It’s been a ride fraught with annoyance and broken things, this one.
Mind you, considering that it’s been shelved for so long it’s a surprise that anything works at all.
Decided to carry on with things, and as the entire computer is fairly modular, pulled the VDU module out.
It’s buried somewhere in the mass of wires going to the CRT and yoke.
Removed, it’s fairly sensible. Two boards, mounted at right angles, with the flyback transformer up on top, potted in some sort of silicone rubber.
It’s built from some fairly high grade stuff, stamped Motorola. The green capacitors tested good but were replaced anyway with new film caps of the same or better specification.
New caps installed (hard to see) and ready to go.
Arty shot of the back of the tube, guns and the back of the yoke. I like the way CRT electron guns look.
Powered up and met with a much brighter raster, however still nothing resembling information on the screen. Not even junk and trash.
Lifted the power supply out, again modular design.
Some of the transistors have tin whiskers. Luckily they are all 2N2222A so have cheap modern replacements in plastic.
Tested the supplies and determined all but the +5 are good (That’s +12, -12, +15, -15, +5), as shown with the dummy load light bulb installed on the -12V rail.
Traced everything out, accidentally put a transistor in backwards (old vs new case EBC CBE mixup) and it seems to have finished off this device, potted in the same sort of rubber the flyback is.
After research, this appears to be a 2N4441, a silicon controlled rectifier- a thyristor. It’s used to shut the current off in case of an overload current draw. I think the one hanging off is a higher power version, and it’s been having troubles and gone short.
I’ve ordered a new filter capacitor (10,000uF 40V) to replace the original and a new thyristor with as close to the specs but higher current capability to the 2N4441.
This will either kill or cure the +5v rail. If it cures, we may have life in the boards yet.