Star Tech Journal

14
STAR*TECH JOURNAL/FEBRUARY 1984
BALLY MIDWAY
TROUBLESHOOTING ELECTROHOME & WELLS GARDNER MONITORS (PART 3)
We continue our 4-part series with "Deflection
Circuitry". Part 1 covered the power supply
section. Part 2 covered "Lighting the Screen".
Part 4 will appear in next month's issue.
DEFLECTION CIRCUITRY
Sync Signals
The sync signals from the logic boards are
needed to synchronize the video information
from the logic boards to the monitor's vertical
and horizontal oscillator for correct picture
stability. The two signals are referred to as the
vertical and horizontal.
The vertical frequency is running at approx-
imately sixty Hertz ( cps, cycles per second).
While the horizontal is at a much raster rate
( about fifteen thousand seven hundred and fifty
cps 15,750).
At the point where the incoming vertical
sync signal meets the monitor's vertical oscil-
lator, the frequencies have to be at the same
rate in order to have a stable picture. Being that
either signal is faster or slower than the other
causes the picture to roll up or down.
A vertical hold control is provided on the
monitor to change the oscillator' s frequency if
needed to stop the picture from rolling. With
the picture still rolling, isolate whether the
problem is in the monitor or logic boards.
Here again, the horizontal incoming sync
signal needs to be at the same rate as the
oscillator. Ifnot, possible drifting of the picture
across the screen may occur. The picture may
be tearing or breaking up and causes the
display to go into diagonal lines.
The horizontal hold or frequency adjust-
ment on the monitor changes the frequency of
the oscillator for stability. Not being abJe ·to
correct the sync back to normal, isolate whether
the logic or monitor is at fault.
A raster scan monitor's vertical and hori-
zontal oscillators are free running, meaning
that no incoming sync signals are needed to the
oscillator in order for it to operate. An oscillator
circuit has a loop in which the output is
connected to the input. This is known as
feedback. An amplifier whose input is dependent
upon its output is called an oscillator.
The two connectors arriving from the logic
boards connect to the monitor's interface ( input
section) are a six and a three-pin connector. On
the six-pin connector, the first three pins contain
the color video information ( red, green, and
blue). Pin four is for ground.
Both monitors are capable of receiving
either positive or negative going sync. If the
sync signals from the logic are positive going,
then pin five would be used for the vertical
incoming sync signal and pin six for horizontal.
The Bally/ Midway games generate from
the logic boards a negative going sync. Inputs
to the monitor for the negative sync are on the
three-pin jack. The six-pin video connector and
the three-pin sync connector should plug in
right beside each other on the interface section
of the monitor.
On Electrohome, the incoming sync signals
are amplified and inverted through the monitor's
sync interface transistors. These transistors
correspond with the schematic's X301 X302
X305, and X306. Two of the four are ~sed fo;
the vertical (X302, X306), and two also for the
horizontal ( X301, X305 ). The logic's incoming
sync signals are then coupled from the transis-
tors to the sync IC, IC501. Inside the IC
package contains sections of the vertical and
horizontal oscillator circuit.
The Wells Gardner monitor uses a diode
network on the interface board to add together
the sync signals. The combined sync signal
then arrives to the horizontal/vertical board
where the signal is applied to the base of the
sync amp (TR308). Following the sync amp,
the signal goes to the vertical and horizontal
oscillators.
To separate the horizontal from the vertical
sync signal, a low and high pass filter is used. A
low pass filter before the vertical oscillator
blocks high frequency signals and passes the
low. The vertical sync being only about sixty
cps, the signal passes to the oscillator. Before
the horizontal oscillator circuit, a high pass
filter is used to pass the highs and rejects all low
frequencies.
Vertical Circuit
The vertical circuitry can be divided into four
groups. The vertical oscillator, driver, amplifier,
and output stages are needed to provide picture
stability and vertical deflection.
One purpose of the oscillator is for vertical
picture stability. The second is to produce the
proper waveform to drive the output section for
vertical defelction.
The oscillator is free running, meaning that
no input signals are needed to trigger the
oscillator on. This is why no signals from the
logic boards have to be connected to the
monitor in order to display a raster.
Frequency of the oscillator runs a little
lower than sixty cps. When the oscillator is at
the same frequency as the vertical sync signal,
the picture will be synchronized ( or stable).
Unstable vertical sync displayed on the
screen has the picture rolling upward or down-
ward. To correct, adjust the vertical hold
control located on the monitor. With the picture
still rolling, isolate whether the wiring, logic or
monitor is at fault.
The waveform produced to drive the output
stage is a sawtooth ( ramp waveform). Its rise
time should be linear. This allows the horizontal
scanning lines to be evenly spaced from the top
of the screen to the bottom.
A buffer is placed between the oscillator
and the output. This is the purpose of the
driver. Sometimes, the driver acts as another
stage of amplification. Next, the vertical amp
gives the signal a boost.
Controls in the vertical circuit consists of
the vertical hold, linearity, and height. The
Wells Gardner monitor vertical circuit omits
the linearity adjustment.
The hold control adjusts the oscillator's
frequency to that of the vertical sync from the
logic boards. This prevents the picture from
rolling up or down.
A linearity control, if used, makes the ramp
waveform linear. A non-linear picture causes
the characters or images on one side of the
screen to be larger or wider than those on the
other side.
The height or size adjustment changes the
amplitude of the vertical signal. To increase or
decrease the size of the picture from top to
bottom, this is the control to adjust.
Electrohome's IC 501
Let's briefly discuss the integrated circuit, IC
501. Inside the IC are many tiny circuits for the
horizontal and vertical section of the monitor.
The horizontal side of the IC is powered up
at pin eleven. Pin eleven is connected to a B
plus feed resistor (R509). The B plus is also
referred to as B 1 on this monitor.
As the horizontal oscillator begins to run,
the necessary drive signal is generated and sent
to the input of the flyback transformer. At this
time, the secondary windings are induced,
supplying other voltages to different areas of
the monitor.
The test point (B2) from off the flyback
delivers the twelve volts DC to pin six of IC
501. This voltage provides the VCC (supply
voltagP.) to the vertical half of the chip.
In the chip, the vertical oscillator and amp
make up one half of the chip. The other half of
the IC holds part of the horizontal circuit.
Components outside of the chip off of pin
seven, determine the frequency of the oscillator
- normally runs a tad lower than sixty cps. Off
of pin five, capacitor ( C402) affects the screen' s
linearity.
From pin five, the vertical waveform is sent
on to an amplifier for more amplification. The
output is pin two of the IC. Here should be a
linear ramp waveform about 1.4 volts peak to
peak. This waveform is sent on to drive the
vertical output transistors (X401, X402).
The vertical output section purpose is to
amplify the waveform and to drive the vertical
windings in the deflection yoke. A linear wave-
form is sent from pin two of IC 501 to the
output transistors (X401, X402). When one of
these transistors is on, the top half ( from top to
middle) of the screen is deflected. While that
transistor is off, the other is on controlling the
electron beams from the middle half of the
screen to the bottom.
A fusible resistor (FR401) passes power
to the output transistors (X401, X402). With
an open FR401 , loss of vertical deflection will
result. Here a thin white line will run down the
middle of the screen. The line will be going
across the longest distance of the CRT. If
FR401 is open, check the output transistors,
diode (D401) and possibly capacitor (C412).
With one of the output transistors bad, replace
the pair.
Other vertical collapse problems may be
caused from the wiring to the yoke. Possibly
the yoke itself. Capacitor ( C41 l) is in series
with the yoke and the side pin transformer.
These components could bring a loss of vertical
deflection.
Continued on page 19.