Service manual

Wells-Garnder Color Vector Monitor Guide
Page 52 of 73
3 - X CHANNEL CURRENT LIMITER
The X channel of this monitor must do more work than the Y channel. This modification limits
the amount of current that can be delivered to the load and the X channel output transistors are
moved so they may be more efficiently cooled by convection currents.
A. Remove the plug from J700 and reconnect at J600. The cable of P700 will need to be
lengthened before this can be done. This move connects the transistors on the bottom
chassis plate to the Y channel.
B. Transistors mounted on the side of the chassis panel may now be connected to J700.
These are now the X channel outputs transistors. The side panel allows more air currents
to pass over the transistors and consequently more efficient cooling. [If this monitor is
mounted vertically in a Tempest, the Y channel output transistors are already oriented for
optimum convection cooling and do not need to be moved.]
[notes from Zonn:
* Moving the transistors seems more like superstition to me. Aluminum is a better
conductor of heat than steel, so it could be argued that the X drivers should remain
where they are since the aluminum can dissipate heat faster than the steel frame. If
things need cooling, install a fan.]
C. Remove the emitter lead from the transistor sockets on the side and place a 0.2-ohm,
3-watt resistor in series with emitters of Q705 and Q706 (as in Schematic 3.1). [I believe
they mean the transistors at physical location Q605 and Q606. If you have just switched
the X and Y Channels, it would make little sense to make the modifications to the
transistors driving the Y Channel.]
D. Place two SK3081 diodes in parallel with the base-emitter circuits (Schematic 3.1).
NOTE: When the currents through the emitter resistors reach 3 amps the voltage drop is such
that the diodes across the base-emitter circuit will start to turn on. This action will drain current
away from the base circuit and prevent transistors Q705 and Q706 from passing too much
current.
[notes from Zonn:
* Limiting the output current on the output transistor may not be a bad idea, if it does
not slow down drawing speeds enough to affect the display. Though it seems silly just to
do the X transistors. During high-speed moves, both X and Y channels will have to pass
the same amount of current. While it's true that the X channel is larger than the Y, and
therefore, on the average, will have Longer moves, and therefore possibly get hotter, the
instantaneous current peaks on both are the same. Since it's most likely these peaks that
would blow the transistors, you'd think both X and Y would be protected.]