STAR*TECH JOURNAL/JANUARY 1984
8
TROUBLESHOOTING
WILLIAMS' GAMES:
INTERMITTENT RESET
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Intermittent resetting is an annoying symptom that occasionally shows up in older-model games.
It can be particularly difficult to trace down, since the reset itself is actually a legitimate system
function, not a problem.
Before we go any further, let's describe the symptom for those who may not be familiar with it.
It's very simple: A kid is playing your video game and suddenly it blanks out and comes up with the
scanning rug pattern (RAM/ROM test). When the game is ready to play again, it comes up in
Game-Over Mode rather than where the kid left off. The game isn't down, but the kid loses a
quarter.
If this is your symptom. actually what you've seen is the system's reaction to the real problem.
So if you try to troubleshoot the reset itself, you'll be fooled into looking for the wrong thing!
Resets occur when: (I) the microprocessor is in a loop and this is detected by the watchdog
circuit, or when (2) the voltage at ZR! drops below 6.8vdc. A loop is detected by the watchdog if
the microprocessor fails to clear itself after 13 3 milliseconds ( 8 vertical scans). In either case the
reset you see is not the problem, nor is one problem always the cause of the reset. Treating the reset
itself is both frustrating and fruitless. In other words, don't bother repairing a reset - it obviously
works fine!
Component aging is a matter you should consider when investigating the cause of resets.
When some components fail, they fail totally by either shorting or opening the circuit. These parts
may be easy to find because they refuse to produce the signals normally associated with them ( or
maybe they're actually burned up, so finding the problem couldn't be easier). Filter capacitors are
a good example, They swell and ooze, plus they may either drop voltage or open up and allow
ripple into the circuit.
On the other hand, many components don't seem to fail in such an obvious way. 74LS374 data
latches are an example of a part that usually fails intermittently. A failure here could show up as a
RAM error or it could produce intermittent resetting. But more on that later.
Environment has to be considered. Where is the game located? Does it get adequate
ventilation? Is it subject to line glitches from motorized appliances on the same circuit? Are you
running the game off a three-wire outlet? Dry atmospheres tend to promote static electricity, one of
the main enemies of computerized equipment. Static discharge ( or a line transient, for that matter)
can reset your machine.
Check the power supply. Sometimes a visual check of the power supply will turn up the source
of the problem: a bulging or dripping 18,000 uF electrolytic capacitor or a melted connector, for
example. Parts that run warm are OK, but those blisteringly hot should be replaced with haste. Are
the three output regulated DC voltages correct? Check for AC here too. You shouldn't see any, or
the circuit's defective. If all seems OK here, check if one of the RAM chips on the CPU board
could be shunting the regulated voltage down to one or two volts!
Swapping Boards. Maybe a circuit board is ailing. Try to restore game operation by replacing
the suspected board with one from a working game. But never attempt to operate a good CPU
board with a power supply that you suspect. Bad power supplies can cook RAM chips like
popcorn! If the power supply is good, try swapping the CPU board and then the ROM board.
Remember to make these substitutions one at a time so you'll know which board is hiding the
culprit.
Auto-Cycle Mode. The Auto-Cycle Mode helps you locate intermittent memory chips. (See
your manual for procedures.) Bad RAM, ROM and CMOS chips may actually pass the power-up
and diagnostic tests several times before they fail. Rather than tie your bench up all day as you wait
for one of them to fail, you canjust leave the game in the Auto-Cycle Mode overnight. The next
morning your game may be displaying an error message from the four-hundredth time the test ran!
Replace the bad chip and you should have a good game.
Further component-level troubleshooting. We've mentioned a few components on the power-
supply board that could cause resets. Other components, mostly on the CPU board, could also
produce the symptom.
Decoder RO Ms. Try swapping the plug-in decoder RO Ms on the CPU board ( some Defender
games only have one) with ROMs from a working game. Make sure you don't interchange the
vertical and horizontal ROMs. Because these chips are responsible for control and pseudo-
address signals in the RAM circuit, an intermittent one could cause a reset. By the same token,
check for intermittent decoder-ROM sockets.
Address Bujfers. Two chips that may cause intermittent resets are both buffers. These are the
high-order address buffers. Both chips are type 8T97. Noise on the high-order address bits or
stuck bits wil prevent the horizontal decoder ROM on the CPU board from outputting the proper
psuedo-addresses that are used to scan the RAM circuit.
A failure of the vertical count buffer chip could disable bits on either the video address bus or
the microprocessor data bus. Check for noise or stuck bits.
Data latches. There are three data latches in the RAM circuit on the CPU board. These are
either 74LS374 or 74LS373-type IC chips. These chips may fail in a very subtle way. Their
propagation-delay time may lengthen slightly. What this means to the circuit is that the three data
latches may get into a "race" condition that winds up snarling the program into a loop. When this
loop is detected by the watchdog, it dutifully resets the microprocessor. At this point sometimes a
RAM error message appears on the ROM board LED display.
It is recommended to try the change included on recent boards starting withJoust video. This is
the addition of 470pF, 50v disc capacitors from pin one of each latch to ground.
Reprinted courtesy of Williams "Logic Supply"