1 Dec
2013
1 Dec
'13
1:18 p.m.
Been a while since I worked on CRTs on my video games... but IIRC if
the HV drops the picture will bloom as the yoke can deflect the beam
further before the electrons strike the face. If your picture is sharp
but dim, my first guess would not be an HV problem. When I was working
on my arcade games, I picked up for a pretty good price a voltage probe
for CRTs.
Have you measured the filament voltage to the CRT ? not sure what
VT220s would be, but I IIRC on arcade stuff is is I think 6.3V.
Are you able to easily swap a tube to see that the dim problem follows
the electronics and not the tube ?
-- Curt
On 2013-12-01 15:26, Chris Elmquist wrote:
I recently "saved" a small pile of VT220 and
VT100 terminals from a
barn in western WI. They were grubby and have various issues but I
am
enjoying cleaning them up and making a few good ones from some bad
ones.
One of the VT220 is a very new vintage unit. Circa-1987, with some
components with date codes 1990. It has a totally different logic
board than earlier models I've worked on-- this one having a large
PLCC
package in the center of the board which I assume is a custom device
integrating the video controller and much of the support logic since
there are considerably fewer parts on this board than the older ones.
Still an 8031 CPU though.
This unit also has a totally different video monitor circuit than the
older ones and my problem lies somewhere on that board-- I am
getting
a very dim image even though the CRT itself appears to be in very
good
shape with no burn-in and otherwise crisp display. I think I've got
a
high-voltage problem but without schematics to match this board, it's
kind of a crap shoot to start debugging.
So, wondering if anyone has newer schematics or technical reference
for
the VT220 than we find out on bitsavers? The bitsavers docs are
circa-
1983 so I am looking for something 1987 or later when it appears they
did a considerable redesign of the guts of this terminal.
Chris