today i picked up a tektronix 7603 mainframe with a 7d02 logic analyzer module off craigslist for the princely sum of $25 usd. the owner said it hadn't been powered on in at least a decade. after cleaning, i booted it up and she ran just fine - for the most part. a couple 7d02 firmware ROMs were throwing some errors, so i went off to track down ROM images and some pin-compatable chips. when i came back the screen had died. occasionally it flashed a bit of display, a few characters for a second. it hasn't since.
checking all the psu rails, they're all in +1% tolerance except one: the 5v rail. it's showing around 4.2v, and the datasheet minimum for a lot of the 74xx ic's on the display board are 4.75. now, i've only just dabbled in electronics, and mostly digital logic at that, so a power supply of this complexity is way above my head, and after reading the service manual a couple times i'm even more lost. going off the schematics every single other voltage is correct, and i cannot make heads or tails of how the feedback circuit that's supposed to be regulating the +5 output works. Q988 appears to be where the +5 is created, and it's getting the proper unregulated +8v from the power supply. in fact, every single voltage i've checked is correct except for the +5 rail and it's sense circuit. highlighted is the only place on the entire board i've found an actual reading of +5v. all of the other +5v (and +5 sense feedback) read +4.2v.