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Hi all,
I'm one more of those people who can't pass by when they see the shape and colors of something Tektronix. Now I'm working on a 2467B and while it seems to be working well, there was one thing I wanted to resolve before finishing my calibration efforts. It's a late model I think; some date codes are from 1995, the serial is B053790. I started measuring the test waveforms from the service manual (070- 6863-01) in the trigger, sweep, horizontal amp and Z-amp sections (reson below). What I am confused about is that several sweep gate and sweep waveforms initially look okay on my scope but I don't see the same levels. And looking at the schematics, the test waveforms don't make sense to me. Waveform 31, for example, is supposed to reach -0.2V. But I assume U700 outputs 0V or 3V on the SG# pin, and Q745 cannot pull it lower than GND. What does? CTO by charging the "** part of ECB" capacitor?! I normally work in the digital domain and I always struggle with these analog things. For the B trigger, waveform 38 is easy, it's just 0V or 3V on SG#. But then waveform 39 follows and quite unexpectedly drops down to - 0.5V. Again, the PNP transistor doesn't seem to explain that. These A and B "sweep gate not" signals are interconnected and also enter U985 and U975 (the "measurement PAL"). The latter seems to get pretty warm, but I see no obvious malfunction. I read the circuit description on page 3a-21 of the manual but it did not give me an answer. So any help making sense of these waveforms is greatly appreciated! As for the reason for looking into this, I had some difficulties doing the "high drive focus" and "writing rate threshold" adjustments on page 5-6 of the manual. Triggering seemed unstable, although this might be normal (the manual talks about a "flashing sinewave"). Later I noticed that at fast sweep times, something like 2 DIV of the trace seemed to be missing between the triggering edge and the start of the trace on the display. An explaination was given on the other list [1] about how this is expected behavior due to propagation delay in digital logic and I can actually see the triggering edge, thanks to the delay line. I'm attaching some screenshots/pages from the manual for the reader's convenience (or at least I'm trying). - Michael [1] /g/TekScopes/topic/11350057 |
