Questions on convergence...
ard at p850ug1.demon.co.uk
Mon Aug 14 16:35:42 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
No, not at all. AFAIK all monitors with microprocessor control use PIL
CRTs. I jsut find it easier to turn presets than to shift the yoke.
Now, as to what killed the delta-gun CRT... Well, Think of the effects of
external magnetic fields on the beams, and remember that the electron beam
moves perpendicular to the field. Resolve the earth's magnetic field into
vertical and horizontal components. The vertical component of said field
causes the beams to move horizontally, the horizontal component moves
BOTH components of the field cause convergence errors on a delta-gun CRT.
But on a PIL tube, the horizontal component of the earth's magnetic field
just shifts the picture vertically (slightly), and as the phospors are
put down in vertical strips, it doesn't cause a convergence error.
Now remember that the vertical component is essentially constant in a
given area, the effetive horizontal component changes if the CRT is
turned round, etc.
So a delta-gun tube has to be converged in the place it's going to be
used. A portable unit is almost impossible. A PIL tube has to be
converged at apporximately the right latitude, but apart from that, it
doesn't matter if you move the unit.
> 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
You're lucky. I've never seen a modern-ish TV (that is, a TV recent
enough to be a PIL CRT, but not a plasma or LCD panel) that was
> 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
If it's suffered a shock, it's possible the 'aperture grille' (the
equivalent of the shadowmask) has come loose inside the CRT. This would
cause purity errors, and probably convergence errors too. In fact do we
know the OP's monitor has good purity (display a screen of each of the
primary colours -- red, green, blue -- does it look even, with no odd
> 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.
> > > 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.
I tend to switch between dots and cross-hatch.
> > 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
Mine will generate a grid of vertical lines, and a similar grid of
horizontal lines. The logical AND is dots, the logical OR is a
crosshatch. All 4 of those patterns are switch-selectable, along with an
8-bar greyscale, the obvious 8 bar colour bar pattern, and a plain
raster. IIRC, you can enable/disable each of the primary colours for all
patterns, so the plain raster can be used to give the 3 primary colour
rasters for purity adjustments.
> instruments. It uses unijunction transistors as frequency dividers, and has
Mine at least uses ICs (mostly TTL from what I remember, along with a PAL
encoder chip and a sync generator).
> > 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.
Yes, like the semi-professional TV-rate Barco I have here. The first nice
feature (if your workshop is as full as mine) is that it's not 'monitor
shaped'. It's a cuboidal metal case, so you can stack things (at least
manuals, etc) on top of it. Normal monitors are a pain to store.
This Barco is cery well made. It's all on plug-in PCBs inside, with an
externder board neatly stored in a spare slot so you can work on said
PCBs. Oh yes, the manual is worthy of the name, with full schematics,
waveforms, testpoints, etc.
That's a monitoe I'd put a lot of time into to keep it running. I'd spend
rather less on a generic PC monitor.
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