DEC Rainbow PC100

[luminescentsimian]

I scored this historical oddity today. 1983 dual-processor (8088 and a Z80), 256k of RAM, dual 400k 5.25" floppy slots sharing a single drive motor, a 10MB HDD, and 12" monochrome CRT. Oh baby. While taking it apart to clean and inspect I discovered that it was a completely tool-less and modular design. Other than the obligatory yellowing everything looks really clean and in good conditions, but the power supply pops it's built-in breaker as soon as the switch is flipped.

I'm not super comfortable poking around the mains side of the power supply but I poked around with everything unplugged and couldn't find any diodes failed short or dried out caps. The breaker pops so fast I'm not even sure what I could see with a multimeter in that short blip. Does anyone have any tips for diagnosing it?

MB with both CPUs:​

A close-up of the dual floppy drive with the red marks on the drive and disks to help you orient them properly. Because they share the same spindle motor the bottom disk has to be put in upside-down.​

 

[Kai Robinson]

I love oddities like this! Cool floppy drive design for space saving...are both available to one cpu, or are they split one for the Z80, one for the 8088?

 

[Androda]

As for the breaker popping instantly, the presumption is a short to ground.

With everything unplugged (not plugged into the motherboard either), use your multimeter on resistance mode (or beeper mode if it has that) to check in the power supply for shorts to ground from the output voltage rails. Also check for a short to ground on the input side.

Next, if none are found, check for shorts to ground on the motherboard itself. Basically check from the voltages on the connector to ground.

 

[luminescentsimian]

I love oddities like this! Cool floppy drive design for space saving...are both available to one cpu, or are they split one for the Z80, one for the 8088?

Both CPUs can access the floppies. The floppy controller is the only thing attached to the Z80 bus and the 8088 has to talk to the Z80 in order to access it. Some sort of shared memory shenanigans.

 

[luminescentsimian]

Progress! After lots and lots of staring at hand written schematics and removing components starting from the output pins I found two ICs that were failed-short. An "8UV48A" power transistor and a "FE16C" common cathode diode array, and the main circuit breaker I took apart to look for damage and couldn't get back together. The PSU fired up without anything connected without tripping the breaker. The voltages on the output pins were a bit wild but after I plugged in the hard drive to provide a load they stabilized. Plugging in the motherboard didn't provide any exciting sparks so I went ahead and assembled everything and plugged in the CRT. The CRT is powered by 12V over the video cable so I hadn't been able to test it yet.
It works, though the CRT does seem dim and a little unstable. It gets through a lot of the POST but stops because I don't have a keyboard for it. Now I guess I need to rig up a keyboard emulator with an Arduino