Apple Lisa No Video

[AlexTheCat123]

I have an Apple Lisa 2/5 that was working fine up until about a month ago. The screen started randomly collapsing to a horizontal line and eventually got to the point where it was unusable because it was a horizontal line all of the time. After a few weeks of troubleshooting, I found that the problem was a bad trace on the CPU board that made the vertical sync signal intermittent, but after fixing this trace, the screen now displays nothing at all. The heater in the neck of the tube is glowing, but there is nothing on the screen. When measuring voltages on the video board, I discovered that the 5 volt line is actually around 2.7-3.7 volts depending on when I measure it. I also noticed that the power supply lines that are supposed to be receiving 100 and 300 volts from the video board for the brightness and focus pots are receiving zero volts. I looked on the video board schematic and it looks like these voltages come from the flyback transformer, but I don't know enough about the flyback and how it works to be sure of this. Could this mean that I have a bad flyback or bad circuitry that drives it? Any help would be greatly appreciated!

 

[fri0701]

Have the original capacitors on the video board been replaced? I had the horizontal line issue on my 2/5 until I bit the bullet and replaced all electrolytics on the video board. Even if it's not the entire problem, it's a good thing to do preventatively to keep things working reliably. Although, the 0 volts on the +300V and -100V lines does seem suspicious...

Just to be thorough, I'd also check that the various plastic connectors that connect into the video board are all making contact. Sometimes the plastic on those can get a little bent out of shape from the heat, and the wires inside aren't actually connected although everything visually looks fine.

 

[AlexTheCat123]

Have the original capacitors on the video board been replaced?

I have replaced all of the capacitors as well as the TDA1170 chip and nothing changed.

Just to be thorough, I'd also check that the various plastic connectors that connect into the video board are all making contact.

I'll check the connectors right now to make sure that they're making contact. Do you have any tips on getting a multimeter probe down on the bottom part of the video board? It's really hard since there's a CRT in the way.

 

[fri0701]

Do you have any tips on getting a multimeter probe down on the bottom part of the video board? It's really hard since there's a CRT in the way.

You're braver than I am - I'd be too nervous to poke around the high voltages in the CRT cavity when the system is running.  :lisa2:

I don't have a video board handy right now, but the large square plastic connector near the video board card edge has given me a lot of trouble in the past in terms of getting it seated correctly.

 

[AlexTheCat123]

I checked/reseated the connectors and they seem to be making contact just fine. What should I try next? I'm guessing the problem has something to do with the lack of the 300V and -100V voltages or the 5V bus that is too low.

 

[fri0701]

Do your PSU output voltages look to be within spec?

 

[AlexTheCat123]

All of the PSU voltages seem to be fine.

 

[aplmak]

Have you replaced the potentiometers on it? I've replaced mine on all my Lisa's.. I'm not sure if John Woodall is still selling them or not..