MCRII Power Supply Functional Description/Troubleshooting
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POWER SUPPLY BOARD - INTRODUCTION: This Power Supply Printed Circuit Board
(PCB) assembly has been designed in such a way that it can be used for all
games of the MCR II series and Vector Scan Systems. Although some games do
not draw the full amount of power these supplies are capable of generating,
there are others that do. So, the supply was designed to operate the biggest
power hog. This Power Supply PCB generates quite a few different voltages,
both regulated and unregulated, which are used to operate a number of
different types of componentry. The +5V and +12V supplies are high current
sources carefully regulated by a standard fold-back current-limited circuit
design which compensates for both line voltage and load fluctuations, and
prevents burn-out due to an overload or shorted circuitry. In addition to
the regulated supplies, two unregulated supplies are also incorporated to
power incandescent lamps and audio circuitry.

SENSE AND COM. LINES: One interesting feature of the power PCB is the use of
the SENSE and COM. LINES to detect any IR drop which might occur in the
ground line between the power PCB and the CPU PCB. IR drops across a ground
line can cause several problems, including the annoying hum bar which rolls
up the monitor screen infuriating operators and players alike. Essentially,
both the SENSE and COM. LINES are connected together just after they enter
the CPU PCB. Because the SENSE LINE is connected to the COM. LINE at this
point, it can be used to detect any IR drop which might occur between the
power PCB and the CPU POB. And, because the SENSE LINE is used as the COM.
reference for the entire power PCB, any IR drop which occurs across the COM.
LINES simply offsets the entire power supply system by that amount and
thereby eliminates any problematic conditions which might otherwise occur.
Commonly, only the SENSE LINE for the +5V is used, and the SENSE LINE for
the +12V is then connected to it with jumper JW2. In cases where more than
2 Amps is sourced by the +12V supply, it may be desirable that the +12V
supply use its own SENSE LINE, in which case jumper JW2 is deleted.

Jumpers JW1 and JW3 can be used to control hum in the audio. Their only
justified use is when there is a considerable distance between the Filter
Assembly and the Power Supply PCB. Another option to control this hum is by
connecting the SENSE LINE(S) to the filter assembly via pins 7 and/or 9 of
connector J3.

THE TRANSFORMERS: The Game Logic Transformer is unique in the video game
industry because it doesn't have taps to compensate for high or low
voltage. It is a constant-voItage transformer which adapts itself
automatically to line voltage variations and fluctuations. The output
voltages stay constant with line voltage variations from 100 to 125VAC
(MTOO-00089-AOOO) @ 60Hz, and from 200 to 250VAC (MT00-00090-A000) @ 50Hz.
This is accomplished with the aid of a separate resonance winding across
which an external 3.5Mfd capacitor is placed.

*** WARNING *** This winding has an output voltage varying from 800 to 1OOOV
peak-to-peak!

This transformer reduces the line voltage to two center tap voltages: 8VAC
and 15VAC, each having its own winding. This prevents high current draw on
one voltage from influencing the other voltage.

A second transformer is used for isolation between the color monitor and
line voltage. This line isolation also provides an AC voltage for the
unregulated power supply and the auxiliary circuits, when implemented.

FILTER ASSEMBLY: The 8VAC and 15VAC from the Constant Voltage Transformer
secondary windings are full-wave rectified by two power rectifiers each,
MR-1120, and then filtered by a 100,000Mfd capacitor for the pre-regulated
8VDC and by a 55,000Mfd capacitor for the pre-regulated 15VDC. Both circuits
are protected with a fuse: 10 Amp for the pre-regulated 8VDC and 6 Amp for
the pre-regulated 15VDC. Both capacitors are discharged through a
drainage resistor when the filter assembly is disconnected for any reason.
Both center taps are connected together in this Filter Assembly to ensure
that they are the same voltage level.

POWER CHASSIS: This is a combination of the transformers with the Filter
Assemblies in order to comply with U.L., C.S.A., and V.D.E. safety regula-
tions. It also incorporates the line filter, all line fuses, a safety switch,
and the line power distribution. The Power Chassis will become available in
the 3rd quarter of 1982.

5V SOURCE: The pre-regulated 8VDC from the Filter Assembly is placed at the
collector of the 2N3772 pass transistor and more or less of this voltage is
allowed through depending on load and other factors. The actual regulator in
this circuit is the LM305 which operates a 2N2905 amplifier. The LM305 senses
the output voltage across the 160 ohm resistor, uses this wave form to
control the base of the amplifier transistor which is necessary in this
circuit to provide enough current for the TIP 31 transistor, which in turn
provides the current to operate the pass transistor. If the LM305 senses a
drop in voltage, it turns on the amplifier transistor, which in turn
activates the TIP 31 and the pass transistor. When the pass transistor is
activated, it allows more voltage through to the output to compensate for the
drop in voltage. The circuit also senses the amount of current through the
0.16 ohm resistor If the current exceeds the safe limit determined by the
value of the resistor, the amplifier is shut off which turns off the TIP 31
and thus the pass transistor. This limits the current to a safe level.

In order to minimize the power dissipation of the pass transistor, its
pre-amplifiers and the LM305 are powered by the pre-regulated +15VDC.
They are protected by a separate on-board 3/8 Amp fuse because in case of
failure, the current draw would not be large enough to blow the 6 Amp fuse of
the pre-regulated +15VDC. The 10 ohm/5W resistor is used to drop the
voltage to prevent overheating of the pre-amplifiers and the LM305.

The SENSE LINE is used as the reference for the LM305 to compensate for any
IR drop in the COM. LINE as discussed earlier. Adjustment provisions
have been made by incorporating a voltage divider network with 2 resistors
and a 100 ohm trim pot. By adjusting the trim pot, the LM305 can be further
offset from the SENSE to compensate for any minor deviation. The resulting
voltage is further filtered with a 470Mfd capacitor. The ferrite bead is used
to prevent high frequency oscillation of the pass transistor, which would
result in its self-destruction, usually by shorting the collector to the
emitter. This would provide an unregulated 8VDC to the logic, destroying
the logic chips.

+12V SOURCE: This power source operates in a similar manner as the +5V
source, with a few minor differences. It has only one pre-amplifier, the
current limiting resistor is 0.18 ohms, and no power dissipation minimizing
technique is used, i.e. the LM305 and 2N2905 are powered by the same voltage
as the pass transistor.

UNREGULATED SUPPLIES: The Power Supply PCB also generates an unregulated
unfiltered auxiliary voltage which can be used to power indicator lights.
This voltage is developed by full-wave bridge rectification of the voltage
of the secondary winding of the line-isolation transformer. This
semi-sinusoidal wave form is then taken directly to the control circuitry
of the indicator lights. Although this source is not used in some games, it
is used in others where certain lights have to be controlled by the logic.

Furthermore, the above voltage is used to generate another voltage, the
audio voltage. This audio voltage is developed by the same initial process
used to create the unregulated unfiltered auxiliary voltage, however, it is
further filtered by the 47OOMfd capacitor before this undulating voltage is
sent to the Audio Amplifiers on the Game PCB.

NOTE: In some games the regulated +12V is used for audio.

ADDITIONAL FEATURES - (NOT USED IN ALL GAMES)

-5V SUPPLY: The 8V winding is again full-wave rectified, but since the
cathodes of the two 1 N4001 diodes are wired to the transformer secondary
winding, this wave form is negative with respect to COM. This negative wave
form is filtered by the 2000Mfd capacitor and placed at the input pin of the
7905 integrated voltage regulator. It can be further offset from -SENSE by
adjusting the 100 ohm trim pot to compensate for any minor deviation from the
specified -5V level. The resulting fully regulated voltage is further
filtered by a l0Mfd capacitor to prevent load fluctuations from disturbing
the operation of the regulator The 7905 has internal overload protection.

RESET LINE: This part of the Power Supply PCB provides a power-on-clear
signal when the game is first turned on OR after a momentary power line
failure. Since random information is loaded into many parts of the computer
when power is first applied, this signal is necessary to clear this
meaningless data away so the computer can start operating with a "clean
slate". The reset circuit is divided into 5 distinct sections, each with its
own function:

A.	The OPTO-ISOLATOR provides voltage isolation from the line isolation
transformer and is relatively insensitive to line voltage fluctuations and
secondary voltage selection. The transformer output voltage can be varied
from 9VAC to 14VAC. The output of the OPTO-ISOLATOR is a semi-square
wave of 5V. (See AC Synch. on "Wave Shapes of Auxiliary Circuits" Figure 3.)

B.	The PUMP CHARGER circuit detects if two or more AC cycles have
dropped out (AC line failure). Its output will go to logic "1" approximately
40 milliseconds after the line voltage starts to drop out. When the
line power is turned on initially, it acts as a power-on-reset circuit. Its
output will rise with the +5V and stay at logic "1" at least until the +5VDC
has stabilized, then it goes to logic "0". (See Pump Charge-out on "Wave
Shapes of Auxiliary Circuits" Figure 3.) The output pulse is transferred to
two circuits; the ONE-SHOT circuit and the LOGIC "OR" circuit.

C.	The ONE-SHOT circuit is triggered by the positive edge of the pulse
from the PUMP CHARGER and stretches it to at least 75 milliseconds. (See One-
Shot-out on "Wave Shapes of Auxiliary Circuits" Figure 3.) This ensures that
there will always be a proper reset pulse, regardless of the pulse length
from the PUMP CHARGER circuit. (It does not respond when the power line is
switched to "ON".) Its output pulse is transferred to the LOGIC "OR"
circuit.

D.	The LOGIC "OR" circuit operates just as its name implies. When it
receives a logic "1" signal from either the PUMP CHARGER and/or the ONE-
SHOT it will pass it through to the AMPLIFIER.

E.	The AMPLIFIER, consisting of the 2N4401 transistor, is used to
provide sufficient current at logic "0". Its output is RESET which is used
by the logic PCBs. (See RESET on "Wave Shapes of Auxiliary Circuits"
Figure 3.)

F.	A.C. SYNC for assemblies A082-90412-D000 / A082-90421-C000 and later
versions. This 555 circuitry performs three functions at the same
time:

1.	lt is a -2 frequency divider; the output of which is 50 or 60Hz,
depending on the Line frequency.
2.	It is a wave shaper. The output signal is squared and approximately
2ms long at Logic "0".
3.	It is a current driver. It can sync or source 50ma and can directly
interface with TTL.

BATTERY SUPPLY: This supply is used to provide current to a "Bookkeeping"
RAM on the logic PCB when the line power fails or is turned off. Normally,
with the line power "ON", 15 VDC is supplied via the 0.22 microHenry
inductor; the 82 ohm voltage dropping resistor, and a blocking diode. The
voltage is regulated with a diode going to +5VDC. The Nickel-Cadmium
rechargeable battery is charged via the 270 ohm charge current limiting
resistor. When the line power is "OFF", the "Bookkeeping" RAM is supplied
with a "stand-by" voltage of 3.6VDC to a 4.2VDC for a duration of at least 30
days.

No matter how advanced our technology becomes, it seems that a technician who
understands power supplies can be successful at troubleshooting most
electronic equipment. The reason for this is that many of the problems that
occur are directly or indirectly related to the power supply.

At this time, we are going to look at the Bally Midway A082-90412/13 and
A082-90421/22 type Power Supplies as seen in Omega Race, Kickman, and
subsequent games. Since power supplies are generally not field serviceable,
we are going to concentrate on diagnosis of a bad power supply. First, we
have to know what the power supply's function is. Bally Midway's power supply
is made up of several smaller supplies. They are: +5 volts regulated, +12
volts regulated, and unregulated audio and lamp voltages, and possibly a
battery supply and/or 5 volts regulated.

The only purpose of a power supply is to give the proper voltage(s) and to
supply enough current at the proper voltage(s) to fulfill the requirements of
the game. These power supplies have current limiting features on all
regulated sub-supplies. If the current demand is larger than its capabilities
first the voltage will drop, then it will shut itself off until the excess
load is removed. It is quite possible that a power supply may be diagnosed
"BAD" when in reality there is a short somewhere on one of the Logic Boards
or elsewhere in the system. Symptoms of a bad power supply can be anything
from no picture to specific functions not present on the screen. The best
thing to do when you suspect a bad power supply is to check all the voltages
with your meter. Keep In mind that just because all the power supply voltages
are present, this does not mean that the supply is good. A power supply
voltage can contain certain fluctuations that only an oscilloscope can detect.
In this case, substitution, is a good check. Approximate measurements can be
made in the Power Supply PCB but accurate measurements MUST be made on the
Logic PCB between the filter and the load.

*** CAUTION MUST BE TAKEN WHEN MAKING THESE MEASUREMENTS! IT'S POSSIBLE TO
DAMAGE LOGIC COMPONENTS BY SHORTING POWER SUPPLY COMPONENTS! ***

+5 VOLTS REGULATED: This is the highest current supply on the board. Most of
the devices on the game boards use this supply. To check it, put your meter
ground probe on point "A" and its positive probe on point "B". You should
read 5.0 volts to 5.3 volts. This supply is adjustable with a potentiometer.

NEVER ADJUST THIS POT WITHOUT A METER CONNECTED AS ABOVE!!

+12 VOLTS REGULATED: This supply is used for peripheral circuits. To check
it, put your meter ground probe on point "A" and its positive probe on
point "C". You should read 12.0 volts to 12.3 volts. It too is adjustable
with a potentiometer.

UNREGULATED SUPPLIES: This is a supply used for audio and lamps. This supply
can read from 9 volts to 16 volts. To check it, put your meter ground probe
on point "E" and its positive probe on point "F".

-5 VOLTS REGULATED: This supply is used by auxiliary circuitry and/or
memories. To check it, put your meter ground probe on point "G" and its
positive probe on point "A". You should read -4.9 volts to -5.2
volts. It too is adjustable with a potentiometer.

BATTERY SUPPLY: This supply is used by the "Bookkeeping" RAM. To check it,
put your meter ground probe on point "A" and its positive probe on point "H".
When the line power is "ON", you should read 5.0 to 5.3 volts. When the line
power is "OFF", you should read 3.6 volts to 4.2 volts.

If all the voltage checks read good, there still is the chance your power
supply is bad. With an oscilloscope, it can be determined whether a supply
has a ripple oscillation, or other type of failure.

If one of the supplies reads lower than it should (for example, the +5 volt
supply reads +3 volts), turn off the game and pull the connector off that
goes  to the logic board(s). Short the Power Supply's -SENSE output to COMMON,
turn the power back on, and check the supply again. If the supply is good,
there is probably a bad device on one of the logic boards that is shorting
the supply.