HELP! ( totally OT) - Hitachi Monitor problem - test-drb@ccmp.vtda.org

28 Oct 1999

please see embedded comments below.

Dick
-----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: Thursday, October 28, 1999 2:37 PM
Subject: Re: HELP! ( totally OT) - Hitachi Monitor problem

...

 Hopefully someone will know about this . . . I've got this really
 "comfortable" 20" monitor (Hitachi CM2085) which developed a rapidly
 fluctuating left and right margin, which apparently is related to its
 pincushion controls in some way. 

Well, I know I don't have the service data for this one. And it's
somewhat unusual for such problems ot be related ot the pincushion
circuitry -- more often it's related to the hsync stuff -- the PLL (if it
uses one) isn't locking, something like that.
 Having attempted to locate a source of service data for this beastie for
over four months, during which time it sat on the floor of my already
overcrowded computer room, where I, mobility-handicapped as I am, had to
step around it, I decided to look at the adjustments.  This fault came about
VERY gradually, having been noticeable during the warmup phase for a couple
of years, but until more recently, when the duration of this problem grew to
over half an hour, I figured that it might be in some way related to the
aging process.

Like most CRT's it has several hundred electrolytic cap's, and I'm loath to
remove them all so they can be tested, though it may come down to that.  I
bought this thing in late '93 and it was manufactured in '91, so I have to
assume it's aged a bit.

The individual scan lines seemed to change both randomly and rapidly, from
one to the next, in width, giving a jittering effect to the margins.  On a
monitor of this size it's not easy to see both margins simultaneously, but
it apears that when the left margin jitters to the right, the right margin
jitters to the left.  Now that I've fiddled with the adjustments to such
extent that the jittering is completely gone, the screen has a very
substantial pincushion, somewhat more pronounced on the left than the right,
though the display is VERY stabile.  The width control on the front of the
monitor currently has no effect.  I find I'm unable to restore the
conditions to their original state, not that it would help.

There are a few adjustments I've not seen before.  A couple, such as
side-pin, Hsize, and Vsize have a second adjustment called "sub.-whichever"
e.g. sub. side-pin, etc.  These seemed to work in pairs with the normal
adjustment.  I'm baffled as to why the front panel controls don't have the
same effects as before.

Since I paid out a couple of K-bucks, albeit nearly 7 years ago, (mostly
because I like this monitor) I'm loath to scrap it.  A prefectly good new
monitor of comparable size, brightness, focus, linearity (and weight), etc.
costs about $400 nowadays, unless one wants to pay for the name.

It's surprising that one can't easily order service documents for these
monitors, but I guess it's my inability to understand how business is done
these days.

...
   Not being in
any sense expert in repairing monitors, I fooled around with
 the various accessible controls, (after removing the plastic outer shell) 
I've never been a believer in tweaking adjustments. If a device stops
working, then some component has failed, and an adjustment is not going
to bring it back.

On the other hand, as here, sometimes seeing the effect of adjustments
can narrow down the fault area. But it's normal to need a schematic for this.
...

> such as "side-pin" among others, and managed to get the horizontal line
> length to remain relatively stabile, though the screen now has a huge
> pincushion distortion, which I can make go away somewhat, but when it's
> gone, the rapid left/right margin fluctuation comes back, though somewhat
> differently than before.
>
> Is anyone familiar enough with this monitor or with monitors of this type to
...
   make any kind
of suggestion which might help alleviate this problem? 
First thought -- a dried up capacitor somewhere which is allowing 2
circuits to couple. Say that's injecting PSU ripple or horizontal scan
ripple (a lot of supplies probably come from the flyback transformer)
into some power line somewhere.

I'd start (given that you don't have any service data) by testing all the
electrolytics with an ESR meter.

-tony