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This was long in the making !


Almost 5 years ago, I purchased a Lisa 2/10, well technically a Mac XL, from someone on Craigslist. The story was actually very cool. The seller said that he was working as a cameraman for NBC News, and he was the one filming the 1984 Mac launch at the Flint Center in Cupertino, when Sculley talked him into buying the XL. So I loved the history behind it. It also came with a box set of Lisa software (System and Office) which I have never seen anywhere since (942-0595-A). 


The Lisa was not working, but it was in near mint condition, and I got in on the cheap.  I actually got it to work fairly quickly, after discovering a short in the fan of the widget assembly. However, a few days later, it died again after I accidentally opened up the back panel while in use, triggering the EPO. This somehow fried the PSU (it shouldn't), and after many random attempts to revive it, I could not find what the problem was.


Fast forward 4 years of the Lisa gathering dust in my garage, and last week, I decided to try again and see if I could fix this dead XL. I bought one of these neat and cheap USB oscilloscopes, and made good progress. Since I could not get any good DC voltages from the PSU, I had a hunch that the pulse width modulator chip Z2 (SG3524) on the PSU was defective, so I changed it. Great move as the Lisa woke up, made all the right beeps, and booted up. However, the video was quite a bit messed up. So, next step I took was to look at the video board and mesure voltages there. I noticed that resistor R1 was VERY hot to the touch, and thought that the voltage regulator U1 (7824CT) was probably leaking current to the ground. I changed it (costed me 20c at a great store - HSC Electronics -  in Santa Clara, CA), and that fixed that problem, as R1 was now cool. However, I had messed up something else in the process, as the video was then seemingly dead, only showing a bright spot in the center of the screen. It looked like both vertical and horizontal deflections were not working. I checked with an external composite monitor that the logic board was in fact spitting out a valid TTL video signal (picture on the external screen is not great, as it's not syncing properly, but I could recognize the home screen).


Well, this took me a few days to figure out, but I eventually realized that I had switched around the two 3 pin connectors from the CRT to the board :wacko:. After fixing this mistake, I finally had a working screen. However the contrast was extremely poor, and countless adjustments of the potentiometers at the top of the board could not correct the issue. This is when the USB scope really helped me, as I was able to follow the TTL signal all the way through to the CRT, and realized that the signal really deteriorated past transistor Q2 on the video board. Changing Q2 and Q3 (thanks HSC electronics again) did the trick, and my XL now works great with a very crisp screen.


So, this was very gratifying, and I cannot thank enough the fellow(s) that reverse-engineered the Lisa circuitry, and also all the members of this 68kmla and Lisalist forums. Without it, this clearly wouldn't have been possible.


To conclude, I have a few comments/questions to the community


1) The connector from the board cage to the widget assembly says "PROTOTYPE" on it (see photo). Does this mean anything ? 

2) I once was touching the Lisa chassis while turning it off, and I got a pretty good shock. This could have explained why I had all all these issues. Has anyone experienced the same thing ? Do I have a faulty home ground ?

3) I cannot get the front bezel to close neatly on the right hand-side "hook". It looks like the floppy drive protrudes a bit too much and prevents the bezel to engage all the way before it clicks. Has anyone encountered the same problem ? 

2018-09-22 11.48.19.jpg

Edited by pinto_guy
Wrong quote
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