Could CR566 start to oscillate for some reason? If CR566 is shortcut or passive restive or open then the 300 mV signal at CR566 can't be explained. When the 6 mA supplied to CR566 after the switch of C556 does not produce a measurable signal at CR566, then an additional current switch in Q566 (less than 10 mA) will not produce 300 mV across CR566.
Albert
toggle quoted message
Show quoted text
--- In TekScopes@..., "Max Mazza" wrote:
I've checked the schematic, "A' trigger generator".
The anode of CR556 should steps to about + 500mV. It's thearming diode.
Once it switches in high voltage state, then CR566 can commutate.
CR566 also steps to + 500mV
From your measurements, I conclude that CR556 is surely good, but CR566 not. The TD commutation is of the type you observe on CR556: clean, steep rising to +500 mV
Remember that CR566 switches only AFTER CR556 switches.
If a pulse reaches CR566 with CR556 still in low voltage state, CR566 doesn't commutates.
I hope that, since you have written the voltages with the - (minus) sign, you simply have made a mistake....
The "A trig sensitivity" simply regulates the quiescent point for both diodes. In low voltage state, it should be presumably around a few tens of mV.
You can play with R565 (with NO signal applied to scope inputs) to observe that increasing the voltage (rotating (slowly!) the pot toward the +5V TDs should commutate in their high voltage state, while rotating toward the -8V should restore their levels to +5/80 mV or so.
You have the 475 schematics, I presume. See schematic page 5 for more details.
CR566 can be supposed open, not only shorted. In the open case, you can equally observe a dirty pulse like your, because is the TD that makes (generates) the steep voltage swing of +0.5V. This is precisely the reason for which TDs are used.
Max
--- In TekScopes@..., "stan_katz" wrote:
Max,
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/div...auto...trigger 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.
stan
