Part of the wiring harness before removing it from the chassis.

There’s two main power harness that run to most of the systems in the machine, which are thankfully still in good condition. As parts of them are hardwired directly into the transformer, it would have been a big pain to replace parts of these. They were pulled out to inspect, but looks like they’ll just get reinstalled as-is since there’s no damage.

The I/O pod removed from the back of the machine.

The I/O harness runs from the front of the card cage to a removable “pod” containing all of the ports on the back of the system. The mice that were living in this machine ate large sections of insulation off the wires of this harness (to use as nesting material I assume?), so a lot of it is going to have to be re-wired. I spent the afternoon today beeping out each connector and drawing a schematic of the harness in EAGLE, which should help keep track of everything as I rebuild the harness.

Part of the I/O harness showing damaged wires from mice.

Next step will be to figure out which specific wires need replacing, and how much wire I need to get. Then it’ll be getting a whole bunch of stranded wire and crimp pins for the connectors and replacing wires one by one!

Completed EAGLE schematic of the I/O harness.

Working on reversing the PSU at HackLab.TO.

Reverse engineering older power stuff like this is not that difficult overall, just tedious at times. It’s a double-sided board, no ground planes, and mostly fairly thick traces, so following traces around isn’t that hard. All the components are through-hole (or bolt-on, in the case of the big power parts), so reading values off (most of) the passives is easy too.

I started by taking some good shots of the board, turning them to B&W and increasing the contrast, then printing out one of each side of the board. I found a fairly random spot to start (the first transformer tap input) and followed where the trace went, drew that into the schematic, and marked it off on the paper copy to show it’s been transcribed. Then repeated that for the next few hours. When traces go under other parts, a combination of bright light through the board and my multimeter on continuity mode helps find where they go, and I took the big caps off the board to help with that too, since they just unbolt.

Tracking sheet I use to mark off each trace that’s put into the schematic, after the first few hours of reversing.

The hardest part so far is that most of the components have Xerox-specific part numbers on them, so I can’t just look up datasheets to find pinouts. TO-3 transistors have a fairly standard pinout, so I’m assuming they use that one until I get the schematic a bit more complete. I’m still not sure if they’re PNP or NPN, but as more of the schematic gets completed it should become more obvious which they are, as should the pinout.

Thankfully the crowbar chips have the original part numbers (MC3423) which lead me to a datasheet. This finally explained what all the SCRs on each board are for, which is to crowbar the supply in case of overvoltage, not power sequencing on startup like we’d guessed. If the voltage on a rail goes too high, the crowbar chip triggers the SCR to short out the voltage rail and blow the fuse.

An example schematic of how the MC3423 can be used, which was helpful in figuring out what the designers were doing.

At this point the connectors, a small part of the rectification/regulation circuitry, and the crowbar circuitry for the +24V rail is reversed out. Since this board has 4 rails and 4 of the crowbar controller chips, the next 3 crowbars should be easy to reverse, just copy/pasting in EAGLE and beeping out each trace to confirm that it matches the same design.

The schematic of the PSU’s linear board at the end of the first few hours of reversing.

Each rail has an LED and a labelled meter probe test point.

It seems like they didn’t pay much attention to safety on connectors back then, as there are a number of connectors with 120V next to low voltage rails! I’ve documented all the pinouts that I’ve identified so far in the documentation section of this site.

Shining a bright light through the board helps the traces stand out, and makes it easier to see traces on the opposite side of the board.

I’m actually considering putting schematics together for at least sections of the linear board, since it would help me understand how it works as well as assist in any required troubleshooting. It’s complex enough that it would be at least tens of hours to do just the linear board though, so I may just do small sections. This is one of those times where I need to decide whether I just want to understand enough to troubleshoot anything that comes up and get the system working, or whether I want to understand how all the bits work in detail. I tend to lean towards the latter, since reverse engineering boards and understanding the circuits within is something I really like doing.