VT100 flyback / geometry
ard at p850ug1.demon.co.uk
Mon Oct 8 16:58:29 CDT 2007
> > Every VT100 I've worked on -- in fact every monitor of any type that I've
> > worked on, has been electrostatically-focused. There's a preset on the
> > video board that adjusts the votlage on one of the CRT pins (I forget if
> > it's pin 6 or pin 7). The 2 rings on the back of the yoke are both for
> > picture positioning.
> Yes and no. They are magnetically focused CRTs as the model number
> implies; it's an MW type (Mxx-yyW) made by Philips, an
> electrostatically focused CRT would be an AW-type. And there are four
Not according to my 1988 Philips CRT Databooks!
The coding is explained as follows :
D : Oscilloscope tube, single trace
E : Oscilloscopt tube, mulitple trace
F : Radar display tube, direct view
L : Storage display tube
M : TV Display tube for professional application, direct view
P : DIsplay tube for professional applicaiton, projection
Q : Flying spot scanner tube
First number : Face diagonal (cm)
Second number : Design number
Letters : Screen Phosphor (W = White, P4)
There are many Mxxxxxx CRTsa in the books, _all_ of them are
electorstatically focussed (one of the electron gun electrodes is
described as the focus electrode).
I've not seen a mgnetically-focuessed CRT since I repaired an _old_ Bush
TV. Old meaning System A, 405 lines. The focussing in that was controlled
by a pair of ring magnets positioned arround the CRT neck, there was a
mechanial arrangemetn to move them closer or further apart, thus varying
the strngth of the magnetic field.
> rings on the neck arranged in two pairs. One pair (the one near the
> deflection coils) controls the centering, and the other pair (towards the
> beam system near the socket) controls the focus. But additionally the
To do magneitc focussing, you meed a pretty strong magnetic field running
along the axis of the CRT. I doubt you'd get that from a couple of
> focus is fine tuned with a potentiometer on the video board. BWT the focus
> and centering rings are described in the VT100 technical manual.
> I think that most high quality CRT systems like monitors are magnetically
> focused; I've seen such CRTs several times in video monitors and
I;ve never seen one. OK, most of my high-end monitors are colour, and you
don't try magnetic focussing on a colour CRT (there's enough prolems
getting the convergence and purity right!), but nonoe of the monochrome
monitor service manuals I've read mention magnetic focussing. I've never
seen a yoke with anything other than centering rings on it.
other than the VT100, can you give me a definite pointer to a
terminal/monitor service manual that uses magneitc focussing?
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