STAR*TECH JOURNAL/FEBRUARY 1984
24
Troubleshooting E/ectrohome & Wells Gardner Monitors (Part 3) continued from page 19.
A low B plus supply will cause the high
voltage and all other voltages to be low. With
low high voltage, the picture may bloom or be
out of focus. Before jumping into a specific
circuit, make sure the monitor's B plus is in
tolerance.
The horizontal output transistor operates
the sweep of the right side of the screen,
whereas the damper controls the left. A problem
which is only on one side of the screen will
cause the component associated with that side
to be suspect.
The horizontal output transistor in the
Wells Gardner monitor can be checked out of
circuit with an ohmmeter. Use the back-to-
back ratio method.
Electrohome's output transistor(X04) is a
special NPN package. It has a damper diode
built inside of the case. To check, the emitter
collector junction should read like a diode. The
last junction is the base to emitter. Here, back-
to-back, it should measure between forty and
fifty ohms both ways.
High Voltage Section
The high voltage at the picture tube's second
anode plug serves two functions. First, the
voltage potential at the second anode attracts
the electrons to the front of the CRT. This is
needed to light up the screen. The second
purpose is to allow the necessary current return
path to ground.
Normal high voltage is between 22.5 KV
(kilovolts) and 25.5 KV. To measure, a high
voltage probe is needed.
In case of a lower than normal high voltage,
the picture may show the symptom of blooming.
As the high voltage decreases, the entire picture
will enlarge in size. Sometimes the screen
expands to a point where the picture gets
dimmer and finally blanks out Usually, turning
the brightness control up makes the blooming
condition worse. Refer to the symptom diagnosis
in the General Troubleshooting section for
causes of blooming.
Grid Bias Voltages
As in examining the flyback circuit, other
voltages are needed for proper biasing of the
CRT. This would consist of the focus and
screen ( G 2) voltages.
The focus voltage is normally about twenty
percent ( 20%) of the high voltage. Whereas the
screen is between four and five hundred volts
DC. The control grid in both monitors is kept at
ground potential. The cathodes ( electron guns)
on Wells Gardner are biased at about one
hundred volts DC. Electrohome is approxi-
mately one hundred and sixty.
On the cathodes, as the DC voltage de-
creases, the screen begins to light up brighter.
An extremely bright screen which cannot be
darkened by the brightness control to a black
background may be caused by a low supply
voltage to the cathodes. A dominant single
color on the screen can be from a shorted
output transistor or a shorted cathode inside
the tube.
Finally, the last element inside the tube
receiving voltage is the heater or filament. Its
purpose is to boil off the electrons from the
cathodes. There are three heaters ( one for each
color). Remember, they're connected in parallel
with each other.
An external heater voltage is taken from a
winding of the flyback transformer and is
approximately six point three volts AC (6.3).
The heater voltage can be measured at pins
nine and ten of the CRT socket located on the
neckboard.
In order to get the necessary voltages from
the fly back transformer, the horizontal drive
signal must be present at its primary winding.
The horizontal output transistor provides the
AC drive signal which is about nine hundred
volts peak to peak.
An open circuit in the horizontal section,
between the horizontal oscillator and the flyback,
will cause no drive and thus no voltages from
off the flyback. A problem here would result in
no filament or high voltage and the B+ measures
too high.
The Electrohome's flyback circuit also
provides other voltages to specific stages in the
monitor.
On the schematic, look to the right of the
flyback transformer (T502). A plus twelve
volts is taken off the diode (D508). This twelve
volts powers up the vertical side of the chip, IC
501. Before the supply voltage reaches pin six
of the IC, the twelve volts also branches off and
feeds the sync and color interface transistors.
The sync interface transistors correspond
to the schematic's components X301, X302,
X305, X306, whereas the color transistors are
labeled XlOl through X106 on the main
monitor PC board. The blanking and beam
limiter ( X303, X304) also requires this twelve
volts DC.
A reference point (RH) off of diode (D503)
supplies the neckboard with a hundred and
eight volts DC. This source goes to the color
video output transistors(XlOl, X102, X103).
The Wells Gardner monitor's flyback
generates both a thirty and fifteen volt DC
supply. The vertical section requires the two
supplies in order to operate. On the interface
PC board, only the fifteen volts supply is used
to the color and CRT cut-off transistors.
The neckboard receives a variety of DC
voltages. Fifteen volts to bias the base emitter
junctions of the color output transistors (TR401,
402, 403). A hundred and sixty volts to the
collectors of the output transistors. Eight
hundred and ninety for the screen bias ( G 2 ),
and about twenty percent of the high voltage is
provided to the focus grid.
In essence, the flyback is in one aspect a
secondary power supply. This leaves less strain
upon the main power supply. Remember that in
order to receive any of the output voltages, the
drive signal must be present at the input.
Troubleshooting
Troubleshooting the high voltage section is the
same concept as in checking the input and
outputs of a transformer. In none of the output
voltages are present, suspect an absence of the
horizontal drive signal at the input. This could
be a loss of drive beginning from the oscillator
and through the driver circuit, or a loss to the
horizontal output stage to the input of the
fly back. If the primary winding is open, no B +
will be reaching the horizontal output transistor.
With some output voltages present and a
few missing, check the components in common
with the circuit that is associated with the
missing voltage sources. Check for open
windings, poor solder connections, open
resistors and diodes.
Missing a certain voltage will be an open or
break in the winding or anywhere to its desig-
nated output. If the output (load) itself is
shorted, it may load down the source voltage
very low or to zero.
The high voltage and focus voltage is to be
measured with a high voltage probe. All other
DC voltages can be checked by a standard
VOM ( volt-ohm-multimeter).
*
CORRECTION TO PART 2 OF THIS
4·PART SERIES
Page 23 of our January '84 issue (Vol.
5, # 11) ... The third paragraph reads
" ... eight volts peak-to-peak .. . " This
should read " ... eighty volts peak-to-
peak ... " Please correct your Journal.
We regret this proofreading error. Ed.
"Star Wars" continued from page 13.
NOVRAM FAILURE
PROBLEM: In self test, the hardware error
screen displays "NON VOLATILE RAM
AT 1 E". This is an intermittent problem which
occurs most often when the game is cold or on
the initial power-up.
SOLUTION: The solution to the problem is to
replace the 1000-ohm resistor R9 on the Main
PCB with a 470-ohm resistor. You may alter-
natively piggyback a 1000-ohm resistor in
parallel to the existing 1000-ohm resistor. If
you have a game with a serial number higher
than those listed below, your game already has
this mod. However, this is a very limited
problem and you will probably never see it
even if your board does not have the mod.
Upright Serial Number UR 9751
Sitdown Serial Number SD 1880
VECTOR-GENERATOR PCB
SHAKY VIDEO
PROBLEM: On some games you may exper-
ience shaky video after a 15-minute warm up.
The video will first start to shake in the high
score screen. The words "PRINCESS LEIA'S
REBEL FORCE" will start to flutter at first
and will worsen to an up and down movement
of about an eighth of an inch. At worse
condition, the scores will also move back and
forth.
SOLUTION: The solution to this problem is
to replace the lOK resistor R83 on the vector-
generator PCB with a 20K resistor.
COLOR X-Y MONITOR
ZERO OHM RESISTOR JUMPERS
The BROWN zero ohm jumpers used on the
deflection and high voltage boards are bad and
should be replaced with a piece of wire when
servicing either of those boards. The WHITE
and TAN jumpers are good and don't have to
be replaced. Zero ohm resistors look like
resistors but are designated on the board with a
silkscreened "W" followed by a number. A
common problem with the brown jumpers is
WI or W2 on the deflection board opening up.