Re: 7854 with strange intensity/focus problem (to me)


unclebanjoman
 

After a few days of use I have to say that the 7854 works quite well except for one thing.
Turning the readout intensity CW the intensity increases but the characters become progressively more blurred.
This does not happens (or happens in a very minimal manner) with the real-time (SCOPE) trace or the digitized trace (STORED INT)..

Seems that the auto focus circuit does not works very well.
I've performed some component checking on the A20 board but all seems O.K.

Today I performed a complete check/calibration of Z-axis circuit, following all the steps, without success. All waveform and voltages are as per manual, with the exception of the step B10 (manual's page 4-53).
For that step a 067-0587-02 signal standardizer is required. I own the early 067-0587-00, having a max rep. rate of 250 KHz instead of the 1 Mhz required.

I tried anyway, but the result is puzzling me somewhat. Cranking the intensity fully CW (readout off, real trace only) the trace become very unfocused but I'm unable to reach a reasonable focused trace adjusting R63 (Focus Gain).
Turning R63 from one end to the other, my DVM connected at TP83 reads between +123 V and -9V.
According to the manual this voltage could be -12V or even more negative, but I reach only -9V at most. And the trace still remains unfocused. The best result is obtained with R63 fully turned CW obtaining a DVM reading of -9V but in this case, returning to normal intensity, and turning on again the readout intensity it is more blurred, worse than before!

I've checked Q77 and Q83 in-circuit with a VOM (ICE 680R) but they looks fine to me.

Focus Output level R70 works fine: I obtain 123 V as per manual.

II am uncertain whether to think about some problems with the CRT focus grid or problems with the autofocus circuit.

Max

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