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Help needed: Repairing an IBM 8514 Color Monitor

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username1

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May 29, 2017
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Hello everyone!

I've recently acquired an IBM 8514 (The CRT, not the display adapter) and upon powering it on, I heard internal arcing for a second, and then nothing. I opened it to look for the culprit and believe I found it: capacitors in the power supply had leaked on top of diodes, which were heavily damaged. Replacing the capacitors is easy, but I need help finding the correct type of diode to replace the damaged models. I want to ensure I replace these parts with the correct equivalents so I don't damage anything.

Due to the display adapter sharing the same model name, finding information on this monitor online is difficult at best, and I've been unable to locate schematics or a service manual with parts listings. The two damaged diodes (D848, D847) appear to read "01 7N" on both, but I'm unable to find much useful information beyond that. If anyone know which diodes should be used in this power supply, please let me know so I can replace them. Thank you!
 

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Are you sure the diodes are bad? That capacitor didn't leak, that's old adhesive which has degraded. It turns corrosive and conductive with age, and must be removed. Check the rest of the monitor for similar adhesive. It starts out as a light tan rubbery substance which hardens and darkens with age (accelerated by heat).

If the diodes did fail, then you can get some idea of what's needed based on how they are used.
 
That's good to know, thanks! I'll make sure I clean up any other places I find that gunk. Unfortunately, when I was cleaning up the board, the corroded legs of the diodes basically fell apart, so I do need to find suitable replacements. I still haven't been able to find a service manual or schematic for this monitor, so I don't have much information to go off of. I've been looking at schematics for the IBM 8513 to try and cross-reference parts, but the PSU is integrated into the main-board and is a different design.

I've attached photos of the PSU on both sides with the location of the diodes marked with red squares, hopefully that's helpful. If anyone has any advice on what to do with this monitor, please let me know. I'm comfortable soldering and replacing parts, I'm just at the impasse of not knowing which type and rating of diode should be used to avoid damage. I'd really like to get this neat CRT working again. Thanks for your time!
 

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In that location and by the look of them they are unlikely to be zeners. So if I could not identify those diodes, I would simply replace them with a BY448, as very likely this diode would out spec those original diodes. Probably a 1N4007 ot UF4007 would also work.
 
I think you are likely right about that Chuck. And if they are the BAX12 diode, low ratings of 90V 800mA and in the low voltage start up circuit, so easily replaced with 1N4002, 1N4003,1N4004, 1N4007 etc.

.......Though looking at the pcb tracks, they might not be as it looks like the two diode anodes are connected together and into a filter capacitor . Hunting around I can't seem to find a place on the 3179A monitor where two small diodes have their anodes connected to a moderate sized electrolytic cap, so maybe the two schematics are different.

In any case likely a 1N4007 would be playing it safe as a replacement.
 
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Looking at the new photo of the foil side, you are indeed correct that the anodes are connected together on what I assume is the B+ side (note the great big honking C851). Hmmm...
 
Can confirm it's the anodes connected, not the cathodes. Would you still recommend a 1N4007 knowing this, or does that change your thoughts? Looking through the schematic of the IBM 8513 I've uploaded, I didn't spot the same diode layout, but I might have missed something.
 

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username1 said:
Can confirm it's the anodes connected, not the cathodes. Would you still recommend a 1N4007 knowing this, or does that change your thoughts? Looking through the schematic of the IBM 8513 I've uploaded, I didn't spot the same diode layout, but I might have missed something.
Click to expand...

Those original diodes look fairly small and likely have lower ratings than the 1N4007, both in current and max voltage. I have a large bag of BY448's and use these as generic replacements for unknown diodes of this sort of size (unless they are Schottky types), as they are pretty hard to destroy in most circuits. But I think in this case, by the look of it, the 1N4007 would unlikely fail.

You could possibly use a lower voltage rated diode, like the 1N4002 or 1N4004, but the thing is we don't know because there is no schematic.

In some cases its worth tracing out the schematic, if the questions are more complex.
 
Hi. Did you manage to get your 8514 going and find out what the problem was? I have similar behaviour.
 
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