Re: Vertical Hold problem

From: macmoni <macmoni_at_email.domain.hidden>
Date: Wed, 31 Aug 2005 07:57:07 +0200

Hi there,

> One of my Lisas exhibits the same vertical hold problem. It also seems
> related to the pot that controls vertical hold on the little board that
> sticks up above the CRT (with the top cover off).
That's in more of half of the Lisas with this screen-error the cause for the scrolling - I repaired the years ago So one can say this is the main spot, you will have to focus your attempts.
Often the pot is simply out order, caused by dust or sometimes oxidated metal components of the pot.

The second main fault is a undervolt-problem. Check the voltage-regulator at the right bottom of the analog-video-board.

The third main fault is a dto. overvolt problem, which unfortunately destroys the pot and/or the pre-resistor, which is connected serial to this pot. Sometimes you can see it shining orange (when the room is darkened) an smell a little "burning air" :-)

In case one simply try to turn (adjust 270) the pot several times or better try to clean its carbon surface carefully and then adjust it to that position it has been before.
In case two and three replace the voltage-regulator of the analog-video-board.

> also seems to go away as the machine warms up, so I don't know
> if this indicates that the pot itself is the problem (oxidation, bad
> solder joint) or if I have some other component like a capacitor
> going.

This will be a very likely a dying voltage-regulator on the analog-video-board; so try to fire up your Lisa, let the computer run approx 10 minutes and then try to measure the temperature of the voltage-regulator; it must be untypical very hot !!! But be aware of high-voltage near the tube's connectors from the flyback coil below the tube even after the power is shut off!!!

... or it indicates an overcurrent-problem of the coil, which you can find near the plug for the tube-deflection on the analog-video-board (approx. in the middle of it); try to adjust this coil or perhaps you will even see melted coil there...

greetings TOM from Bavaria

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Received on 2005-08-31 08:27:57

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