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StarTech Journal 
December 1983
Volume 5
Number 10
Pages 14 and 15
=========================================
WELLS GARDNER X-Y MONITOR/FINAL SOLUTION

By Wayne McGuire, Brady Dist, Charlotte, NC

We at Brady Distributing Co. pride ourselves on the service we have been
able to offer our customers. The following modifications are a product of
our shop personnel and, in our estimation, a final solution to the Wells
Gardner X-Y monitor problems.

Over the past three months, we have modified every Well Gardner X-Y monitor
to cross our bench and the rate of repeated failure is such that only one
has returned with a deflection problem. These changes make the 19K6901and
19K6103 very reliable monitors and much better suited for the enviroments in
which they are operated.


BRADY MODIFICATIONS FOR WELLS GARDNER QUADRASCAN COLOR X-Y


1 - INPUT PROTECTION CIRCUIT PCB ASSEMBLY

To ensure continued curcuit operation, Atari Feild Service suggests that you
perform thefollowing modifications.

A. Remove the lead connecting the IPC board to +27 volts on the display
printed cirduit board. The lead is near Q1 and Q3 (see Fig 1.1).

B. Replace this lead with a 1k ohm, 1/4 watt resistor (see Schematic of Fig 1.1)


2 - POWER SUPPLY CIRCUIT

The Low Voltage Power Supply is to be bypassed in this modification. The
input stages of the deflection circuits used in this monitor are of the
differential type and exhibit a high common mode rejection. This
characteristic will filter out any noise or ripple of the power supply.

A. Cut diodes D104 and D105 out of the power supply curcuit ( Schematic 2.1)

B. Make a Rat Tail splice connecting the leads at pins 1 and 4; also, splice
the leads at pins 5 and 7.

C. Cut the remaining wires flush with the plug (leads at pins 3 and 6)

NOTE: This modification will require readjustment of the monitor color,
focus and signal level from the logic board. This is due to the higher
supply voltage now available at the deflection and color stages. this
modification will remove about 45 watts of power from the bottom plate of
the monitor chassis and cause a noticable reduction in temperature.


3 - X CHANNEL CURRENT LIMITER

The X channel of this monitor must do more work than the Y channel. This
modification limits the 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.

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)

D. Place two SK3081 diodes in parallel with the base-emitter curicuits
(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 curcuit will
start to turn on. This action will drain current away from the base curcuit
and prevent transistors Q705 and Q706 from passing too much current.

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 220MF 50V capacitors.

B. Replace transistors Q901 and Q902 with transistor type 2N2102. 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 50MF 200V capacitor.

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 these modifications. The
vents will allow convection currents to cool the high voltage section
reducing the thermal stress to these curcuits. Also, if the power supply
modifications are performed, this entire modification becomes mandatory.


 


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