Service manual
Wells-Garnder Color Vector Monitor Guide
Page 53 of 73
4 - HIGH VOLTAGE POWER SUPPLY
These changes to the high power supply should reduce incidence of failure due to high
temperatures and/or low voltage power supply failures.
A. Replace capacitors C901 and C902 with 220F 50V capacitors. [Original capacitors
are 100uF 50V.]
B. Replace transistors Q901 and Q902 with transistor type 2N2102. [Originals are
MPSA06.] Place a heat sink for a T0-39 package on Q901. Take care not to allow the two
transistors to touch.
C. Replace C905 with a 50uF 200V capacitor. [Original is a 33uF 160V.]
D. Solder an SK3081 diode across capacitors C910 and C905 with the polarity of the
diodes opposite that of the capacitors.
E. Cut vents into the aluminum cover of the high voltage unit (as illustrated in Figure
4.2).
NOTE: The last procedure is the most important of [the HV] modifications. The vents will allow
convection currents to cool the high voltage section reducing the thermal stress to these circuits.
Also, if the power supply modifications are performed, this entire modification becomes
mandatory.
[notes from Zonn:
The experiments I did on the high voltage were different than those described.
One thing I noticed about the high voltage sections is that transistor Q900 is dropping 19.6
volts! Based on a 2.1v drop across R901, Q900 is passing .538 amps, at 19.6 volts that's ~10
watts. (This could be a bit high since R900 supplies more than just Q900 with current.)
That was quite a bit, and since I did not see anything else that needed the full 25v, except
for T901, (It looks like they run the output HV transformer with ~30v), I installed a pre-
regulator on the +V side. Since they were using a 13v zener as a voltage reference, I chose a
15v regulator (7815 or equivalent).
This was a few years ago, but I remember it working pretty well. The heat dissipation on
Q900 went *way* down (everything ran nice and cool), and I do not remember any adverse
effects on the HV output (vectors were fine and nice and stable.). With a +15v pre-
regulator, Q901 would only be dropping 6.9v, or less than 4 watts. This also significantly
drops the current demands on Q901.
I do not currently do this since it was just "tacked on". I got really busy at that point and
never pursued it any further, so I never tested things like the Star Wars explosion, etc. --