Star Tech Journal

25
STAR*TECH JOURNAL/DECEMBER 1983
BALLY MIDWAY
TROUBLESHOOTING ELECTROHOME & WELLS GARDNER MONITORS (PART 1)
We begin part 1 of this four-part series with the power supply section of the
Electrohome and Wells Gardner monitors. Parts 2 through 4 will appear
in our January, February and March '84 issues consecutively.
POWER SUPPLY SECTION
Power Supply and Degaussing Circuit
Both Electrohome and Wells Gardner monitors use a power supply with a
regulator circuit. Wells Gardner has a half-wave line power supply, whereas
Electrohome has a full-wave bridge. Full wave means that both halves of the
AC input wave (positive and negative) are used
The DC voltage on both monitors leaving the power supply and arriving
to the input of the regulator circuit is about one hundred forty /fifty volts DC.
This could be easily checked on the monitors. On Wells Gardner, the test
point would be on the collector of the power regulator transistor TR502.
Electrohome also measures the collector of the power transistor X04,
2SC1106. The collector of both transistors is the case.
Fuse F901 should be checked on Electrohome if no voltage is present.
Its value is one and a quarter amp. The B+ adjust controls (VR501 on Wells
Gardner and R909 on Electrohome) has been preset at the factory and
should not necessarily be altered
The test point for the monitors B+ is at the emitter of the power regulator
transistor. Wells Gardner should measure one hundred twenty-seven VDC,
and Electrohome will check normally at one hundred twenty VDC. A B+
voltage too high or low or none at all indicates a problem somewhere other
than the adjustment of the B+ pot.
The regulator circuit uses a series or "pass" transistor, which acts as a
variable resistor in series with the load The pass transistor's base is
controlled by other components in the regulator circuit.
On Electrohome X902, the error amp samples the B+ line (B 1 ). As B 1
begins to increase, the increased voltage at the base of X902 also begins to
increase the forward bias of X902. The collector of X902, forward bias
X901, and the base current of X04 will all decrease, maintaining the B+
voltage around one hundred twenty VDC.
Going in the other direction with B1 decreasing, decreases the base
voltage of X902. X902 forward bias is less; therefore, the collector of
X902, the forward bias of X901 and the base current of X04 increases as
needed. The Wells Gardner monitor operates in the same manner.
Let's look at the Audio/Power board on Wells Gardner. The transistor
TR551 is not used. This device is to be used in conjunction with an audio
circuit. Individuals can get this easily mistaken as part of the regulator
circuit.
Resistors R501 on Wells Gardner and Electrohome RO 1 shunts some of
the output current around the pass transistor. This provides less power
(heat) to be dissipated in the device.
Wells Gardner monitor(model K4600 series) is equipped with a three-
amp line fuse, F601. Electrohome has a three-amp line fuse (F902), and an
amp-and-a-quarter fuse (F901 ). Usually when one of these fuses is open,
possibly a power transistor may be defective (power regulator or horizontal
output transistor). Check and replace if needed
1. MONITOR DOES NOT POWER UP
First check and make sure the AC line voltage is present to monitor. If so,
look for open fuses on the monitor PC board Suspect also the transistors,
and current limiting resistors. Burnt foils or poor solder connection may be
found on the PC board. Keep in mind, in case of an open fuse, the power
regulator and horizontal output transistor may be shorted. This may save
time by not having to change the fuse a second time.
2. LOW B+ VOLTAGE
The monitor may not come on at all with a low enough B+. Other
indications of low B+ would be a smaller than normal picture. This is with
the sides, top and bottom of the picture not being completely filled out. There
may be somewhat of a picture, but is probably wavy.
It could be the picture is full and looks good, but is wavy or exhibits hum
bars.
The problem could be in the logic board's power supply. To detect, listen
for a noticeable hum in the speaker. If so, the game logic power supply is
probably at fault.
The problem being in the monitor would be in the power supply.
Suspected components are the transistors, diodes, and electrolytic capacitors.
3. HIGH B+ VOLTAGE
A high B+ could activate the monitor's protection circuit. When the circuit
is on, there is no load on the regulator. The F,+ will not regulate without a
load. The monitor is in a shut down condition, but remember, there is still
power going to its circuits, Refer to the HVL (protection circuit) in regard to
a high B+ problem.
The flyback transformer acts also as a secondary power_ supply. This
transformer receives a large alternating current from the honzontal output
circuit. The fly back then steps up the voltage through a number of windings
in the transformer. Diodes are used to get the needed DC voltage. This
leaves less strain upon the actual power supply.
The higher voltages go to the CRT' s grids ( screen and focus) and second
anode. The color transistors on the neckboard require about two hundred
volts from the flyback circuit. The vertical deflection circuit is powered up
also. CRT filament voltage is taken straight from a winding in the
transformer. The AC voltage to the filament is about 6.3 volts.
Finishing up the power supply is the degaussing circ 1 1it. This circuit is
connected to, but separate from, the power supply. The power supply is not
dependent upon the degaussing circuit in order to operate. The AC input
voltage is paralleled to the degaussing circuit input.
The degausser's purpose is to demagnetize the shadow mask in the CRT,
when powered up. The shadow mask is made of metal and has slotted holes
through it. The CRT's electron beam, when scanning the front of the tube,
passes through the slots of the mask. Each beam from its color gun strikes its
own color phosphor. If part of the shadow mask is magnetized, the beam,
instead of striking its own color phosphor, will strike one of the other two.
The monitor can be easily magnetized by turning the game's direction.
Another way could be from the earth's rotation or an electromagnetic device
interfering nearby. To correct, simply degauss the unit.
The degaussing circuit's output is connected to a thick coil and is
mounted around the bell of the picture tube. When the coil is activated, an
electromagnetic field is induced and neutralizes any magnetization developed
on the tube.
The degaussing circuit is automatic. When the monitor is first turned on,
the degaussing circuit is on for a few seconds. A thermistor is connected in
series between the AC input voltage and the degaussing circuit.Until the
thermistor heats up to a certain temperature, the coil is on. The device is
positive temperature.co-efficient. The resistance of the thermistor increases
with temperature.
Sometimes an automatic degausser is not strong or close enough to
counteract the magnetic field. In this case a manual degaussing is needed to
demagnetize the monitor. To degauss manually, use an external degaussing
coil.
No Raster (Lit Screen)
Under a no raster condition (meaning no lit screen), there are two major
factors to check in the monitor. Knowing whether either one, both or none of
the factors are present begins to isolate the problem. Check to see if the
monitor has the following:
1. Filament ( or heater) lit inside the neck of the picture tube. To check, look
through the glass neck of the tube located by the neckboard. The filament
when lit will light up as an orange glow.
2. High voltage to the second anode plug of the picture tube. To measure the
voltage, a high voltage probe would be needed. The voltage on both
monitors' second anode will be about twenty-five kilovolts (25,000
volts).
CF/ 1
· ~
11
FILAME"N'T
Continued on next page.