last modified Jan 13, 1998
Note that Stern copied Bally PMU boards so any test for BALLY is the same for STERN!
Older Bally coil Cross-Reference
Subject: Bally ROM/PROM Jumper Combinations (save this!)
Well, I found it. Buried in my files from 1985 came this tid-bit from
April 2, 1985 Service Bulletin...
"Game: ALL BALLY MIDWAY ELECTRONIC PINBALL MACHINES THAT USE AN
AS-2518-35 M.P.U.
SUBJECT: ROM/PROM(eprom) M.P.U. JUMPER COMBINATIONS
JUMPERS FOR 'E' JUMPER NUMBERS
----------- -------------------
U1 2316 ROM OR 9316 1-4, 2-6, 7-8, 9-11, 12-36
U2 2316 ROM OR 9316 13-15, 16A-19, 31-32, 33-34
U6 2316 ROM OR 9316
_________________________________________________________________
U1 2716 PROM 1-5, 2-4, 7-8, 11-25, 10-12,
U2 2716 PROM 13A-14, 31-32, 33-34, 16A-19
U6 2316 ROM OR 9316
_________________________________________________________________
U1 2716 PROM 1-5, 2-4, 7-8, 10-12, 11-25,
U2 2716 PROM 13A-14, 16A-34, 31-32, 33-29
U6 9332 ROM
_________________________________________________________________
U1 2716 PROM 1-5, 2-4, 7-8, 11-25, 10-12,
U2 2716 PROM 13A-14, 31-32, 33-34, 16A-19,
U5 2716 PROM 26-29, 28-30
U6 2716 PROM
_________________________________________________________________
U2 9332 ROM 4-12, 7-8, 10-11, 13A-14, 29-33,
U6 9332 ROM 31-32, 16A-34
_________________________________________________________________
U1 2716 PROM 1-5, 2-4, 7-8, 11-25, 10-12,
U2 2716 PROM 13A-14, 31-32, 33-35, 16A-18
U6 2716 PROM
_________________________________________________________________
U1 2716 PROM 1-5, 2-4, 7-8, 11-25, 10-12,
U2 2716 PROM 13A-14, 31-32, 33-34, 16A-19
U6 2316 ROM
_________________________________________________________________
U1 2716 PROM 1-5, 2-4, 7-8, 11-25, 10-12,
U2 2716 PROM 13A-14, 31-32, 33-34, 16A-19
U6 2316 ROM
_________________________________________________________________
U1 2316 ROM 1-4, 2-6, 7-8, 9-11, 12-36,
U2 2316 ROM 13-15, 16A-19, 31-32, 33-34
U6 2316 ROM
_________________________________________________________________
U1 2316 ROM 1-4, 2-6, 7-8, 9-11, 12-36,
U2 2316 ROM 13A-19, 16A-18, 31-32, 33-35
U6 2716 PROM
_________________________________________________________________
U1 2716 PROM 1-5, 2-4, 7-8, 11-25, 10-12,
U2 2716 PROM 13A-14, 31-32, 29-33, 16A-34
U6 9332A ROM
_________________________________________________________________
U1 2716 PROM 1-5, 2-4, 7-8, 11-25, 10-12,
U2 2716 PROM 13A-14, 31-32, 33-34, 16A-19,
U5 2316 ROM 26-29, 28-30
U6 2316 ROM
_________________________________________________________________
U1 2316 ROM 1-4, 2-6, 7-8, 11-9, 12-36,
U2 2316 ROM 13-15, 16A-19, 31-32, 33-34,
U5 2316 ROM 26-29, 28-30
U6 2316 ROM
_________________________________________________________________
U2 9332A ROM 7-8, 4-12, 13-15, 9-11, 16A-34,
U6 9332A ROM 29-33, 31-32
_________________________________________________________________
U2 2532 PROM 4-12, 7-8, 10-11, 13A-14, 29-33,
U6 9332A ROM 31-32, 16A-34
_________________________________________________________________
U2 68764 PROM 4-13A, 7-8, 11-35, 9-12
_________________________________________________________________
U2 2532 PROM 4-12, 7-8, 10-11, 13A-14, 16A-29,
U6 2732 PROM 31-32, 33-35
_________________________________________________________________
Ed Schmidt
Asst. Technical Service Manager
ES/dd"
>>Well, I hope that is of some help to you folks... was nice to get it
------------------------------------------------------------------------------
Subject: Bally (Stern) -17/-35 CPU LED test.
* Original Subj: Bally (Stern) -17/-35 CPU LED test. was Re: TECH: Bally Six
Million Dol
From: adoerfer@welkin.shr.dec.com (_bill)
Date: 25 FEB 95 13:10:50
Organization: Digital Equipment Corporation
Message-ID: <3inse0$9ac@nntpd.lkg.dec.com>
Newsgroups: rec.games.pinball
In article <3igaf8$cg8@delphinium.cig.mot.com>, defeo@rtsg.mot.com (Tony P.
DeFeo) writes...
>
>Could someone with a tech manual for MPU boards of this
>era (circa. 1978) please E-Mail me what it means when
>the LED only flashes twice (on normal powerup it should
>flash 7 times, once for each self test, I believe).
>Actually, if you could, I would like to know what all the
>startup self tests are for future reference.
>
>Thanks in advance!
>
>Tony DeFeo
>defeo@cig.mot.com
I've seen these articles posted.
(this applies to Bally AS-2518-17 and -35 and Stern MPU 100 type boards)
The short version and opinions:
In brief, the flicker tests that it can power up (I check fuses and conectors
and voltages if that fails)
1x checks Rom/prom (U1,2,3,4,5,6,7,8 - take care to replace with equivalent
chip or you may have to re-jumper)
2x checks ram and Nmos (U7, U9, U10 or 11 - I would do 11, 10, 7 9 myself,
take care not to bend the pins)
3x checks cmos and ram (U8, 9, 10 11 from my experience u8 is OFTEN the
culprit, and being a relatively cheap and hard to
find part, get some spares - especially if you
don't ground your machine) P5101L-3 go the most
and are the easiest to damage, in my opinion
4x checks PIA (U10,9) if it gets this far, almost always U10
and once you get u10 replaced, you usually find
u11 went, too. 6820(or equiv) are for me the second
most frequent things that go
5x checks PIA (U11,9) here it's almost always U11
6x checks display interrupt (U9,U11,U12,C16,C17) Here I find it usually to
be a physically bad board, and replacing chips won't
help much, although U9 might still be defective.
7x checks zero crossing circuit (U11, U10, U14)
The more detailed version
A) 1ST FLICKER:
On power-up, the MPU chip (U9), requires that +5±.25VDC be applied BEFORE
the
RESET line is allowed to swing from 0 to +4.8 VDC. It also requires the
presence of a two-phase, non-overlapping clock pulse. If these conditions are
met, and if the MPU chip itself is good, the LED on the module flickers
briefly. F.O. 560 page 29, deals with fault localization if the LED indicates
a
fault (LED always 'on' or 'off').
The brief flicker indicates the operation is proper. The MPU has gone out
to
memory. It has obtained the starting address of the Self-test from memory.
The
flicker indicates that it then went to that address and started to execute the
Self-test program.
The Valid Power Detector circuit on the MPU module works with the +5VDC
regulator, Q20, on the Solenoid Driver/Voltage Regulator module to prevent the
reset line from going high until +5VDC is proper at the MPU chip. Q20 is
guaranteed by the manufacturer to go into regulation when +7.5VDC is applied
to
its input. This means that when the game is turned on and a sufficient period
of time (milliseconds) has passed so that C23 (11,700 mf) has charged to a
+7.5VDC level, Q20 switches into regulation and supplied +5VDC to the MPU
chip.
However, Q1 in the valid power detector circuit does not allow the MPU chip to
turn on until some time later. The zener diode (VR1), in series with the base
of Q1 delays application of of the reset voltage until C23 charges to an
8.9VDC
level(+8.2 across VR1 and +.7V base-emitter junction drop across Q1). At that
point in time, Q1 and Q5 go into conduction, and the reset line at the MPU is
caused to go high (+4.8VDC). Only then is the MPU chip 'on'.
The importance of the Valid Power Detector circuit can be appreciated when
the following fact is known:Should the reset line be allowed to go high before
the +5VDC is applied and proper, or should the 5VDC supply fail and go out of
regulation, the MPU can jump out of the program. The reason that this happens
is that the MPU goes out to the program memory bank U1-U6 for instructions.
The logic levels are wrong because the +5VDC is not proper. The MPU
misinterprets the data, jumps out of the program, and writes its own program!
The MPU at that point in time is like a train that has left the tracks. It
can
end up anywhere. The difference is that the train eventually stops. The MPU
may continue as long as the clock circuit continues to run.
If the MPU chip jumps out of the program, it is said to be in 'run-away'.
While it is creating it's own program by going out for instructions and
executing anything that it gets from memory that looks like an instruction, it
invariably overwrites the Bookkeeping functions in U8, the non-volatile
scratch
pad RAM. An indication of 'run-away', then, can be false data in bookkeeping
functions. Probable cause: faulty Q20, leaky C23 (high ripple) or leaky zener
diode,(VR1, MPU module).
B) FIRST FLASH:
The MPU chip next goes out to program memory bank U1 through U6 (Read Only
Memory). It tests each chip, one at a time. When it finds the bank correct,
it flashes the LED the first time to indicate its finding. A fault in the
memory bank then is indicated by the absence of the first flash.
How the MPU tests each memory chip can be illustrated by the following: In
a
game with chips U2 and U6, the MPU will first go to U2. It will fetch the
first byte in U2, it will add it to the second byte in U2. It will add to the
sum the third byte in U2. It will continue until it has summed all 2048 bytes
in U2. If the sum is '0000 0000', the MPU proceeds to U6 and repeats the
process. If U6 has a sum of '0000 0000', the MPU causes the LED to flash the
first time. Fault in either U2 or U6, of course, is indicated by the absence
of the flash.
The contents of each chip have byte locations called checksums reserved for
this test routine. There is one checksum byte reserved in each 512 bytes of
program memory. The programmer at Bally must insert a byte with the proper
value in each checksum byte location to force each 512 byte checksum to equal
'0000 0000'.
During the life of the electronic game, if a chip in the program memory bank
U1 through U6 fail by so much as changing a single bit in its 2048 X 8 bits /
byte = 16,384 bit contents, it will be detected during the MPU test. The MPU
will not allow play until the defective chip is located and replaced.
C) SECOND FLASH:
The MPU chip, through the program, goes out to NMOS RAM U7(Read/Write
memory).
It erases the contents of the first byte in U7 (U7 is 128 full bytes of
'scratch pad' memory). It then tries to read back the word '0000 0000'
(indicating 'erased' or cleared), If it can read it back it continues to add
1, write and read until, 256 tries later, it writes the word '1111 1111'. If
it can read this back, it has determined that the first byte in U7 is good.
It
repeats the process for each of the 127 remaining bytes, one byte at a time.
If, at the end of these 256 X 128 = 32,768 tests, each time the MPU writes,
it
can read the same word correctly, the MPU chip causes the LED to flash the
second time.
If U7 is defective, the MPU will not allow game play until it is replaced.
It is to be noted that there is a pause between the first and second flash
of
the LED during the MPU Self-Test. This pause represents the actual time
necessary to do the 32,768 individual test cycles involved in testing U7.
D) THIRD FLASH:
The MPU chip goes out to the CMOS RAM U8 (Read/Write memory). It makes a
copy of the contents of the first half byte in U8. This is necessary because
U8 is the battery supplied, non-volatile memory location where the bookkeeping
functions are stored. It then erases the contents of the first half byte in
U8
(U8 is 256 half bytes of 'scratch pad' memory). It then tries to read back
the
word '0000 XXXX' (where XXXX are bit locations to be ignored). If it can read
it back, it adds '1' to the previous word (new word, '0001 XXXX'). It
continues to write and read until it reaches and reads the word '1111 XXXX'.
When this is done successfully, the MPU restores the original contents to the
first byte located in U8. It then makes a copy of the contents of the second
byte and repeats the process. It repeats the process for each of the
remaining
254 bytes, one byte at a time. If, at the end of these 256X16 = 4096 tests,
each time the MPU writes, it can read the same word correctly, the MPU causes
the LED to flash the third time.
If U8 is defective, the MPU will not allow game play until it is replaced.
E) FOURTH FLASH:
The MPU chip, thru the program, now tests the first PIA chip, U10. Each of
the two PIA chips (U10 and U11) is identical and interchangeable. The test
for
both is also identical. PIA chip U10 is accessed by means of decoder inputs
RS0, RS1, CS0, CS1 and CS2. These inputs are used to control the PIA. By
means of these lines, each bit of two full byte sets of ports can be
initialized to be either an input or an output. These ports are labeled PA0
thru PA7 and PB0 thru PB7 on the MPU schematic.
Also by means of these lines, two ports, CA2 and CB2 can be initialized as
either inputs or outputs. Finally, two additional ports, CA1 and CB1,
designed
to be used as inputs only, can be initialized to trigger on a positive or
negative going edge. These ports on PIA's U10 and U11 are used as interrupt
inputs, to be discussed later.
To determine if each of the PIA chips is good, the MPU chip, thru the program,
does the following:
1) It accesses, by means of inputs RS0, RS1, CS0, CS1 and CS2 each of the two
full byte registers used to store the port initialization information. It
does this, one register at a time. After it completes the first register,
it
repeats for the second. It goes thru 256 tests similar to that used to
check
each byte in U7 (Second Flash). If, each time the MPU writes a word into
the
register, it can read the same word back, it continues to test to
completion.
If the MPU finds a fault in either of these two full byte registers, it
stops
the test and will not allow game play until the PIA chip is replaced.
2) It accesses, by means of inputs RS0, RS1, CS0, CS1 and CS2, each of two
full byte registers used as data output registers when PA0 thru PA7, PB0
thru
PB7 are used as outputs. It does the same type of test on each register
described in 1) above. Again, if no faults are found, the test is continued
to completion. A fault detected stops the test. Game play is not
permitted
until the PIA is replaced.
3) It then accesses. by means of inputs RS0, RS1, CS0, CS1 and CS2, first the
CA2 and then the CB2 port. The port is initialized as an output. the port
is then written into to see if it can store a '1' and then a '0'. When both
ports are found good, the MPU causes the LED to flash the Fourth Time.
The CA1 port on U10 is tested manually each time the Self-Test switch on
the inside of the front door is activated. The CB1 port is tested later as
the Seventh Flash.
A total of 4 X 256 + 4 =1028 test steps are required to test each of the
two PIA chips. However, there are internal buffer amplifiers used with the
PB0 thru PB7 output register and CB2 port register which cannot be tested by
the MPU(access is only to the register - if the buffer is open, it does not
interfere with the register's ability to be written into and read from by
the
MPU). It is this uncertainty that reduces the accuracy of the MPU Self-
Test to 99.5%.
F) FIFTH FLASH:
Identical to the test procedure and results detailed for the Fourth Flash
except:
The CA1 port on U11 is tested later as the Sixth Flash.
G) SIXTH FLASH:
The MPU chip, thru the program , monitors PIA1 port CB1 (U10). If
transitions from high to low are detected, the MPU decides that the zero
crossing detector is working. It then causes the LED to flash the Sixth time.
If U14 fails and the CB1 line is stuck high or stuck low the MPU chip will
not allow game play until the chip is replaced. It is to be noted that the
zero crossing detector circuit input is the +43VDC line to the solenoid
common.
If the fuse in that line (F4 on the Power Transformer module) is blown when
the
power is turned on, the MPU will not allow game play until the fault on the
+43VDC line is corrected.
H) SEVENTH FLASH:
The MPU chip now monitors PIA2, port CA1 (U11). If transitions from high to
low are detected, the MPU decides the Display Interrupt Generator is working.
It causes the LED to flash the Seventh Time. The MPU then enters the part of
the program that deals with the initialization of the game prior to play.
If U12, a 555 timer, or any associated circuit component, fails, the MPU
chip
will not allow game play until the fault is corrected.
I) GAME INITIALIZATION:
The MPU chip, thru the program, now initializes the two PIA's, U10 and U11,
assigning to each port its role as either an input or an output, as required.
It then clears out U7 (Read/Write memory)
The MPU now 'takes a picture' of the settings of the fixed switched S1 thru
S32 on the MPU board. It stores this 'picture' in memory (four bytes) in chip
U7. With this record safely in memory, PIA U10 is now free to do other things
during the course of play.
The MPU next jumps to a subroutine which turns on the 'Game Over' feature
light, lights the 'Ball in Play' light and the 'Credit Indicator' light if
there are credits stored in memory. It resets drop targets and activates
saucer kickers or any kicker associated with a playfield device that can trap
the ball and keep it out of the outhole. It then energizes the coin lock-out
solenoid to allow the game to accept coins (unless credit maximum was stored
in memory). Playfield and backbox feature lites associated with and
appropriate to animation effects are now turned on.
With the game tested and initialized, the MPU now divides its time between
monitoring momentary switches for closures (coin switch, credit button) and
updating displays (lamps and score registers).
------------------------------------------------------------------------------
Subject: Re: Tech: Bally (or STERN) repair procedure
To: All
Spotted this out on the range...
GC> From: zubla@shc.uiowa.edu (Greg Chapman)
GC> Date: 11 Oct 1995 05:30:24 GMT
GC> I bought a working 1980 Bally Space Invaders pin at an auction.
GC> It worked for six hours of abuse at the auction and so of
GC> course when I get it home it doesn't. :^)
GC> I am using Bally Electronic Pinball Games Repair Procedures
GC> number 560-2, mostly because it says 1980. I have the ones
GC> before and after it too.
GC> 1. How do you find which manual is for which machine?
I generally find that the latest Repair Procedures that covers that style of
CPU is the best, as it contains any corrections to previous editions.
GC> I turn the machine on: flicker, flash, pause, 5 MPU LED blinks.
GC> 5 VDC at TP1, 12 VDC at TP2, but nothing at TP3.
GC> I turned to p.18 Power Transformer Module A2 Diagnostic Table.
GC> This leads me to discover that fuse F4 (32V, 5A, 3AG) was blown.
GC> I replace it, retry, blows the same fuse again. So now I am
GC> supposed to locate defective solenoids.
Nah, what you have is either a shorted bridge rectifier OR the MOV (thats the
red disc just above at the 1 O'Clock position on the fuse/bridge/transformer
board). Take your trusty ohmmeter and set to a low value resistance and
connect it between TP5 and ground. You should see nothing lower than 100
ohms, if so you probably have a shorted MOV, if not then a shorted diode so
take the ohmmeter/multimeter leads and connect them to any pair of the
bridge rectifier pins. With the fuse out and the plugs all removed you should
not see anything lower than, say, 100 ohms across the pins, note that these
pins are just a small nipple like tip sticking out of the pcb that are around
one of the three bolts that hold this pcb to the metal frame. If you find a
low resistance point, then try unsoldering one leg of the red MOV (looks like
a minerature lolipop) and check again. These bridge rectifiers can be
replaced with any 25AMP 100PIV bridge that you can get, just use some
jumper wires if not same style and make sure you put the new brdige on it's
own metal heatsink. I've used Motorola's MDA 2502's for years, but I beleive
they are out of production. Check the surplus stores for spares.
Rough ASCI drawing (from memory) BALLY/STERN Fuse - Rectifier board
-------------------------------------------------------|
| |
| TP1 TP2 TP3 TP4 TP5 |
| _____ |
| [___] | <--- MOV
| . . . . . . _._ |
| .{}. .{}. .{}. | R | | <----- Screws &
| | e | | Bridge rectifiers
| |--| |--| |--| |--| |--| | s | |
| F1 F2 F3 F4 F5 | . | |
| |_._| |
| |--| |--| |--| |--| |--| |
| |
| ||||| ||||| J1 ||| |||||||| J2(3?) |(not sure of locating
| ----------- ------------ | pin or pin count,
| |||||| |||||||||||||||| J3(2?) | you get the idea
| ----------------------- | though)
|------------------------------------------------------|
Some day I shall tidy this up!
Enough already!
:-#)>
------------------------------------------------------------------------------
Subject: Re: TECH: Xenon Flippers causing reboot?!?!?!
To: All
Spotted this gem...
MC> From: martyc@postoffice.ptd.net (Marty Chamberlain)
MC> Date: 1 Jan 1996 18:36:32 GMT
MC> Hi, all! I'm currently working on bringing a Bally Xenon back to life.
....
MC> ball is served, etc. All
MC> looks good until you use either flipper. Upon hitting
MC> either flipper, the
MC> machine's displays go blank and it "reboots". Very strange!
This is a classic symptom of a bad primary filter cap on the +5VDC regulator.
The magic points to note were the phrase "either flipper". This basically
eleminates the diodes on the coils and a CPU problem. If you check across the
large filter cap on the solenoid/power supply board (around 12,000/25VDC) with
an AC voltmeter you should see about .3VAC or less and around 11VDC with a DC
meter. Please do not use an Autoranging style of meter for this as I have
found that they are unreliable in checking for small AC ripple on a DC line.
If the ripple is greater than .4VAC then replace the filter cap....
(Nasty ASCII diagram)
__________________________________________________________
| _____________________
| |High voltage filter |
| |____________________| | | | | |
| x | | | | |
| x (heat sink and misc) _____ | | | | ( ) |
| x (high voltage stuff)| | | | | | ( )
| x_____ | | | | | | ( ) (and so on...)
| | | |12000| | | | | ( )
| | | <-Relay |25VDC| | | | |
| |____| | | | | | |
| x |_____| | | | |
| x <-connector ^This^ ^^Large metal heat sink^
| x is the
| x suspect!
You get the idea.....
:-#)>
------------------------------------------------------------------------------
Subject: Re: TECH: Xenon Flippers causing reboot?!?!?!
To: martyc@postoffice.ptd.net
Hi, Marty!
Got your note...
U> To: jrr@jukes.wimsey.com
U> Date: Sun, 7 Jan 1996 16:27:00 -0500
U> Thanks for the tip...here's what I did...
U> With the game in "Game Over" mode, measured AC volts
U> (with meter in "manual",
U> non-autoranging) across the terminals of the large
U> filter cap on the solenoid
U> driver board as you suggested. Measured .843VAC.
^^^ Way too high!!
U> machine doesn't reset.
U> Just an observation.
U> I then swapped a spare solenoid diver board in,
U> measured the filter cap voltage
U> as above. Measured .835VAC. Symptom still existed,
^^^Again much too high
U> but it seemed to take a
U> little more "flipper ON time" to get it to happen.
U> I then took the solenoid driver board from my perfectly
U> functional Medusa, and
U> put it in the Xenon. Symptom still existed. I didn't
U> measure the filter cap
U> voltage on this board because I was too frustrated at that point.
U> Do you have any other ideas? I'm about at the end of my rope!
Well, check the voltage at the cap! You might have a bad connection on the
brown plug on the upper right of the board. That's where the grounds and
+11VDC come in. Are you screwing the board down with the two 8-32 screws
on each corner? (this improves the ground connection). You might have an
open diode on the 11VDC bridge rectifier, what is the DC reading at the
filter cap? (with everything plugged in)...If you have a 3 amp 50PIV diode
lying around you can jumper the corner terminals of the bridge, by that
I mean connecting the band end of the diode to the + terminal of the bridge
then connecting the other end to each of the two AC inputs, one after the
other, while watching the DC voltage. IF it goes up then the bridge is bad.
Now do the same for the bottom half of the bridge if no voltage increase,
but this time the non-banded end of the diode is connected to the - terminal
of the bridge and again you jumper one at a time to each of the two AC inputs.
This will test the bridge fine as usually only one diode in the bridge fails
open at a time. The usual voltage when the bridge has an open diode is
about 8VDC at the primary filter cap and about 1VAC ripple.
Hope this helps! ( am posting to the group for information)
:-#)#
------------------------------------------------------------------------------
RESET Problems:
Hi, I thought I'd pass on this little tip, if your LED is on continuosly
when you turn on the game, OR, sometimes the game won't start, you need
to suspect your RESET circuitry. A quick and "dirty" test for a good
reset operation is to short to ground the junction of R1 and R3. This
will force a reset in a good board, and you should see pin 40 of the CPU
IC going low. If not suspect the transistors, I find Q1 is the usual
failure (2N3904) often from battery corrosion (alkaline-neutralize with
white vinegar), if Q1's collector doesn't go to at least 4VDC when the
R1/R3 junction is grounded, then Q1 is bad...
:-#)#
Back to 1st Flicker
Got this from rec.games.pinball on November 13, 1997:
areizman@aol.com replied to a question
"TECH: 8-ball Deluxe - Outhole Kicker Problem"
by Don.Shoemaker@nortel.com...
>he Outhole Kicker will not eject the ball from the Outhole into the shooting
>lane. As such, the game is unplayable. I think the problem may be with the
>
>Solenoid Expander circuit
That Sol expander was a little toy of mine. Mike Miller and I put that baby
together when those darn playfield designers began to go through more coils
then the driver board could handle.
The reason for the series diode is without it current will pass through the
coils and fire two at the same time. (look carefully at the schematic and
you'll see what I mean.)
As far as your intermittent problem goes, first lets start with the
obviouse. Does the outhole score? If not then you have a switch problem. Next,
check the solder connections on the expander board, 15 years of vibrations can
fracture the 156 pin soldering and make the expander intermittant.
Finally, the MOC3011 does not draw enough current to turn on the scr that
drives it. To get around this problem we wired a lamp in parallel with the
circuit. If that lamp is not lighting then you cannot consistently drive the 3011.
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