Questions on convergence...

Roy J. Tellason rtellason at verizon.net
Sun Aug 13 18:10:53 CDT 2006


On Sunday 13 August 2006 06:05 pm, Tony Duell wrote:
> > I've done that sort of thing,  on TVs,  and what you describe there ain't
> > old. TVs that were still using all vacuum tubes when I worked on them
> > back in the days I had my shop -- now they were OLD!  Convergence used to
> > be a real PITA, because you had static convergence that was done by
> > moving magnets on the neck of the CRT (not to be confused with the purity
> > magnets!) and then you had dynamic convergence,  which was typically
> > 12-16 separate adjustments, all of which interacted to some extent.
>
> Actually, having grown up with delta-gun CRTs, I find them much easier to
> set up than the in-line type. Tweaking presets is a lot easier than
> tilting the yoke and hoping....

As in it's handled in software?  I have not done any convergence adjustments 
since those days,  the stuff I'm dealing with any more hardly seems to need 
it touched.  Perhaps the only thing off in the original poster's monitor is 
those static adjustments,  if it suffered some kind of a physical shock,  
which I would guess is at least worth looking at,  seeing if something is 
obviously out of place.  I didn't see any response to my point about whether 
this happened all of a sudden or gradually over time,  though.

> [...]
>
> > I used to try and fix monitors.  The last time I had one that I wanted
> > working I ended up tracking down some guy in the area who would work on
> > them,  taking him 3 or 4 of the monitors that I had around,  and telling
> > him that he could keep them if I got the one fixed that I wanted.  As it
> > turns out he didn't fix it right,  and I've not felt like being bothered
> > to go back there and hassle it since then.  I did open that one up,  but
> > there was no way I was going to attempt that repair.  <shrug>
>
> Why not? Admittedly modern monitors are horrible to work on, with one PCB
> that's totally inaccessible....

This one in particular was a NEC Multisync um,  3a I think?  After pulling the 
back cover off you couldn't get at much of anything,  except by taking it a 
whole lot further apart,  which involved undoing both side panels but to 
swing them out meant that you had to disconnect a lot of stuff,  so there was 
no way obvious to me to get in there and do test points and such to find out 
what was going on.

When I got it back from the guy I hooked it up to a dos-based machine and got 
multiple images,  as if the frequency it was trying to display wouldn't sync 
properly or it couldn't hit the right frequency at all or something.

I remember looking at that time for what might be online for that monitor,  
which turned out not to be all that much.  Anybody know if this has changed?

> > The other thing is,  to do a proper convergence needs a signal that's
> > going to give you a stable pattern,  although I suspect that's easier to
> > do with a computer driving it than not,  I had to buy a little generator
> > for TVs back
>
> Indeed. With a computer, it's trivial to generate a cross-hatch pattern
> (the main one needed for convergence).

I always used to start with dots,  myself,  for the center convergence.

> For TVs, you either buy/build a cross-hatch generator (I remember
> building one from a kit about 15 years ago, it was only a handful of ICs,
> one of which was a TV sync generator chain). Or you use a home computer,
> suitably programmed. There were programs for the BBC micro, C64, etc,
> published in the magazines.

The one I bought back in 1974 (!) does include the dot, crosshatch, and color 
bar patterns as well as a blank raster,  for purity adjustments,  but only 
fixed sizes of them,  not fewer or single ones,  which I saw featured on some 
instruments.  It uses unijunction transistors as frequency dividers,  and has 
three trimpot adjustments on the back which needed to be tweaked often as it 
wasn't terribly stable.

> > Monitors are apparently moving into the "not worth fixing" category any
> > more.
>
> To be fair, very little of the stuff we talk about in this list is 'worth
> fixing'. Why spend hours/weeks restoring a PDP11 when a PC has a lot more
> CPU power? Well, we all know the answer to that...

It depends on how much effort you want to put into something.  With a pile of 
monitors around here and no specific need for _that_ one,  I didn't pursue 
it.  I did put it in storage,  though,  in a pile of monitors that's in 
there...

> I feel that the original monitor (and disk drives [1], keyboard, etc)
> are as much a part of the machine, particularly (in the case of the
> monitor) on a workstation, as the CPU. Therefore, I'd want to repair the
> monitor if at all possible (even if, say, I have to re-wind a transformer
> to do so,,)

Some units are no doubt easier to work on than others.

> Now no-name SVGA monitors are another matter. I do _NOT_ want any of
> thsoe, I don't repair them, etc.

The ones here all seem to have names on them,  Packard Bell,  Emachines, 
Zenith Data Systems, MaxTech...   :-)

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin




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