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
Froggie the Gremlin