Hello, I am new here. Figured I'd jump in with something outrageous.
I've acquired a non working Classic and a Classic II with early (EAB) and late (LAB) analogue boards respectively. I have recapped both ABs and LBs.
The LAB would blow the fuse immediately, eventually 'solved' by borrowing the QP2/IRFBC40 from EAB unfortunately this still didn't result in a booting LB (see below).
The EAB would under-voltage, including +31V DC rail (approx 20V unloaded, 16V loaded). This results in a dim and wobbly display.
With the EAB the C1LB would checkerboard, and the C2LB would white screen.
Once revived the LAB showed stable voltages, but would still checkerboard on C1LB, and white screen on C2LB. By this time I had already established the the C2LB could boot successfully with good power so I am at a loss as to why this is.
How do I know the C2LB is good? well I removed the power wires from the EAB and soldered ATX pins on the end and inserted them carefully into the correct voltage rails of a spare PC power supply, this left the AB to power the local electronics + the CRT, but the LB was now powered from ATX.
Although the screen was still dim and wobbly, I was able to see C2LB boot (yay), unfortunately I still got a checkerboard C1LB (boo).
So what next?
Well I reinserted the power wires back into the EAB, but then tapped in a molex HDD connector for 12V and 5V, leaving the -12V on a wire to plug into the ATX power connector. This meant the EAB was now 'receiving' power from the ATX board on those rails as well as the LB. I then took a DC-DC Boost converter and tuned it as close to 31V as possible (actually it maxed out at 29V) and soldered this onto the output pin of IP2/LM317.
Guess what? It works! I now have CRT power provided by the ATX supply, and it's bright and stable, and the C2LB boots like a champ.
Sadly the C1LB still checkerboards, this is a shame because it's the one I really want to see working. But at least I think I can be pretty certain the fault is with the LB not the PSU.
Oh and another thing! The first 7 rows of the ATX power connector are keyed in a compatible way with the 14pin connector on the LBs. Only the latch doesn't line up so a spot of hot glue may be required to keep them in place. Taking an ATX power extension cable and cutting off the excess contacts would happily plug in between the AB and LB.
What does this mean?
I figure if I picked up a couple of ATX power extensions, I could make up a harness that could simply be inserted between the AB and LB with a seperate connector to take power from an ATX supply.
This would provide +12, +5 and -12V to both boards as required.
The only hardware modification required would be to inject +31VDC into the AB on the output of IP2/LM317, this would be provided via a suitable (common negative) DC-DC boost converter powered off the ATX 12V.
To my eye the circuitry on the AB looks like it should be pretty happy with being fed power the 'wrong' way. Except maybe IP3/LM79L12 which could do with a bypass diode like IP2 and IP4 both have? (but pointing the other way, cos negative), although I don't believe this gets used anywhere on the AB so the harness could be designed not to pass this back to the AB.
Guess I better top my tall story off with a picture, turns out it's not that easy to take a photo that shows both the screen and the lack of power cable into the back of the AB, but here goes. The white shield helps show the lack of power into the AB. ATX power wires are soldered onto the back of the AB opposite the loom which runs to the LB.
I'll post updates here when/if I make some progress.