I think I'm making progress! I have patched every bad trace that I can find on the I/O board and now the power light will turn on and stay on when I hit the switch.
Woohoo! Congrats!
If I press the power switch again after turning the computer on, it will turn back off, but only if I let it run for about 20 seconds before trying to power off. There is still nothing on the display, though. I don't even hear the high-pitched whine that CRTs make when they are on, so I suspect that there is something wrong with the circuitry that drives it. Does the CRT turn on when you press the power switch or does it require a signal from a board in the card cage before it energizes? If it requires some sort of signal before it turns on, then I probably have a problem with the CPU or I/O board, but if it turns on with the rest of the computer, then I may have to mess with the video board to make it functional.
You might still have power supply issues, wetware RAM may be unreliable, but from what I remember the +/-33V lines are what power the CRT.
If you take the top off and turn the lights off you should see the back of the tube glow a bit where the cables go in. Don't do this just yet, read the rest of this.
I'm assuming you've already played with the brightness and vhold controls in the back of the Lisa?
So the contrast is controlled by a latch on the I/O board attached to one of the VIAs, but if the CPU board is dead or it has no RAM, it won't tell the VIA to set the contrast to a visible setting. So if there's a problem anywhere in that chain, you'll also get a black screen, but it could be lots of other issues from not being able to access RAM, or a dead CPU board or anything else like dirt on the contacts of the motherboard or broken traces between the CPU board, motherboard, or the connector or cable going to the analog board.
So next we'll want to try get some video output, any output, even if it's just turning up the brightness all the way and getting a white glow from the CRT. If you can't get the CRT to light up at all, or glow in the back, could point to the power supply.
If you're getting just one dot on the CRT and nothing more, it could mean that the flyback transformer is dead, or could be the VSROM or associated circuitry on the CPU board.
see:
https://lisafaq.sunder.net/single.html#lisafaq-hw-vid_jumpy and especially read the whole video section and pay attention to the safety warnings. I'd suggest wearing insulating gloves when you do that and keeping one hand behind your back. CRTs are very dangerous and they can kill you. If you know all this already, and you seem to, then great, I don't mean to be telling you basic stuff, but I don't want some kid, a few years from now, who never saw a CRT in their life before, reading this forum and then sticking their hand in around a CRT and getting themselves killed.
Now, the next step would be to either purchase a flat plastic screw driver kit - I mean the blade is made of plastic or other non-conductive material, not just the handle of a screwdriver. You could also use a really small spudger with a flat end, or to cut a plastic knife into the blade of a flat screwdriver.
If you don't feel like cutting a plastic knife yourself, you could buy something like these which are ceramic bladed, but it's a waste of money. (
https://www.amazon.com/REAMTOP-Slotted-Ceramic-Alignment-Screwdriver/dp/B06XGF8Q8Q/ )
The reason you need this tool is that the next step will deal with you opening the top case and poking around the analog video board and should the screwdriver fall in, you don't want it to a) break the CRT or b) short something out, or worse c) electrocute you, so a plastic disposable knife is the best tool for this job, but you'll need to cut the blade into the right size to fit into the trimpots that you can use to adjust the video board controls using diagonal cutters.
Now, after you remove the front face of the Lisa, remove the card cage, and go underneath the top of the Lisa at the top, you'll find some screws. Undo those and you'll be able to remove top of the Lisa's case. You can see this (in reverse) here:
https://youtu.be/ZXL2Ku23nSc?t=360 and a better one here:
https://youtu.be/0r-zLeYygzc?t=1293 (at those time codes).
You'll need to insert something into the power cut off switch at the bottom left of the Lisa's face, like some rolled up PostIt note or whatever, and also into the hole at the back of the power supply where you see the back door has a bit of plastic sticking up. This will allow you to turn the Lisa on without these covers, which is dangerous and why those power cut off switches are there.
Reinsert the card cage after you've done this and now the Lisa should be topless, and backless
and yes, ofc, faceless.
Power it on. Wear gloves and with one hand behind your back, use a magic marker to mark the original position of all the trimpots - you'll move them very slowly and gently while the Lisa is on and one at a time. The trim pots may have some wax on them to keep them in the factory position, but you'll be breaking that seal if it's still there.
Next use the plastic screwdriver to gently and slowly move the trim a little bit at a time pots back and forth to try to adjust the video. If you've got any sign of video - even if it's just a white glowing screen, great, if not, you've got more debugging to do. If one trim pot doesn't produce results, bring it back to the original position that you used the marker to note it.
Since you don't have a working speaker you don't know if it's beeping, or if there's a RAM issue or whatever. It might be worth fixing the two transistors just to hear any warning beeps. Beeping would be a good sign that the CPU and the boot ROM are good even if the RAM might not be or something else is wrong. Perhaps you could trace the sound output pin off the VIA - should be the output of the shift register, maybe CB1 or CB2 or CA1 or CA2, sorry too lazy to look it up right now, and then attach a logic probe between that pin and ground to that and listen to the beeps, if any, that way.You can find what the warning beeps mean here:
https://lisafaq.sunder.net/single.html#lisafaq-hw-rom_beepsI could suggest trying to get an external monitor attached to the video out RCA jack, but likely that will be near impossible even if the video is fully working since that output is so weird - I've tried and haven't gotten very far with normal SD video stuff.
So what this tells you so far is that the COP421 is working and powering on and off (most of) the system. What you don't know right now is, 1. is RAM good? 2. is the CPU board fully good? 3. Is the CPU ROM good? 4. is the rest of the I/O board good (though that shouldn't affect video except for the contrast latch and DtoA.)
If either the CPU board is bad or the RAM isn't accessible, or the contrast latch on the I/O board is bad (or not set by the CPU) you'll also get a black screen and some beeping.
If you have a logic analyzer or even a logic probe you can poke around and see if there's some signals going in and out of the CPU. If you have a fox and hound cable tracing system (like this one:
https://www.amazon.com/Meterk-Multifunction-Instrument-Maintenance-Continuity/dp/B071K8L32H/ ) you can use just the "hound" part which is really just an AM radio to pick up signals from the various boards and you should hear some noise as you move near each component. You won't be able to hear the really high frequency signals from the 68k, but you'll hear enough noise from other parts without even touching any pins to give you a rough idea to know if something is working.
It would really help if you had access to another Lisa so you can swap parts and see what's working or not, but if you're not near anyone with a Lisa, no worries, just keep poking at it.
You've made some really great progress, don't stop now, keep going, you're getting closer. These kinds of debugging things are like an onion where you're peeling off layer after layer of problems, but you don't really know how many layers deep there are to go.
