Review or Discard at Will

Imsai Power Supply Rebuild

Imsai Power Supply Rebuild

Dec 13, 2014

Recently, well once in a while in the last couple years, I’ve had the urge to rebuild some of my old CP/M equipment. Back in the day, the late 70s and early 80s, I ran some California Computer Systems (CCS), CompuPro (aka Godbout and Visan) equipment for a public bulletin board system. Also used these in business mainly for word processing but spread sheets too. I also used my father’s old Imsai S-100 equipment mostly for playing Startrek. I’ve recently been able to boot the CCS and CompuPro systems, however the Imsai was the worse for wear. It blew it’s fuse protection. My house through the years suffered a fire with resultant water damage as well as an ice storm that caused some flooding during the thaw. As you can see in the following pictures the damage isn’t totally catastrophic but significant. I’ve removed the card edge guide rails and the mounting hardware holding the PS-28U unregulated power supply sub assembly to the main chassis. The power supply with capacitors removed.Main frame before cleaning. Interesting, this image from my original Imsai documentation shows what appears to be a half wave rectifier, see heatsync near large capacitors, with two diodes. The image above of my actual board has four diodes. The documentation text does indicate four (4) diodes for a full wave bridge rectifier. Although the water damage, really just some mudding of the dust, was minimal I thought the capacitors may have shorted or leaked, note the white oxide near the connection holes. The diodes near the lower left in this image are two different kinds, the documentation specifics four (4) of the same diodes MOT MR 501. The outer locations CR4 and CR7 are the correct diodes specified while the center two (2) diodes were HEP 7520. Using the diode test setting on my digital VOM I found that the two MR 501s at CR4 and CF7 were shorted. Checking a few substitution charts I came up with a diode I had on hand an IN5408. I replaced all four to have a matched set. Although mechanically very well connected I found two of the diodes in the bridge rectifier with cold solder...

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