(...) and basically ruled out every component but the transformer.
In your first posting you write that you get voltages on the secondary side. Your voltages are 50% too high, but the ratio of the 5V, +/-12 V and 33 V is correct. -5V does not count, this is regulated by a separate 7905. That means primary drive circuit is working, transformer is working, secondary rectifiers are working. But your regulation is not working.
At first: when operating the PSU outside the Lisa, make sure pin N (+5 V sense) is connected to pin 11, 12, M (+5 V output)! Otherwise your controller will measure 0V and continue outputting power until the crowbar triggers.
You already replaced R29, so I assume it is working. This is a common failure, the wiper can have intermittend contact. The Q3 will never conduct, the controller circuit thinks the output voltage is much too low, same as above. Rotate R29's wiper to the Q4 side, then the controller gets the biggest actual value available, which means it should set the lowest output voltage. Target value is 0,7 V at the base of Q3 when +5 V and -5 V have equal magnitude.
If the thing still bangs into the crowbar: disconnect mains, apply +5 V from a lab PSU to the +5 V rail and -12 V to the-12 V rail. Do not apply -5 V to the 7905 output, these don't like to be reverse-powered. Put an 1k resistor in series with the +12 V rail and apply 12 V. The resistor is necessary to limit the current through CR21 when it fires.
Now start with R29 away the Q4 side. Rotate it slowly and see what happens with Q3 and Q2. The voltage at the base of Q3 should rise until it reaches 0.7 V. Then Q3 and Q2 should conduct and the PUT triggers. That means the voltage at pin 3 goes from 12 V to almost zero.
If this works, connect another 1k resistor from pin 3 of CR17 to 12 V line. It might be necessary to disconnect pin 3 from the PCB, so that only the bias voltage is connected. Now repeat the procedure above and check if CR17 triggers, too. Your fault is likely to be somwhere in this area.
The only PUT you can buy today is the 2N6027 from OnSemi. This works as a replacement for the obsolete MPU-131.
Hot dang - I'll give all this a shot in a little bit! Thank you for the testing ideas!
Progress outside the PSU, however! I wired up a bench power supply and connected it via a JAMMA fingerboard cut down to size. To my absolute joy, the Lisa powers up (albeit with problems). The first time it powered up was to just a grid full of garbage, so I pulled the RAM cards and inserted one at a time into MEM1. The first one just gave me another grid full of garbage, but was blinking this time (see videos in zip file below). The second, however, allowed it to partially post (also see zip file below). The status bar at the top is repeating garbage, and it's trying to run the next test (I assume - it keeps trying to draw an X over the next icon, but just keeps restarting the posting process and redrawing the main window over and over), but I do get an error code - 41. I checked the list of error codes and 41 translates to a "Selection logic problem" on the CPU card. I'm not exactly sure what "selection logic" represents, so I'm hoping someone might be able to explain that a bit. I also do not have the I/O board in at the moment, as it's currently being repaired due to typical battery damage; I'm pretty sure it's not required for the machine to boot, but I may have misunderstood that. Any idea where I should be looking next? I'm going to give the hardware manual another read and see if it addresses the topic of selection logic, but any input is always tremendously appreciated.
Either way, things are moving forward and I'm absolutely thrilled!
EDIT: Okay, after checking the owners guide, it looks like error 41 is caused by the I/O board being missing. I'll get it cleaned up the rest of the way and repaired and go from there.
