TECH: Williams System 11a Power Supply Issue

[b***@yahoo.com]

I have a Fire! table that was working perfectly until it was turned
off. When I turned it back on a few minutes later, all the GI lights
came on but nothing else happened. I had a similar issue a few years
ago which was caused by a low 5V output and ended up replacing most of
the related components on the power supply board, which fixed the
problem.

Now, however, I'm having a different issue. The +12V test point is
showing around 1.5V and the -12V test point is showing almost -40V
(relative to the back of the backbox). The +5V test point is showing
about 0.6V. I measured the voltages coming from the transformer on the
wires that plug into 3J1. All the DC voltages are correct. The AC
voltages (pins 10 and 11) show 10VAC relative to pin 12, which seems
low but is most likely due to the 200V minimum range on the cheap
multimeter I had to buy (since my good one is on the other side of the
country :) ). When the plug is connected, pins 10 and 11 still show
20VAC relative to each other, but show 0V relative to pin 12. The
diodes in the bridge rectifier test okay on the board.

My guess is there's some sort of ground problem, but I'm not sure where
to look. Can anyone suggest what would cause these symptoms, or give
me some ideas on what else to test? Thanks in advance.

-Brian

[Action Pinball]

I'm unsure about your AC input voltage- IIRC, this game takes input to the
5vdc regulator circuitry from the 18/19vac winding in the transformer
secondary- ie: the two gray wires. I'd remove the 12-pin input connector
from the PS board and check these with meter on AC to make sure that voltage
is in spec.

If so, continue upstream- checking components along the way- there's not
much in this circuit so not too complicated.

One common cause of failure is one diode in the rectifier shorting and
blowing only 1 of the fuses that protect it (7 amp slow-blow). So check for
this. Remove both fuses and check physically with meter to be sure they're
good. Check rectifier while fuses are out of circuit (or input connector is
removed). This sounds like a classic blown fuse or open/shorted rectifier
problem, so I'd double or triple-check things in this area.

Beyond that, the filter capacitor is usually a must for replacement if it is
old/original. Look for bulging on one end of the cap which can indicate old
age/overdue for replacement. You could also have a bad 2N6057 regulator
transistor or IC1 regulator chip (LM723).

Don't forget to check solder joints on the back side of the connector pins
on the board- both input and output connectors. Cracked joints are
commonplace in these old games and you can't expect any reliability if these
haven't been inspected and overhauled.

Be sure to leave the output connectors off (both 6 and 15 pin connectors at
top of board) while doing your testing and repairing, to make sure nothing
else in the game interferes with the PS board or so the PS board doesn't
damage anything else if high voltages are present on low voltage lines

I'd focus on getting the 5vdc working first then move on if anything else is
still out of spec, but most likely you are seeing multiple symptoms of just
one single problem.

Hope that helps!

Ray J.
--
Action Pinball & Amusement, LLC
Salt Lake City, Utah USA
Web: www.actionpinball.com

We're serious about pinball. Anything else is just for fun!

Post by b***@yahoo.com
I have a Fire! table that was working perfectly until it was turned
off. When I turned it back on a few minutes later, all the GI lights
came on but nothing else happened. I had a similar issue a few years
ago which was caused by a low 5V output and ended up replacing most of
the related components on the power supply board, which fixed the
problem.
Now, however, I'm having a different issue. The +12V test point is
showing around 1.5V and the -12V test point is showing almost -40V
(relative to the back of the backbox). The +5V test point is showing
about 0.6V. I measured the voltages coming from the transformer on the
wires that plug into 3J1. All the DC voltages are correct. The AC
voltages (pins 10 and 11) show 10VAC relative to pin 12, which seems
low but is most likely due to the 200V minimum range on the cheap
multimeter I had to buy (since my good one is on the other side of the
country :) ). When the plug is connected, pins 10 and 11 still show
20VAC relative to each other, but show 0V relative to pin 12. The
diodes in the bridge rectifier test okay on the board.
My guess is there's some sort of ground problem, but I'm not sure where
to look. Can anyone suggest what would cause these symptoms, or give
me some ideas on what else to test? Thanks in advance.
-Brian

[b***@yahoo.com]

Hi Ray,

Thanks for your reply. The voltage between the two AC wires on the
12-pin input connector is around 20VAC (both are around 10VAC relative
to pin 12, which is ground on the schematic I have). I double-checked
the two 7A SB fuses, and double-checked the rectifier with the fuses
out. Everything looks good there. All filter caps were replaced when
I had a 5V problem a few years ago, and none look blown.

While checking continuity between the connector pins and the rectifier,
I noticed something that seems odd to me. With the power off, there is
continuity between the two AC wires on the plug side of the 12-pin
connector (pins 10 and 11) and pin 12 (ground). Is this normal?

Thanks,
Brian