8 Oct
2007
8 Oct
'07
2:21 a.m.
On Sun, 7 Oct 2007, Tony Duell wrote:
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
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
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.
Christian
Are the sloping sides of the trapezium horizontal or
vertical?
They are vertical, i.e. on the left and right side. The width gets
wider from top to bottom.
Trapezium problems can be caused by the deflection
yoke, of course
(shorted turns or other prolems in one coil of the pair), but before
worrying about that, I'd make sure none of the supply lines on the video
board have a sigificant ripple at one of the scan frequencies.
I'll check the power supply voltages. This VT100 is the replacement for
the other one with the (as I think) defective flyback.
The only
adjustable magnets are the two sets of rings on the neck, one for
the position and one for the focus (it's a magnetically focused CRT as
most monitors are).
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.