-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Sunday, November 07, 1999 12:10 PM
Subject: VR241 vertical linearity problem
>I'm still working on that Rainbow, and today I've tried it with a DEC
>VR241 colour monitor.
>
>The good news is that the graphics card (and GSX) works fine. And the
>graphics are rather impressive compared to the (contemporary) IBM CGA card.
>
>The bad news is that the VR241 has a fault. I feel somewhat strange
>asking this here, as I'm normally the one to answer such questions, but
>here goes...
Hmmmm
>
>The fault is in the vertical scan. The top 2 lines (or so) are spread
>out, so that the top row of characters appears to have black lines
>running through them. In other words the vertical scan is very non-linear
>at the top.
If part of the picture is cut off with a horizontal line then it is either
a video timing problem or a fault in the deflection circuitry preventing the
beam from being where it is supposed to scan in enough time.
>
>Adjusting the vertical linearity control improves things a little, but
>reducing the spacing at the top increases it at the bottom. What is odd
>is that the vertical position control affects the fault as well -- the
>problem gets worse as the picture is moved up the screen. This seems to
>rule out a problem with the ramp generator, since the vertical position
>control operates by applying a DC current to the yoke.
This is likely a vertical deflection problem - possibly a bad capacitor,
bad connection, bad flyback/pumpup diode.
>The problem gets
>better as the monitor warms up.
>
Ahh,, a dried up electrolytic capacitor is most likely.. if it got a lot
worse then it would be a semiconductor.
>I've not dived into it yet -- and I suspect the electrolytic in the
>vertical output stage. I am wondering if anyone's seen this before and
>can suggest a possible cause.
>
Easiest thing to do is substitute a good capacitor for each electrolytic in
the vertical output circuit.
(if you don't have a scope). If you do have a scope then work your way
through the vertical output circuit and locate the problem.
john
>-tony
>
Received on Sun Nov 07 1999 - 11:54:16 GMT