14
STAR*TECH JOURNAL/JULY 1982
MIDWAY'S
COLOR TV MONITOR REPAIR (PART 2)
We continue our three-part article with Theory of Operation.
Part 1 appeared in our June issue.
We will conclude next month.
Part 1 - Introduction and Symptom Diagnosis
Part 2 - Theory of Operation
Part 3 - Troubleshooting and Differences Between Monitors
THEORY OF OPERATION
To understand what goes on inside the monitor, large general
groups of circuits will be examined instead of laboriously
analyzing the branches and small circuits that make up these
groups. This will help avoid confusion and aid in a basic,
concrete knowledge of what makes up a monitor.
THE POWER SUPPLY
The AC going to the monitor from the game transformer is just
like the voltage and current from your wall outlet. It jumps up
and down going positive and negative sixty times a second. But a
monitor needs nice, smooth DC; direct current, not alternating.
So diodes chop up the AC and a big electrolytic capacitor filters
it out to make it even smoother. Since the monitor is a big piece of
electronic equipment, with many circuits demanding a lot of
power from the power supply, there are also zener diodes and
transistors to help maintain a nice, constant, smooth voltage so
that the monitor circuits don't jump around. And this is what
happens when you see a wavy picture. There is AC creeping
through the power supply, so it must be malfunctioning. If the
voltage from the power supply is too low, the other circuits will
be starved for power and you may see a small, wavy picture, or
none at all.
Some circuits receive voltages that afe higher than what the
power supply should put out. But they come from the flyback
transformer which will be discussed later.
THE INTERFACE SECTION OF THE CHASSIS
The interface section of the chassis is fairly easy to identify. It is
right by the place where the video jack( s) from the logic board( s)
plug into. There are sets of transistors that receive the separate
red, green, blue, and sync informatiori from the cables that come
from the logic boards. The circuits jack up the voltage and match
impedances, or in other words, prepare the logic board outputs
for the circuits that will really amplify them for the output
devices such as the yoke in the case of the sync, or the picture
tube that shows the colors.
An interesting aside is that our sync is composite negative
sync. That means two things:
L The sync is a negative going wave form.
2. There are two pulses going at different speeds over the
same wire:
a. Vertical wave forms at 60 times per second ( or Hertz) and
b. Horizontal wave forms at about 15,750 times per
second (Hz).
The sync is amplified by a sync amplifier transistor and sent
on its way to the oscillators. The sync or timing information will
be explained along• with the oscillator shortly.
The color information is sent via wires to the neck board
where the main amplification occurs. This will also be discussed
later.
VERTICAL AND HORIZONTAL DEFLECTION
After the sync signal is amplified by the sync amp, it goes to two
different sections, the vertical and horizontal circuits. Basically,
the sync signals are for timing so ·the picture doesn't mess up
since it is assembled like an orderly jigsaw puzzle, but so fast that
you can't see the electron beams for each color painting the
picture on the screen. This will all become clear soon. For now,
we will follow the 60 cycle component of the sync as it goes on its
journey to the deflection yoke.
The 60 cycle pulse goes to the vertical oscillator to make sure
this circuit goes back and forth ( or oscillates) at 60 times a
second. Without this pulse keeping the circuit at the correct
speed, it may get lazy and oscillate at 58 cycles or lower, or get
ambitious and oscillate at 62 cycles or higher. At the wrong
speed, the picture will start to roll up or down.
A Wells-Gardner 13" or 19" color monitor uses transistors
for its sync section. An Electrohome 13" or 19" color monitor
uses an integrated circuit IC501 for its sync section. The idea is
all the same. The output to the vertical amplifying transistors for
all monitors must be a sawtooth wave form, sort oflike a bunch of
pyramids, racing to the yoke's vertical coils at 60 times a second.
Along the way to the output transistors, the 60 cycle pulse is
shaped and amplified to do the job: the yoke magnetically pushes
the electron beam to fill the screen out sideways looking at the
screen with the greatest length going up and down. Or viewing
the screen sitting like a home television set. The amplified
vertical output fills the screen up and down. Watching a monitor
like this, seeing only a horizontal line means a problem with the
vertical coils of the yoke or anything from the vertical output
section on back to the oscillator.
The horizontal section is very similar with a few exceptions.
The horizontal wave shape is more like a square and has a
frequency of 15,750 cycles a second. Again, Wells-Gardner
uses transistors for the horizontal oscillator, and Electrohome
uses the other side ofIC501. Still, the effect is the same. If the
oscillator isn't going at the correct speed, the picture may move
sideways, start to slant, or tear up with slanted thin figures. With
both the vertical and horizontal of all monitors, there are variable
resistors that change the speed of the oscillators up and down.
This way you have controls that can make the correct frequencies
to keep the electronic jigsaw puzzle nicely locked in place. If
you're driving in a car and next to you someone else is driving
their car at exactly the same speed, it will appear that they are not
moving. And this is why the sync frequency and the oscillators'
frequencies must match, so the picture doesn't appear to move.
The correct wave form is shaped and amplified in the
circuitry just like in the vertical section. But the horizontal output
transistor is a large power transistor and not only serves to give
current to the horizontal yoke windings, it also feeds the flyback
transformer.
THE FLYBACK TRANSFORMER (OR HIGH
VOLTAGE UNIT)
The picture tube needs high voltage to light up, and the power
supply can't meet this demand. The fly back transformer receives
current alternating at about 15,750 times per second from the
horizontal output transistor. The "flyback" jacks up its input
voltage and puts out a higher voltage alternating at the same
speed. But, in your "flyback" there are diodes that chop up the
alternating voltage to make it a smooth DC output just like in the
power supply. This is what goes through that thick red wire to
yourpicturetube. THIS AREA HAS ABOUT 18,000VOLTS
ON IT AND IT CAN KILL YOU!!
The " flyback" may be dangerous, but it is also generous. It
has extra output windings which give voltage to the heater pins of
the picture tube, voltage for the vertical deflection circuits, and
picture tube screen-grid voltage. So in a way, the high voltage
"flyback" is like a second power supply.