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


Dan G
 

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

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.
"baseline" is the bottom portion of the waveform. The service manual is right,
but the confusion may have come from the labels on the A21 PCB: there
are TWO trimpots labeled "OUTPUT LEVEL". One is in the lower right
corner of the board -- that's R70, and the other is near the center of
the board -- that's R135.

I have just replicated the adjustment procedure from the manual, using
a 7B92A as the time base in Horizontal Slot B, and a 7A26 in Left
Vertical Slot (adjusted to make sure the trace is as far off the screen
as possible!).

R135 (the correct one!) does indeed directly control the level of the
bottom portion of the waveform, as expected. In my case, the levels
were already correct (10V baseline, 70V p-p), so there was no need
to adjust.


I hope this helps,
dan

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