VT100 flyback / geometry

[Tony Duell]

> > 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 : 

<letter><number>-<number><letter(s)>

First letter
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 
magnetic rings.

> 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 
> terminals.

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?

-tony