Working on a Monroe OC8820

I posted a couple months ago about this OC8820 I found in my father-in-law’s crawl space. I went back and picked it up last weekend and have started to check it out. I started by disconnecting the power supply and checking that voltages looked normal, and they were. So I plugged it in and did and powered it up and not much happened accept for a beep. I stuck a couple floppy’s in the drives, not formatted for this machine just to see if the drives would start up. This time it attempted to read each drive and switched back and forth between the two. So it seems the computer is running and trying to do something. The monitor didn’t show any signs of life though. Adjusting the brightness did nothing, so looks like I have some work to do there.

Then after a few minutes, I heard a sizzle and a nice plume of smoke came out of the power supply. I disassembled the supply, an Astec AA11730, and saw that one of the film capacitors had popped.

So I’m going to have to get the power supply working again and then tackle the monitor. I also need to order on of those floppy emulator devices so I can try loading some software. Hopefully the drives won’t be too much trouble to get working. Here are a few pics.

Here a pic of the power supply showing the bad cap.

This is the main board which includes the floppy drive controller.

And this is the huge video board.

The machine had this tag on the side. Looks like it was used at the Glenview Naval Air Station. Cool!


That failed cap looks like a RIFA - in the case of Acorn computers, these are on the mains side and there for interference suppression, so you can in that case use the PSU without fixing them… once you’ve aired the room a bit.

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Your right, but I think I’ll just wait till the part comes in and get the supply put back together and hooked up. In the mean time I’ll clean up the inside of the machine and check out the drives.

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I’m still working on the repairs of my machine too, if you need help to test something let me know. My power supply and the video are working fine but I need to replace the memory chips and probably some logic ones too.

Looks like there are two more RIFA caps on that board. I’d replace those too while you’re at it.

I got the PS put back together and started it back up, minus the drives. Voltages all look good but still not getting any video. I check the signals from the video board and I’m getting a clean Vsync, but no Hsync or Video signal. When I have more time to mess with it I’ll start probing the board to trace the problem. The fact that I’m getting Vsync means the MC6845P is at least partially functioning, so maybe it’s one of the chips between it and the output to the monitor.

I checked out the drives and they are loaded with dirt. Might be because the cooling fan blows directly into the drives. Geez.