General Category > Lisa Troubleshooting and Repair

Apple Lisa Scrolling and Jumpy Display

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8088mph:
I got my pots from someone else. I can ask if you can't find any. They have different values so make sure you get the right value for the particular one you are replacing. If you want original parts, I still have mine, they probably work fine since the capacitors ended up being the problem but I had changed the potentiometers first.

AlexTheCat123:

--- Quote from: 8088mph on April 20, 2020, 08:54:00 pm ---If you want original parts, I still have mine, they probably work fine since the capacitors ended up being the problem but I had changed the potentiometers first.

--- End quote ---
Thanks for the offer for the pots, but I already ordered a new one (Bourns 3352T-104LF) along with all of the electrolytic capacitors needed to recap the video board before I saw your message. I hope this fixes the problem!

AlexTheCat123:
The new capacitors and trimpots for the video board arrived today. I installed all of the capacitors and replaced the Height and VHOLD potentiometers, but the problem is still there. I can barely see it since the screen is just a horizontal line, but I think that replacing and adjusting the VHOLD trimpot has stopped the scrolling issue. What should I try next to solve the horizontal line problem?

stepleton:
I would recommend checking out some of the additional advice from The Dead Mac Scrolls, linked above in my previous reply. I've seen one or two video boards fail owing to bad semiconductor components---while caps and pots are well worth replacing, the board lives in a cramped, dusty, perhaps hot little space and gives a hard life to its other parts too. The Mac analog board video circuitry is similar enough to the Lisa video circuitry that the advice can carry over, provided you can identify which Mac components are analogous to which Lisa components.

patrick:

--- Quote from: AlexTheCat123 on April 24, 2020, 03:12:29 pm ---The new capacitors and trimpots for the video board arrived today. I installed all of the capacitors and replaced the Height and VHOLD potentiometers, but the problem is still there. I can barely see it since the screen is just a horizontal line, but I think that replacing and adjusting the VHOLD trimpot has stopped the scrolling issue. What should I try next to solve the horizontal line problem?
--- End quote ---

Check the vertical deflection circuit. This is U2 (TDA1170N) and the components around it.

There is a LM7824 voltage regulator on the board. At its output (pin 3 or C2) you should measure +28 V. The voltage can be adjusted with the pot labeled "WIDTH". Do not apply more than 30 V here, otherwise the TDA1170 could be damaged. If pin 1 and pin 3 of the controller indicate almost the same voltage value, the width potentiometer is defective. C2 could be worth a replacement, it is heavily used. But I assume you have already done this.

At pin 2 of the TDA1170 (or at C12) there should be 11..16 V. R24 is getting really hot, it drops the difference between the +28 V supply and this voltage.

At pin 8 you should see (or hear) the 60 Hz VSync from the CPU board. If this is intermittend, you have issues (corrosion?) with your mother board. or with the CPU board / video PROM.

Pin 4 drives the vertical deflection coil. Check the solder joints at the connector, and check (or replace) C19 with a low ESR/high ripple current rating capacitor. It is in series with the deflection coil and carries its full current.

If the width regulator or voltage regulator has failed in the past, your TDA1170N may be flaky. I had to replace this part quite often.

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