Star Tech Journal

S*TJ MARCH 1982 19
hitting it without moving. Transistors Q500
through Q502 and their circuitry affect the
voltages on Q503 to tum the beam current off.
This DOES NOT mean you have automatic
protection against CRT bums from too much
brightness. In fact, it would probably be a good
idea to keep the brightness and contrast controls
TURNED DOWN to the point where the game
looks good but not too bright. If the picture is way
too bright, fine spider web-like retrace lines will
follow the figures wherever they move and you
are headed for a burnt CRT. The brightness
control affects the DC voltage between the
cathode and G 1 of the picture tube. The contrast
control varies the amount of signal to the cathode.
Both control picture intensity.
The High Voltage Generator - or
- EHT Supply
On the side of your monitor is a box-like cage
with a wire that goes to the CRT. This is the EHT
supply. It performs several functions, one of
which is to supply the high voltage for the CRT.
The input to the EHT supply is at pin eight of
P900 where 40 volts AC are fed through a large
resistor, R900. Actually, this is a VERY impor-
tant resistor because it limits the current to the
oscillator, keeping it from taking off on its own
and increasing the high voltage to the point where
X-rays are emitted from the CRT, which is
DEFINITELY NOT GOOD.
Did we mention an oscillator? What's an
oscillator? Well, in this case, it is made up of:
transistor Q903, the primary winding of the
"flyback" transformer, and a few other com-
ponents that toss the voltage back and forth
(oscillate) 25,000 times each second. By doing
this, it electromagnetically induces a bigger
voltage in the "flyback" transformer's secondary
winding since it is bigger. This voltage is rectified
( chopped up) by diode D904 to get 12,000 volts
DC in Electrohome monitors and 14,500 volts
DC in Wells Gardner monitors. This voltage is
used to light up the CRT (picture tube). The
other transistors, from Q900 to Q902 and their
circuit components keep the power to the oscil-
lator steady or regulated, as they say in
engineering. There is an adjustment control,
R905 , to make certain the oscillator is fed the
proper power.
The "flyback" transformer also has an addi-
tional secondary winding which generates more
voltage to power other circuits. At pin three of
P900 there is about 400 volts DC for focus
voltage to the CRT. This can be adjusted with
R909, the focus control. From pin five at the
other side of the "flyback" transformer secondary
winding, there is 90 volts DC for the "Z"
amplifier circuit. In between pins three and five
of P900 there are two diodes and capacitors that
change the AC from the "flyback" secondary
winding to DC just like the power supply. In fact,
that's just what it is, a "mini" power supply.
The CRT - (Picture Tube)
The CRT has already been described indirectly.
However, to make a picture or tum the CRT on,
certain voltages are needed. Otherwise, it won't
work. These are: about 6 volts AC (note that's
AC) is needed for the heater filament in the tube
neck to light up; the electron beam's intensity
must be controlled by the "Z" amplifier's signal,
which is applied to the CRT's cathode; there
must be voltage at GI of the CRT for brightness;
there should be about 400 volts DC at G 2; there
should be focus voltage, which varies, but can go
as high as 400 volts DC; and there should be high
voltage at the anode of the CRT which runs into
the thousands ofvolts (th la voltage can Jump
almost one Inch - ao BE CAREFUL!!)
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Always remember that a monitor can bite
like a snake. Even when it is turned off, capacitors
hold voltage and will discharge it to you should
you be touching chassis ground. The CRT or
picture tube itself is a giant capacitor, so avoid
the flyback anode plug hole. With the monitor
on, the power supply circuit and/or the flyback,
which puts out at least 12,000 volts, CAN BE
KILLERS!! Avoid handling power transistors
(usually output transistors), yoke terminals, and
other high power components when the monitor
is on.
WARNING: That Picture Tube
Is a Bombl
When it breaks, first it implodes, then it explodes.
Large pieces of glass have been known to fly in
excess of 20 feet in all directions. DO NOT carry
it by the long, thin neck. Discharge its voltage to
ground by shorting the anode hole to ground. Use
a plastic handled screwdriver, connect one end of
a wire with an alligator clip at each end to chassis
ground and the other end to the metal shaft of the
screwdriver. Using -ONE HAND ONLY (put
the other in your pocket) and touching ONLY
the plastic handle of the screwdriver (DO NOT
TOUCH THE METAL SHAFT) stick the blade of
the screwdriver into the anode hole. Be prepared
for a fairly loud pop and a flash. The longer the
monitor has been turned off, the smaller the pop
and dimmer the flash. But BE CAREFUL, picture
tubes will liold a very healthy charge fo~at least a
week if not longer. Even after you've discharged
it once, it may still carry a residual charge. It's
better to be too careful than dead, which is why
electronic equipment always carries stickers
referring servicing to qualified personnel. Handle
the side with the viewing screen against your
chest when changing it. ALWAYS wear safety
goggles when handling the picture tube.
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