Re: 7854 revived with a new CRT - how to correctly calibrate Z circuit (was: #157693 -

Dan G

On Fri, Jan 29, 2021 at 09:39 AM, unclebanjoman wrote:

The second problem arose during the calibration of the Good SIgnal Flip-FLop
GSF (page 4-50, section B5 of the manual): using a DVM I set the voltage at
TP103 (board A21) 40 mV less than the voltage at TP122 with no problems.
Using two identical timebases 7B53A in slot A and B, I connected the probe of
my test oscilloscope at TP106, board A21.
7854 set to Horizontal mode B: I see a good pulse signal, with levels approx.
-0.2V and +3V. The signal is stable while rotating B INTEN through its range.
7854 set to Horizontal mode A: I see a the same pulse signal, same levels, but
it's "slippery". For a few instants is stable and triggered, then become
jittery, slips and then return somewhat stable. Adjusting GSF Level, R102
(located on Z-Axis Board A21) has no noticeable effects.
Thinking about some problem in the 7B53 in A slot, I swapped the two plugins:
same result: GSF signal is stable in HORZ MODE B. Unstable when set to HORZ
This is an interesting observation, and one that I was able to reproduce with my
7854 using 7B85 and 7B92A time bases. HORZ MODE A is the key -- it doesn't
matter which time base is installed in which slot.

Observing TP122 with 7A13 (using a different mainframe, of course) in voltage
comparison mode allowed me to see what is happening. Namely, with Horizontal
Mode set to A, there are occasional spurious pulses (about 20ns wide) that
show up out of sequence. They get converted to TTL levels along with the real
pulses, and so they throw off the display and make it look like it is "slipping".

As far as I can tell, U105 is just doing its job here and converting these spurious
pulses along with the rest, since they have the same amplitude. Hence, I don't
think this "slipping" indicates an incorrectly set GSF level. I think as long as you
can observe all the real pulses at TP106, with A and B INTENSITY varied
throughout their range, then the GSF level is OK.


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