475 Anode Lead Sparking


Jeff Davis
 

Hi all,


I've been attempting to repair a 475 with a bad high voltage supply. I tracked the problem down to a bad high voltage multiplier - when disconnected, the high voltage comes back to -2470 just fine. After replacing the high voltage multiplier with one from a parts junker, the CRT anode lead sparks like crazy to any nearby path to ground.


Trying to eliminate operator errors before looking for other causes:
Could this be caused by powering on without the vertical preamp board installed? It doesn't seem like that should cause a problem, and I wanted to see if the parts queen hv multiplier worked before completely reassembling.


The scope has another problem, a bad filter cap in the +5v supply, so the +5 supply right now is about 4.4V. That doesn't seem to be affecting the HV regulation, however. The HV still reads -2470.


Is there something I could have missed in reassembly that might cause the sparking?


What would cause the CRT anode lead to spark like that?


Thanks,
Jeff N0DY

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