VR241 vertical linearity problem - test-drb@ccmp.vtda.org

7 Nov 1999

-----Original Message-----
From: Tony Duell &lt;ard(a)p850ug1.demon.co.uk&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
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