sweep won't trigger properly, though it will free-run no problem.

Here's what I know:

1. The trigger signal is making it to D555, the trigger tunnel diode. I've traced the trigger up to this point. It looks ok. It's thus probably not a switch contact issue.
2. The auto-multi is being triggered, and thus D555 is *probably* okay. I believe this because
  a. I can look at the trigger pulse on the T555 secondary.
  b. The scope will stop auto sweeping when the mode is set to auto for certain positions of the trigger knob. The auto-multi is thus seeing the pulse off T555, and putting Q615 into saturation, putting the scope into essentially the same state as if you had put the sweep mode into "trig."
3. The scope will free-run sweep just fine... so certainly part of the sweep circuit is okay. It just won't _trigger_ properly
4. The tunnel diode D565 is okay. I think this because 3 requires that the tunnel diode switch states (I think?), and because I've put D565 on a curve tracer and it looks healthy
6. The anode of D565 shows no evidence of a trigger pulse.

Any ideas?

[ezalys]

Here’s D555’s anode. This looks identical to the anode of the partner diode on the B side.



Here’s D565’s curve. Notice 2 mA per vert. div. That's 2 divisions (4 mA) + ~2 ticks (0.4 mA/tick) that's 4.8 mA at the break-off point, where the spec is 4.7 mA. I think it's fine.



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[ezalys]

Guesses:

1. Maybe D560, D561, D614, or D616 is bad. If, say, D560 is shorted, D565 won't bias to the right point for triggering. If D561 is open, D565 won't see the trigger pulse. If D614 or D616 is bad... well... idk how this part of the circuit works, but these seem like they're part of the trigger inhibit circuit so maybe they'd cause issues.

2. V625 is doing something bad, or R627 has gone out of tolerance. These would cause the tunnel diode to be biased improperly maybe?

Things to check:

1. Maybe D565's quiescent voltage in the state when it should be ready to accept a trigger. It'd be nice to know the quiescent current, too, but I'm not really sure how I'd access that.

2. Just straight pulling V625 and putting it in a tube tester... or swapping it with it's partner. It's just hellish to access, like many things on this incredibly densely packed scope.

3. Maybe I could just pull the "I" cable. Would this change the Q-point in a dangerous way of anything? Would pulling this wire leave D565 in a ready state?

4. I have a P6042 current probe. Feels like I should stick it on the "I" cable on the A side and B side and compare to see how much current V624B is trying to steal/contribute to D565's biasing. It has a 1 mA/div mode. Probably going to be noisy as hell but maybe with enough filtering it'll be ok.

Any other ideas, folks?

[tautech]

Trigger setting ?
Is that a pulse trigger and with the level you have set you'd expect it to be a falling edge. 

What happens with other trigger menu options and set to a different level on the signal ?
Is trigger Holdoff set to default ?

[ezalys]

The 556 is a large vacuum tube oscilloscope. The pictures I've posted are of a small digital scope I'm using to test.

[tautech]

The 556 is a large vacuum tube oscilloscope. The pictures I've posted are of a small digital scope I'm using to test.

OK sorry.
Try to get the attention of Martin.M as he's one of our old Tek iron experts.