Star Tech Journal

lO _ _ _ _ _ _ _ _ _ _ S~R~ECH
Midway "Pac-Man" Reset CKT
FIGURE 1
7.5
voe
+5V
10MFO ~
25V ;;-.. CID
1K
R28
10K
POR
01
(TO COIN
COUNTER
CIRCUIT)
~
FIGURE 2
SW11
- -
- -
- -
- -
2N3391A
7.5
1 I
AC
IN
p
+5V
1K
V BLANK
7 EP
10
ET
2 CK
3
A
4
B
5 6
C 0
74161
BC
ca
10,000
IOV
10,000
IOV
9
TC
CLR
15
RESET
(TO MICRO-
PROCESSOR)
Ul;
turning "on" the NPN
transistor. During this time,
the collector of 01 is "low",
presenting an active ·POR
(power on reset) to the coin
counter circuit (prevents false
counts during power up) and a
load (Ld) signal to IC 9C
(7 4161) holding the
microprocessor in a reset
condition. After a period of
time (to allow all voltages to
stabilize), C1 O becomes fully
charged and acts as an open
stopping all current flow to the
base of 01, turning it "off" and
releasing both reset signals.
Loss of Displays, 4 76-484 Rock-Ola
On some keyboard/display
boards, the use of copper board
edges instead of nickle plate
has caused a corrosion
condition that leads to
intermittent displays of
"Credit" and "Record Now
Playing."
7
LO
WOR
Symptom:
Logic PCB is locked up
(constantly resetting) and
WDR signal is present (watch
dog reset).
Probable Solution:
C7 or CB (10,000 MFD/1 OV
capacitor) is open.
Theory ( Figure 1):
When power is first applied,
C1 O (1 OMFD/25V) charges to
7.5VDC through R28 and R29.
Resultant current flow
produces a positive voltage
drop at the base of 01,
voe
During normal operation, WDR
occurs to clear the counter
(9C) before it reaches its
terminal count (TC) keeping
Reset inactive.
However, if sufficient ripple
(AC) voltage is present (Fig. 2)
in the 7.5VDC, C10 will charge
and discharge accordingly.
This action will cause 01 to
switch on and off,
consequently resetting the
processing and preventing the
system from fully powering up.
by Chuck Maier
Cleaning the connector on the
board and applying a light
coat of vaseline will prevent
additional service calls.