Comet high voltage wandering any hints?

talon93pete

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Ok,
So I rebuilt the HV section of my PS. Before I did I had dimly lit displays, but garbled. After the rebuild the HV was +74 and -74. Did some poking around, I ordered the 1.2 k resistors for R2 and R4, I replaced them with the original 680 ohm resistors and the voltage came up to +94 and -94.
Sounded ok to me, plugged the display driver board in aaaaannnnnddddd nothing. Not even a faint glimmer.
Probed around some more the - supply was cycling about every second from -34 to -89. The plus was solid +91.
I unplugged the displays, no change, then each of the plugs to the cpu, when I got to j10 on the display driver the fluctuation stopped.
I plugged it back in and unplugged 1j3 on the cpu, still a solid voltage.
I have not gone any further at this point just wondering is someone else has seen this before or could share a tip.
 
I would check the diodes. I had a similar issue and it ended up being a couple of diodes that went out. There's not many of them on there so I would check those. Be sure to check them in both directions.

Edit: Here's a link to my thread. The power supply is for a Sys 11 but I believe yours should be the same.

I see my issue was low voltage and yours is the high voltage. All I can really suggest is to trace the high voltage circuit back from the connectors and test the components along the way.
 
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Yes I did replace all the diodes and caps in the HV section.
It seems to put voltage out just fine-ish till the cpu is connected to the display driver.
I'm thinking something on the cpu is pulling the voltage to ground. There is not much there, some resistor networks and a pia if I remember the schematic right. Its gotta be something thats clocked at about a hertz in attract mode too.
I suppose its possible that a signal from the cpu is activating something on the display driver and thats pulling the voltage too.
 
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