Best place to get info on monitor repair...

Tony Duell ard.p850ug1 at gmail.com
Wed Jun 28 05:50:07 CDT 2017


On Wed, Jun 28, 2017 at 2:04 AM, Warner Losh via cctalk
<cctalk at classiccmp.org> wrote:
> Had a static discharge event with my VR-201 and suddenly the background is
> 'grey' rather than 'black'. By this I mean I see maybe 50% of the color
> (where before I saw maybe 1%) and maybe half a dozen solid diagonal lines.
> This replicates across the two rainbows I tried the monitor with.
>
> This seems like it would be a common thing, but my google fu has turned up
> little. I get why I might not find a how-to on fixing a VR-201 (though I do
> have the prints), you'd think that I'd be able to locate the proper jargon
> to find the answers, but so far goose-eggs. I can't even find a good
> monitor repair FAQ, though I guess I need a primer on terminology as
> well... I've done a lot with video over the years. But it's all been
> digital, I've never repaired analog gear.

'Digital circuits are made from analogue parts' as Don Vonada said. I
still wonder how anyone can understand digital circuits properly without
understanding analogue stuff. But anyway...

This sounds like it should be a simple fault to fix. At least the HV circuitry
is still working (you have a raster, albeit an over-bright one).

My thought is either a problem in the video amplfier or in some of the
supplies derrived from the flyback transformer. The video amplfier is
remarkably complicated, it consists of a CA3046 transistor array
(4 of the transistors are used) and 4 more transistors.

But start by measuring the CRT voltages (not the EHT, so you
don't need more than a normal multimeter). I noted down the ones
in one of my VR201s (working normally, normal brightness picture)
and got :

Pin 1 (Green) Control Grid : -25V
Pin 2 (Yellow) Cathode : 35V
Pin 6 (Red) : 1st (accellerator) anode : 560V
Pin 7 (Blue) : 2nd (focus) anode : 200V

Pins 1 and 2 are the important ones at this stage. Look for pin 2 being more
-ve (lower voltage) that this or pin 1 being less -ve than here. Can you adjust
pin 1's voltage with the user brigthness control, and if so, can you make it
a lote more -ve?

When I have an idea of which area of the circuit is the problem, I
will look into
debugging it more.
>

-tony


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