Hi Albert,
I had a few minutes to do some probing, and it looks like CR364 is being triggered on the leading (falling) edge of the square wave from the collector of Q376 which is fed by the comparator (Q359). R364 has an effect, but doesn't change the situation significantly.
The other possible issue I found was that CR940, and CR950 aren't affected at all by adjusting the delayed trigger level control, or even when switching it to external delayed trigger source (with no external input).
Thanks,
Andy
toggle quoted message
Show quoted text
--- In TekScopes@..., "Albert" <aodiversen@...> wrote:
Hi Andy,
My previous comments need correction. I did measurements now in my 7904A and in this 'scope the whole trace intensity was influenced by the plugin's pot. Then I saw and remembered a jumper for "7900" vs "all other". Didn't try. Maybe that's it!
My description of the Delay Time signal is applicable only when the delayed sweep is "triggerable after delay", so Level in out position. Then the increase in bias of the TDs CR940 and CR950 after end of delay is clearly visible, and stays in effect until both TD's switch to the high level at the next (delayed) trigger event.
With Level pushed in CR968 keeps Q948 in saturation; CR940 remains in the low state all the time. Since (I suppose) you have the problem in this situation the fault might be in the circuit around TD CR364. This TD should switch to the high state at end of delay. Maybe R364 is adjusted for a too high bias current for this TD, forcing the TD in it's high state too early.
I took the delaying sweep saw tooth from TP408 and triggered the test 'scope from that signal. CR364 can be probed at TP365.
How to adjust the bias current (also for the other TDs) is undoubtedly described in the Calibration procedure. I didn't read that part (yet).
Hope this helps.
Albert
