Re: Model 1 video tear (common) problem - how to fix?

On Sun, 27 Jul 2008 10:45:20 +1000, "Ian Mavric"
<ianmav@xxxxxxxxxxxxxxx> wrote:

Hi everybody,

I've got a Mod 1 here which exhibits the video tear (unable to sync
properly) problem which I am trying to fix. In fact, of my 6 Model 1s, 3 of
them have this problem, which makes me think that the problem might be
affecting a large percentage of Mod 1s out there.

Here is the @9 display on the M1 in question:

http://ianmav.customer.netspace.net.au/auctions/Picture%20or%20Video%201114.jpg

The monitor has been adjusted with a good M1, and you can see the picture is
not central (not even close) and the sides concave (both are).

So far on this unit I've changed:

Z6, Z57, Z10, Z11, Z41
C20, C21, C26, C27
Q1, Q2, Q5
Z1, Z2
Just replacing components does not make sence. By doing this you might
introduce other (more serious) problems.


The voltages were adjusted to properly ready +12DVC, +5VDC, and -5VDC, and
with the roms plugged in it asks the normal "Memory Size?"

Given the (apparent) vastness of the problem (I've seen this problem on
probably 25 machines in my lifetime, and most of them I've parted-out being
unable to solve the problem), I'm sure someone out there has fixed it...

Does anyone know which part/parts are causing this problem?

Thx.

FWIW
Given that the monitor is correct i would say it has something to do
with the stability of the synchronization i.e. the frequency of the
mod 1 (X-tal or main osc circuit)
As the lines from top to bottom get shorter i would say that it might
have to do something with the influence of the line frequency.
You do not mention if you live in a 50 or 60Hz country.
Is the line shortening of the lines on the monitor stationary (or
slow/fast scrolling)?
To see if the main oscillator is working correct (steady freq) is not
easy. You can for instance not check this with an oscilloscope. If you
have access to a freq counter you might check if the freq is steady
and has the correct value.
If you have more than one M1 you might try to to use the main
frequency output to force on the bad M1. (Disconnect somehow the osc
from the bad M1. Make sure the connecting wires are not to long.)
It might be possible that the main oscillator does not catch the X-tal
freq. You might try to parallel the x_tal with a small C (20pF) and
see if it catches the X-tal. Even putting a finger on the oscillator
might change the circuitry enough to change the freq and catches the
X-tal.
Good luck
Bu


.

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