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Vintage HP 3721A Correlator : Repair, Restoration and Enhancement

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dazz1:
Hi
HV circuit board drawing and parts locater attached.

There may be multiple faults.
The gain steps about 25%, 40%, 100% which suggests two parts that affect the Y of the trace failing.
The inconsistent instability of the vertical trace indicates a part partially failing.
I have not seen an unstable trace at 100% gain.

The symptoms only affect the vertical Y of the trace.  The X of the trace is steady.  The X is unaffected by Y gain steps or instability. 

I have done in-circuit testing of all the resistors.  None of the measured values were far enough out of tolerance to justify lifting the part to test out of circuit.


tautech:
Eyes are immediately drawn to 3M9 R29.....  :horse:

dazz1:
Here is the circuit diagram for the HV section.

dazz1:

--- Quote from: tautech on October 05, 2024, 07:40:25 pm ---Eyes are immediately drawn to 3M9 R29.....  :horse:

--- End quote ---

OK R29 measures 4.3M out of circuit (pot disconnected).
The FOCUS pot measures 5.3M.
R30 measures 29M out of circuit.

I can't probe R29 with power on.  The volts are too high with R29 sitting at -3kV.  R29 should only have about 126V across it, so well within limits.
I will just change it out. 

dazz1:
R29  is rated to 0.5W and drawing 0.37W.  Should be OK on paper, but my experience is that resistors should be operated well under their power rating.
I am going to replace R29 with a 1W rated resistor.

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