I said the CRT cable (thinking of where the cable leaves the HV unit) but Harvey said it straight forward: in the vicinity of the HV unit .
This way you measure the HV transformer switching frequency (which simply is the SMPS switching frequency in the 7704A) *regardless* whether you see a dot pattern or not in the trace. The probe picks up the strong alternating EM field and not (or not in the first place) a small AC ripple on the DC HV. In self-oscillating HV situations this is a handy way to verify that the HV unit is oscillating.
You can almost sure exclude now hat there was some strange (external? oscillations?) signal entering the Z-axis amplifier causing a dot pattern since in that case you would at the same time have noticed also a running dot pattern (perhaps of different amplitude than the synchronized pattern).
Still it's not sure what causes the dot pattern, a badly decoupled LV somewhere in the Z-axis amplifier, or a badly filtered HV voltage. I would first check the Z-axis output TP41120 for 25 kHz ripple.
Just to be sure, are all shields (SMPS box and HV unit) present? I don't remember if removing the HV shield causes intensity ripple in a 7704A.
Albert
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On Sun, Jul 29, 2018 at 12:07 AM, Tomas Alori wrote:
Oh I am an idiot, I dont think you meant the CRT cable right? The CRT cable
signal would always match the dotting frequency for obvious reasons.
