IBM PS/2 Model 25 CRT - No High Voltage

[Jimmy422]

Hi all! I have an IBM PS/2 Model 25 computer with a built in CRT that is not outputting any high voltage. The computer piece does work, so I know the power board that powers both the computer and the CRT is functioning at least somewhat properly.

When I looked inside the machine I noticed most of the capacitors were leaking, so I replaced all of them out of caution. This didn’t fix the issue but i didn’t really expect it to either.

Here’s what else I’ve checked so far:

All diodes and fusible resistors appear to be OK

All the larger transistors (ones in the packaging where they could screw to a heat sink) tested OK

I don’t see any cold/cracked solder joints on the flyback or any of the power connectors

The 5v, 12v, and 115v power rails coming from the power board are all outputting proper voltages.

There’s a spot on the neck board that is marked 150v, but it only appears to have 115v on it. According to the markings on the power board 150v is not one of its outputs, so my guess is that this voltage is being outputted/boosted by the flyback?

The horizontal output transistor tested OK, I replaced it just to be safe and this didn’t fix the issue.

The horizontal oscillator IC has all the correct voltages at its pins according to its datasheet. However I don’t have a scope to verify the signal it’s generating is OK.

The transformer that works with the horizontal oscillator seems to be OK, and the transistor that works with that transformer tested OK as well.

At this point I’m at a bit of a loss. My guess is that the oscillator chip is bad or the flyback is bad, and I assume that a bad flyback is a death sentence for this monitor, so I’m hoping that’s not the case!

Does anyone have any advice as to what else, if anything, I could check?

Here’s a link to some photos of the boards - if you need any additional photos let me know!

Thanks all!