Service manual

Wells-Garnder Color Vector Monitor Guide
Page 16 of 73
These transistors often go bad and here is a quick lesson on how to check a transistor with a
meter. Unplug the red plugs from the deflection board to isolate the transistor from the circuit.
From the bottom of the transistor, the configuration is:
_
/ \
base -> /o o\ <- emitter
| |
\ /<- the case is the collector
\_/
Pretty crude but you get the idea. Test the deflection transistors as described earlier, one by one,
by placing the multi-meter leads on the tops of the pins of the red connectors or on the transistors
themselves. Neither method requires the removal of the transistor from its socket so do not
bother. You do, however, need to disconnect the associated connector from the deflection board
prior to testing. A detailed discussion of transistor testing in included in Appendix F.
So what symptoms go with what? Well if only one of the deflection amplifying transistors goes
bad, then you will lose that corresponding half of the screen. For example, if Q705 went bad in a
Tempest game, you would be missing the top half of your screen. It will not actually be gone; it
will be collapsed into a line across a 0 axis of the screen. If more than one chassis transistor goes
or either of the power transistors (Q103 or Q102) goes you will get no picture because the spot
killer turns on (and the LED on the deflection board lights) and shuts down the electron beam.
Checking these chassis transistors is one of the first things you should do if parts of your screen
are missing or your spot killer is on.
When replacing these chassis transistors, make sure the socket is centered. Do not forget the little
rubber or clear mica (plastic) insulator that was stuck to the chassis or to the bad transistor you
just removed. You need to grease this insulator on both sides with a special grease called
Silicone Heat Sink Compound. The grease is not strictly necessary but helps transfer the heat
from the transistor to the chassis so the transistors last longer. The grease is relatively expensive,
so you may want to just buy new insulators (they should be about 10 cents each) because most
come "pre-greased".
Do not be nervous about remembering the orientation of the transistors; the pins are off centered
in order to make the connection somewhat idiot-proof (see earlier ASCII picture. They are
obviously "handed" but can be put in upside-down; it is a tight fit and hard to do but possible).
Also, after you replace any chassis transistors and before you power the game up, use your meter
to make sure none of the transistor cases are shorted to the chassis. A continuity test is perfect for
this check. Misalignment of the transistor is very common and will cause fuses to blow and will
probably destroy the transistor. If any part of the transistor is touching the chassis you are asking
for trouble.
If the chassis transistors are OK, most missing pictures are due to problems with the deflection
board (whereas most distorted pictures are due to bad capacitors in the HV unit).