After replacing damaged diode CR111, it’s time to reassemble the PSU and give it a try. Reassembly is a bit easier than disassembly, but still very fiddly, especially getting all the spade connectors back where they are supposed to be. Needlenose pliers are needed, and some patience, and a willingness to poke fingers into the internals to get the plastic system-connector-plug back in its’ slot.
The crap semi-internal bolt I had to drill out in Episode 2 has been replaced by a proper steel hex-driven bolt, soooooo much better!
As this is a “smart” PSU, I cannot test it in isolation – it needs some output load before it will do anything, so let’s get it fitted back into the Indigo.
Fitting the PSU back into the case is very easy – it just slides in on rails into the top right of the case, as viewed from the front, then is held in place with a single screw at the back of the case.
Next step was to power-up the Indigo with just the CPU board, without the graphics board and drives installed, after unplugging everything else in the house, just to make sure the repaired PSU wasn’t dangerous and wouldn’t freak the mains power supply. Went well – fan spins, no burning smells, no sound of capacitors popping, and no smoke!
Next it was time to try it with the system-boards installed. Unfortunately, I had misplaced the SGI-special keyboard cable, in spite of having the actual keyboard to hand. No problem, let’s just try with a serial console instead… unfortunately, Indigo requires a rather special serial cable, which I had also misplaced.
The Indigo doesn’t have DE-9 or DB-25 serial connectors. Oh no, that would be far to straightforward. Instead it has mini-DIN sockets for that. This was fashionable for about 2 years in the early 90s in the UNIX workstation world – the actual cable I was looking for was my Sun serial-adaptor cable for a SPARCstation-2, which uses the same pinouts as a “proper” SGI one (which I don’t actually own). I keep dreaming that, like flared trousers, mini-DIN serial connectors will come back into fashion, “any day now, you’ll see!”
After hours of searching through boxes, bags, drawers, the top of cupboards, et al, et al, I found the darned cable on a bookshelf. Dang! Time to get my trusty P3 laptop computer out, to act as the console. Oh yes, I’ll need a null-modem serial convertor as well, which gives me an excuse to get ye olde serial breakout box into the project!
For those of you that don’t remember 15-pin D-shell Ethernet sockets, you need an external transceiver (formally: “Medium Attachment Unit”) to convert it to more common form such as RJ-45 or 10Base2. A widget like the CentreCOM 210T…
Unfortunately, when trying the PSU for real, under load, the signs were not good. It couldn’t see the SCSI hard-disk drives, much like before, even though their LEDs were lit. It recognises the SCSI floppy-diskette drive, but that only needs the +5V rail. Sounds like the missing +12V output is still, well, missing. Mind you, at least you can hear the funky Indigo power-on sound in the video.
Time to probe the actual PSU output voltages.
On the SGI indigo, the HDDs are mounted on slide-in sleds, which breakout the SGI-custom backplane connector into a 50-pin IDC SCSI ribbon and a 4-pin molex connector. Armed with an empty drive-sled, I now have probe points to check both the +5V and +12V rails, via that molex connector on the sled.
Hmm, with the CPU board and both a SCSI HDD and SCSI FDD installed, the nominal +12V line is showing only 3.51V. With just the CPU and FDD drive installed, we have 2.38V. With just the CPU, we have 2.49V. What the heck! There is obviously some other fault with the PSU, not just the dead zener diode that was replaced. Just to be sure, I probed the other line in the molex connector (should be +5V), and hey presto! it shows 4.97V – which proves I knew which line was which, no mistakes there.
One other person has noted that capacitor C127, next to oft-failing zener diode CR111, is directly linked in series with CR111. Although C127 is not showing obvious signs of damage, I thought I could just see a slight bulge in the top wrap-over plastic coat. Maybe C127 needs replacing too?
I was not completely convinced, but with time running out for RetroChallenge, it was at least worth a shot.
However, replacing a radial capacitor (when you can’t get at the legs) is way more tricky than working with axial components where you have full soldering-iron-access to all the important bits. Eventually, it was a case of ripping the damned thing out by force, then desoldering and removing the stumps.
One replacement 22uF 25V aluminium electrolytic capacitor later, and we have… no change at all, exactly the same behaviour as before. Shucks. There are obviously other things wrong with this PSU, things that only a complete re-cap and re-power-transistor are likely to resolve. Unfortunately, that is way beyond my capabilities. This particular RetroChallenge project is complete, but failed.
It appears that the only way to get this Indigo working again, is to obtain a fresh 25-year-old PSU. Time to give Magnificent Mr. Mapleson a call!