Got me stumped: HV on DE 5000 Power Supply

stangbat

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I'm rebuilding the high voltage section of a Data East 520-5000-00 power supply. This is the alphanumeric power supply that generates -100 and +100v (basically a WMS System 11 copy). Originally it was blowing F3 even with the displays removed. I rebuilt everything except the 100 uF caps at C10 and C11. It still blew the fuse with the displays not connected.

I then replaced C10 and C11 and it now powers up. When I measure voltages at the power supply's output pins with no load, I get -89.1v and +91.5v. Seems on the low side.

When I hook up the displays the negative voltage swings all over from -44 to -80v. The positive stays at 88-89v. I get these same low readings at the PS connector and at the display board connector. The displays flicker, but they obviously won't fully illuminate with the low voltages they are getting.

Edit to add that the incoming AC voltage to the HV section is 95v, which is what it should be.

I thought that some of the components I replaced may have been damaged when I powered it back up and blew the fuse after the rebuild. So I went back and basically rebuilt the HV section again, step by step, to be sure that nothing was damaged. No change, the voltages are still low.

The connectors are fine. No cold solder joints. Good continuity between the PS and the display board. I've gone through the schematics and checked grounds and continuity on the PS board itself. I've run out of ideas. I used to think PS rebuilds were easy. This one is proving otherwise.
 
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