26
STAR*TECH JOURNAL/DECEMBER 1983
Troubleshooting Electrohome and Wells Gardner Monitors (Part 1) continued from page 25.
[REAR VIEW!
If both the filament and high voltage are missing from the picture tube
(also known as the CRT, CATHODE RAY TUBE), then measure the
monitor's B+ voltage. The test point would be at the emitter of the power
regulator transistor. Electrohome's transistor is labeled as X04 on the
schematic and on Wells Gardner the part is marked as TR502. Each of
these transistors is styled in a T03 package and normally has a part number
of 2SC l 106 on its case.
The power regulator transistor is located on a metal bracket to allow heat
transfer off the device. This keeps the temperature in a range that will
prevent overheating and de_stroying the transistor. There is also one other
power transistor on the monitor. This is the horizontal output. It is mounted
on a bracket also. These two transistors are not interchangeable with each
other.
Looking from the back of the monitor onto the main PC board,find the
AC line cord. The monitor's AC cord should be towards the back left side of
the board. The metal bracket on the left side of the main PC board holds the
power regulator transistor. The horizontal output transistor is located at the
extreme right side of the board.
Measure the emitter of the power regulator transistor. The B+ voltage
on Electrohome should measure 120vdc, Wells Gardner 127vdc.
A monitor with no raster, no filament or high voltage will have a B+
voltage of four different possibilities. The voltage will either be normal, zero,
higher than normal, or too low.
1. LOW B+ VOLTAGE
Low B+ voltage usually shows up as a different condition than no filament
or high voltage. Refer to the power supply ~ection where this problem is
handled. Look in the table of contents under B+ voltage low.
2. NOB+ VOLTAGE
In case of the B+ being zero, halve the problem by seeing whether the
monitor is receiving its l 15vac at the monitor's line cord.
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Well, let's take the first half- no AC voltage to the monitor. Check the line
fuse which is between the isolation transformer and the monitor. This fuse is
located in the bottom of the game cabinet Next, make sure the wiring from
transformer to the fuse and monitor is good. Verify the 115 vac to the input of
the isolation transformer and 115 vac is present on its secondary winding.
Missing 115 vac to input of the isolation transformer could mean no
power to the main power transformer. Loss of input to main transformer
would result in no coin or play of the game. A problem in the monitor allows
game play and sounds from logic, but no picture.
Check for bad wire with AC voltage to main transformer and loss of
voltage to primary of isolation. If the primary of the isolation has l 15vac
present, and the secondary is zero volts AC, there is an open transformer,
either in the primary or secondary winding.
AC voltage to the input with no B+ at the regulator transistor- suspect
the fuse or fuses in the monitor. The fuses are located on the monitor's main
PC board.
Electrohome has two fuses. The line fuse F902, which is rated at three
amps, and another fuse F90 l to the power regulator circuit which is rated at
one-and-a-quarter amps. The Wells Gardner monitor has only one fuse
marked F601 and rated at three amps. This is the line fuse.
With one of these fuses open in either unit, replace the fuse. But, before
powering up again, check the two power transistors; one may be shorte
the transistors are removed to test, when replacing make sure the clear mica
insulator is placed between the transistor and the metal bracket. Wh~n
checking a fuse use an ohmmeter. Sometimes the fuse looks good, but 1s
open.
The monitor receiving the AC voltage and with the fuses not open -
check for the DC voltage to the input of the regulator circuit. The voltage
should be near 145 to l 48vdc. This could be tested on the Wells Gardner at
the collector of the power regulator transistor, same as on Electrohome. The
case ofthis transistor is the collector. Usually with no B+, the DC voltage
here would be absent. If so, check for an open current-limiting resistor, foil
traces or poor solder connection.
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3. B+ VOLTAGE TOO HIGH
In case of no filament or high voltage, and the B+ measures higher than
normal, refer to HVL (protection circuit) section. When the B+ reaches
138.5vdc or more on Electrohomeor around 145vdcon Wells Gardner, the
high voltage limiter circuit is activated. This protection circuit being on
prevents the horizontal oscillator from operating. Since no signal is fed into
the flyback transformer, no filament or high voltage is obtained.
GaT - e _
Continued on next page.