Re: Help diagnosing problem.

From: Ray Arachelian <ray_at_email.domain.hidden>
Date: Sat, 03 May 2008 21:48:27 -0400

Patnukem wrote:
> awesome since I do not have a keyboard/mouse or a hd. the screen
> should still turn on without them though?
Yes, so that means everything else should be ok. You need to figure out how to fix the display. So that could be several things: CPU board (the video state ROM in particular), the power supply, and the video board.

The video state ROM controls the entire state machine for the display system. If this is missing or damaged, beyond missing the serial number, the Lisa won't have the timing information to drive the vertical retrace, nor horizontal retrace signals. It could also be some other chip or component that are part of the CPU board, but I think this is unlikely since you would have gotten a CPU board error if that was the case.

So it's likely that it's either the video board - perhaps a cracked solder joint, or bad capacitor. It could also be the power supply as it contains separate power lines just for the CRT and video board. Be careful with these, the CRT and the large caps are kind of dangerous, and even extracting the video board is a bit difficult.

Do you think the CRT warms up at all? If you mess with the brightness knob on the power supply, does it show anything at all? If you turn the brightness up all the way, you should see something on the display. If you don't, then the CRT is probably not getting power (or not scanning.) You might need to turn the lights off to see if it glows faintly or not.

But do check the obvious things first. There should be a set of ribbon cables that come out of the motherboard connector, some of the wires go to the drives, one should go to the video board. Check that the pins of these are clean, both on the motherboard, and the connector. Of course it could also be the CPU board is unclean.

You might also have a Lisa that had the 3A ROM, in which case, there might have been a transformer attached in series with the video board. This would be by itself, possibly taped down somewhere inside the Lisa, but not part of the video board. If so, it should be safe to remove this and reconnect the wires to the video board. Check the CPU board and look at the ROM version. see: for the part numbers. If you see 341-0346A and 341-0347A you have the 3A ROM and should check for the transformer. (It's possible to not have this transformer and still have a 3A ROM).

A 3A ROM is kind of bad news, it limits your Lisa to running MacWorks only, so it diminishes its value, but you might be able to buy (or burn your own replacement) ROMs.

The parallel port VIA might cause problems with the display as it controls the contrast DAC when the DEN signal is off. Try swapping the two VIA's on the I/O board (if they're both soldered). Or if you can find the resistor network that drives the contrast circuit, perhaps a volt-meter might show some output or not. This could help you track down what's bad with the display.

Check the motherboard, if you see any warped or puffed up leads, you might want to test them and if they don't show zero ohms, solder a jumper wire to the joints. The corrosive stuff inside the batteries might have caused something in the video circuitry to break.

The most likely thing to go bad with time would be a capacitor, but of course a bad cap can cause other components to fail. Check the power supply schematics to see which components you need to look at for the video circuitry within the power supply. Check the video board itself, reflow the solder on the joints, it might help.

You might also have a dead CRT, if you do, you're SOL as those are difficult to find replacements for.

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