Someone kindly pointed me in the direction of the HP “Service” manual for a terminal in this family.
According to HP, the recommended course of action is to “replace the main PCB”. Manufacturers have a lot to answer for when it comes to today’s throw-away ideals… it’s not just the consumer.
Horrible photograph, as I was trying to balance the tube (it’s held in by gravity and will alone) and not let the board fall out and not electrocute myself AND take a picture at the same time:
Did manage to liberate this from it earlier. I know it’s the computer equivalent of humming whilst strumming your lips but it does at least show the character ROM is working and there’s a certain amount of life left in it.
It did that twice then refused to do anything else again. Slowly getting somewhere.
Replaced a bunch of burned-up looking PH-33D diodes with an approximate match (1N4006) and got the voltages on the board aroundabout where they should be- (+5.03, +12.45, +38.70, -11.3) and was greeted by a raster and a couple of beeps
Which I presume is an error code (gonna go read the manual) but since turning it off then on again the raster has now shrunk and it doesn’t beep any more. More investigation required.
But, have a picture, because, well, pictures.
haha oops. I was using the wrong “ground”, I mis-read the labeling, it was somewhat hard to read with the back of the case on.
Poked about today and found the +38 good, the +12 at about +11, the -12 at about -10 and the +5 at 0, which is kinda odd because it uses the +5 as reference, as far as I can see. Or maybe it’s the +12, it’s not that easy to trace the board because it’s covered in a big sticker and only silk-screened on the component side.
Working by the logic that if there’s somewhere reasonable voltages present (at least in the right ball-park) then the power supply is mostly doing alright and the +5 is being dragged down to ground somewhere else in the circuit.
This’ll be fun to trace, only 80% of the board uses the +5 rail…