[lisa] Re: screen stability fix

From: James MacPhail <gg__at_email.domain.hidden>
Date: Thu, 09 Mar 2006 15:39:56 -0800

>What about the little wobbles that go down the screen?

In addition to the dirty contacts issue that macsimski reported, I believe there are a few causes of this problem:

  1. Minor fault on the video board
  2. Marginal or bad capacitor in the power supply
  3. Heavier than usual load on the 1.2 Amp PSU, such as an internal hard disk

Swapping the PSU may help distinguish (a) from (b).

You can test (c) by temporarily removing the second memory board, XLerator, hard disk, and expansion cards and see if the problem remains. If reducing the load eliminates the problem, consider:

If you have a heavily loaded PSU (or for long life in general), I suggest adding a small fan over the PSU to draw air through it. I use a 12V fan running on 7V, and it makes a great difference to the PSU temperature with minimal added noise.

HTH, James

X/Lisa Squiggly video fix

WORKING AROUND THE VIDEO CIRCUITRY OF THE X/LISA WILL EXPOSE YOU TO POTENTIALLY LETHAL COMPONENTS. PERFORM THIS WORK AT YOUR OWN RISK! Some X/Lisas display wavering / wandering pixels which is sometimes aggravated by increasing the load on the power supply via memory boards or other additions.

The problem is due to poor filtering in the power supply, which couples the ripple on the 5 volt supply into the supply for the video circuit board.

The problem seems to vary from machine to machine. It is aggravated by dirty contacts on the card edge connectors associated with the power supply, motherboard power and video circuit boards.

The ripple enters the video circuit via the Horiz. Phase control on the video board. The effect of the ripple is minimised by rotating the Horiz. Phase control to the counter-clockwise extreme. If this removes the problem entirely, then the following alternative method of adjusting the horizontal phase of the image may be used to prevent re-introducing the ripple into the video circuit.

Note that video circuits incorporate high voltage charge storage components that can deliver fatal electric shocks even after the computer has been unplugged for some time. Do not work on the video circuitry unless you are familiar with high voltage work.

Note that the neck of the display tube is fragile and easily broken. Because the tube contains a vacuum, there is danger of flying glass should the tube be fractured. Suitable caution and protective goggles are required.

When the Horiz. Phase control is adjusted to the CCW extreme, the image on the screen usually wraps around the edge of the CRT. With the phase control adjusted to the extreme, the addition of a capacitor between the base of Q8 and ground will move the image back towards the center of the screen. (The left side of R54 is equivalent to the base of Q8. The bottom of C5 and the emitter of Q8 are ground.) The amount of capacitance required will depend on the particular machine. 0.02uF (0.02 micro-Farads) is a good starting point. The capacitor should be rated at 50V or more. Ordinary ceramic capacitors are more than adequate.


Turn off the machine, unplug it, and leave it over-night, longer if possible.

Instruct someone nearby that they may need to give assistance in case of an accident!

Treat the CRT neck with great care to prevent breakage and flying glass. Be careful to avoid pulling on the wires that go to the CRT. Stay clear of the red high voltage wire attached to the stinger in the top of the tube to prevent electrocution.

Remove the top cover of the X/Lisa.

WORKING AROUND THE VIDEO CIRCUITRY OF THE X/LISA WILL EXPOSE YOU TO POTENTIALLY LETHAL COMPONENTS. PERFORM THIS WORK AT YOUR OWN RISK! Remove the video board: There are a few connectors that will have to be disconnected from the board. Be careful not to yank the wires as you disconnect them. You will have to unplug the multi-wire cable from the neck of the CRT to move the video board more than an inch or two. There are two phillips screws near the top of the board. The bottom of the board goes into a card-edge connector.

Check the location of the capacitor to be added and install some socket pins or flying leads so that the size of the cap can be changed without additional disassembly.

If you have some confidence and another video board to inspect, you may be able to solder the cap on without removing the video board from the machine. You should still allow as much time as possible for the CRT to discharge with the machine unplugged.

Install the capacitor, re-assemble the machine, and try it out. The Horiz. Phase control should be at the CCW extreme. If the image moved too far, use a smaller capacitor. If you want the image moved over further, use a larger capacitor or adjust the phase control.

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. Received on 2015-07-15 16:36:25

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