7854 revived with a new CRT - how to correctly calibrate Z circuit (was: #157693 - https://groups.io/g/TekScopes/message/157693)


unclebanjoman
 

Hi all,
more than a year ago, I initiated that discussion: https://groups.io/g/TekScopes/message/157693 about my 7854 with severe focus/intensity problem.

Finally I got a replacement CRT guaranteed working some days ago.
I replace old CRT with the new one and.. finally!!! Trace is brilliant, sharp and focused. The readout also is very well focused, even at high intensity.

Undoubtedly Albert Otten was right when he hypothesized the phenomenon of CRT's double-peaking for my case!

Now, with this new CRT, some re-calibration is necessary, due to slight changes in deflection sensitivity, step response, etc. So I started from scratch.
I want to clarify that the scope actually works well, I have not noticed obvious anomalies (for now).

I follow the calibration step-by-step as per manual.
I have come to section B, "Z axis and display - page 4-47" and here I found two things that baffled me.

Section B-3 "Adjust Z-axis DC levels": I correctly managed steps a thru g obtaining an 80 Vp.p. signal at TP122, board A21.
Continuining to next steps I read this:

i. Connect the test oscilloscope 10X probe to TP183 (located on the Z-Axis Board) with the probe ground connected to chassis ground.
j. Set the 7854 B INTENSITY control fully clockwise.
k. EXAMINE—Test oscilloscope display for a waveform baseline between 8 and 12 volts above ground reference.
I. ADJUST—The Output Level adjustment, R135 (located on the Z-Axis Board A21), for a waveform base line at 10 volts above ground reference.

Question: what is "baseline"? The bottom portion of the waveform? If this is the case, R135 has no effects on the level of bottom portion.

Continuing:

m EXAMINE— The test oscilloscope display for a 68 to 72 volt peak-to-peak vvaveform. (Do not move the test oscilloscope vertical Position control.)
n ADJUST—The Z-Axis Ampl Gain adjustment, R125 (located on the Z-Axis Board A21) for a 70 volt peak-topeak vvaveform displayed on the test oscilloscope.

Here I can adjust: I set for 70 Vp.p. But the lower portion of waveform is at approx 7-8 V about GND.

Can be the manual wrong? I think the the regulation at points j/k/l should be made witch INTENSITY fully CCW, adjusting the R135 (Z axis output level) for top waveform level about 10 V from GND. I'm thinking wrong?

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 MODE A .

I don't see any motivation for this behavior since the FF circuit is very simple.
Really I don't know why this happens, since the scope seems to work normally. I will follow the Z signal back to schematics <4>

The 7854 manual seems very unclear in this cases. I do not have another 7854 to compare the signals and I don't remember what the signals on A21 Z axis board was, since my last tests two years ago...

What do you think? Am I doing something wrong or is the manual to be wrong?
Any suggestion is welcome.

Cheers,
Max

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