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My IBM 5154 had some blown capacitors in the power supply (the AC line filters). It worked, though I decided I should replace the caps in the power supply just to be safe. I went ahead and replaced all of the electrolytic capacitors as well (they were out of spec).

Put it back together and it works, sort of. I can clearly see the picture that is supposed to be there but the blacks are not black in EGA graphics mode and there is an undulating wave. In non-graphics (text) mode, the blacks are black but the undulating wave is still there.

Here's a video. How can I further troubleshoot this?

GIF capture by Spektre

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    Current leakage? Did you clean off all flux after soldering? You might try asking over on electronics.se – Alex Hajnal Jul 8 '18 at 1:56
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    Vertical waves = supply voltage ripple. Pincushion distortion, small image and high brightness = excessive EHT voltage. Hope the screen isn't producing x-rays! Suspect the B+ rail has lost regulation. Perhaps you put a cap in the wrong way around? – Bruce Abbott Jul 8 '18 at 4:13
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    Note that if you find any reversed caps they should be discarded and replaced. – Alex Hajnal Jul 8 '18 at 5:04
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    Please be careful. – Alex Hajnal Jul 8 '18 at 5:12
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    As an aside, also look out for cold solder joints (they'll look dull, not shiny). These can be fixed by reflowing the solder at a sufficient temperature. – Alex Hajnal Jul 8 '18 at 5:16
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It was because I forgot to plug a jumper into a socket that switches it from a 240V AC input to 120V AC input (socket M5). I simply forgot to plug it in. That's not the ONLY thing I did wrong, but it's the one that mattered.

Here's a complete list of ways in which I nubbed this up, for future readers benefit:

  • I forgot the 120 V AC plug (see above)
  • I tested the power supply without a dummy load, possibly damaging it and causing all of the output voltages to be way too high (~1.5x, once I corrected the above)
  • I discharged the capacitors with my finger (ouch!)
  • I discharged the capacitors with a screw driver a half-dozen times, probably damaging them (I now made myself a discharging thingy)
  • I forgot to clean off the flux

The good news is, my monitor is as good as new! Well, as good as it was before I decided I would "improve" it by replacing the capacitors (granted, two line filter capacitors had actually blown up, so it wasn't without reason). The picture is still off-center and too blue, but I'll save those adjustments for another day...

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