Vector Monitor HV power...

Over the holiday weekend I had plenty of time to play with "stuff"... One
of the things I did was finally force myself to understand how the HV
supply of monitors works (and to design a "new" one as a test).

I concentrated on the Wells Gardner HV design and the GO-7 power section
(since those two use the same CRT). It looks to me as though the only
reason we're using "custom" HV transformers at all is that they're
insistant on using the differential +-25V DC power supply. The only big
difference from most of the "raster" type power supplies (other than not
being triggered off the Horizontal retrace speed) is that the transformer
that triggers the Horizontal Output Transistor sits with one leg at chassis
ground on a Raster where as the Wells Gardner/Atari monitor designs hold
one leg at -DC rail. (I assume that having the +-DC supplies were just an
artifact of the deflection system, so rather than bring 120VAC out to the
HV cage they just used the DC supplies already available.)

It looks to me that the "special" windings of the vector HV transformers
really only increase the turns ratio to make up for the 50-60V peak to peak
output of the HOT instead of a ~120V P-P on a Raster. (So in theory, if
you replaced an amplifone HV unit with a GO-7 HV transformer-- wired up so
everything matches correctly-- you'd get about 50% of the HV you'd want. I
run my Wells Gardners at about 16KV-- I wonder if you couldn't just turn up
the "HV adjust" a bunch and get somewhere close to 16KV for a usable
display... :-)

Now that I "get it" it looks really easy to make a HV power supply for an
Amplifone or Wells Gardner Vector monitor. All you do is take 120VAC and
run it through the DC rectification front end (like a GO-7) that gives you
about 120DC. Instead of all the horizontal retrace stuff and whatnot you
trigger a couple transistors off of a 555-timer circuit like the Wells
Gardner uses. The transistors kick a little trigger transformer (the Wells
Gardner/Amplifone used step-down transformers to pick up more current) that
triggers the HOT which applies the potential of the +120VDC rail to the HV
transformer. Screen and grid voltages just come off a resistor divider
network. That's all there is to it.

Basically you could make a replacement HV supply using parts from a GO-7
with no problems-- and you wouldn't need to worry about keeping the
deflection yoke and the rest of the "raster" crap attached to the chassis.
I bet the whole thing would cost less than half of a new Amplifone
transformer...

-Clay

Clayton N. Cowgill Engineering Manager
_______________________________________________________________________
/\ Diamond Multimedia Systems, Inc. clay@supra.com
\/ Communications Division http://www.supra.com/
Received on Mon Dec 1 16:27:33 1997