Re: 475 Triggering Issue -- Will recent 468 Triggering Issue Thread Help Me?



Remember, these measurements are made with a student scope that has its own problems and can't measure below 100mv/div. So with 10x probe its hard to read. Amplitude and pulse widths given are very approximate.

I'm still working up the courage to pull out a TD lead. Anyway, with both TDs in circuit:

2v p-p square wave in
.5v/div ... auto ... trigger source lamp extinguished
CR566 -- nothing
CR556 -- approx. -500mv nice commutation approx. 300us pulse width

2v p-p square wave in
20mv/ source lamp lit
CR566...crummy,noisy elongated commutation pulses approx -300mv approx 400us width.
CR556 -- approx. -500mv nice commutation approx. 300us pulse width

Isn't this experiment sufficient to prove CR566 is shorted?
Should I still pull a lead. If so. On CR566 or CR556? And what/where do I measure? I'm confused about the post pull procedure.


--- In TekScopes@..., "Max Mazza" wrote:

--- In TekScopes@..., "stan_katz" wrote:

I've uncovered what I think is a significant symptom. If I inject a 1 volt square wave, at 1khz or above, set v/div to 20mv/div, and play with the position control, I can get the trigger source lamp to light. If I try the same stunt with a sine wave, I need at least 500khz to get the scope to trigger. This symptom seems to play into what Max was suggesting by lifting one lead of a tunnel diode, and then determining if I can get trigger with high slew rate sources. Of course, with the scope setup to the above test conditions, it's useless.

This was exactly the same behavior I noted when debugging my 3B4 timebase unit. With 1 KHz square wave of adequate amplitude, the triggers works. With a sinewave, it must be 100 KHz or above to trigger, with difficult.
If disconnecting the TD the behavior is exactly the same, assume that 99% the TD is open.

To get a definitive certainty, you should check if some signal is present on the anode of TD (with TD in place). If the amplitude swing during the TD's commutation is around 200-300mV pk-pk, the TD is OK. If a few mV, the TD is shorted. If more than 500mV, the TD is open.
Do that check before you detach one TD's lead and then after.
Compare the results.
Let us know.


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