Lisa Video Board

[pinto_guy]

Hello my fellow Lisa geeks ! Can someone help me diagnose this video problem ? The video board was originally OK-ish, but with some jumpy behavior. So I went on to change all 10uF e-caps (2x100V and 3x16V) which has always worked for me. But then I got this screen below. Tweaking all pots does not solve the problem. I went on to swap all the others e-caps, to no avail. I'm suspecting the vertical sweep chip TDA1170N is fried, and I have one on order just in case. And why would changing the 10uF caps create this problem ? Has anyone seen this before ? Looks like the vertical linearity is completely out of whack, but all components of that circuit look fine. Also, the video signal is much noisier on the bottom half of the screen. Thanks in advance


View attachment 2025-05-01 10.58.55.jpg

 

[Brooklyn]

If it happened after you changed caps, go back and check your work. Make sure the right caps are in the right place and not backwards.

Also look for and cracked solder joints or solder bridges.

 

[pinto_guy]

Thanks Brooklyn.

I usually don't check resistors that do not look burnt, but becoming desperate, I did. One of the resistors feeding TDA1170 (R28) showed 470 kOhms when it should be only 56k. After a quick replacement, I got the problem fixed. R28 is actually next to C18 on the board which I had replaced, and it's possible that it got a bit too hot, leading to failure.