S*T J
APRIL 1981
8
********* HVL CIRCUITS*********
Atari "Missile Command" Color Monitors
by Bernie Barranger
Government regulations require that when the high
voltage rises to a point where soft x-rays may be
generated by the CRT, the high voltage must be
automatically turned off. Monitor manufacturers comply
by incorporating a High Voltage Limiter (HVL) circuit.
When the circuit is activated, the horizontal output
stage is disabled. The HVL will remain enabled until the
AC power input is removed.
Diagnosing a monitor with an activated HVL can be
misleading. B+ will rise to a value nearly equal to the
input of the B+ regulator. This may lead one to believe
that the problem is a defective B+ power supply. Not
necessarily so. An insufficient amount of current is
being drained by the inoperative monitor for the B+
power supply to maintain its regulation. B+ will remain
high until the horizontal output stage is operating.
To determine the problem, the first step is to remove
the HVL trigger component (Fig. 1). This will be either a
zener diode or a transistor. Now if the picture returns
and B+ is still high, the problem is the power supply. If
B+ returns to normal, the HVL circuitry is defective.
A no picture situation after the HVL trigger removal
indicates trouble in the horizontal circuits. The oscillator,
driver and output stages should be checked.
Figure 3 lists the possible defective components for
each circuit and monitor.
Note: Do not defeat the HVL circuit. Its purpose is to
protect your paying customers from prolonged exposure
to soft x-rays should the high voltage increase.
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"MISSILE COMMAND" COLOR MONITORS FLOW CHART FOR BlACK SCREEN
VIDEO
/
OR
LOGIC PCB '
PROBLEM
NO PICTURE
FUSES OK
OK
G)
MEASURE B+
HIGH
,v
PICTURE
RETURNS
REMOVE
HVL
TRIGGER
NO
PICTURE
0
CD
MEASURE
B+
J, HIGH
D EFECTIVE B+
REGULATOR
CIRCUIT
w
0
NORMAL
HORIZ. OSC.
WORKING?
,v
OSC.
CIRCUIT
INOP.
NO,
DRIVER
CIRCUIT
INOP.
YES
DEFECTIVE
HVL
COMPONENT
\/
HORIZ.
DRIVER
OPERATIVE?
YES
\II
Fl11re 1 Refer to Figure 2 for circled numbers.
NO ...
,
0
HORIZONTAL
OUTPUT
INOP.
/