interesting and rare circa 1978? or so early solid-state Coffee-Mat Star Battle cocktail pinball machine
that someone brought me to repair a few weeks ago. Not much info on the web on this one.
Transformer OK but there was no voltage at output of bridge rectifiers on the logic board.
Two out of the three bridge rectifiers were bad (opened) - replaced all 3 with beefier ones.
Now, voltage was appearing at the input of the +5V regulator, but still nothing at output.
The -10V regulator also had issues. Ordered proper replacement parts and replaced both.
+5V and -10V voltage supply were now OK, but the two player score displays only displayed '00',
middle displays were still dead, and center saucer hole solenoid was stuck on.
I also realized some CMOS chips were abnormally hot.
Replaced those 'hot' chips (and installed sockets). No more CMOS chips overheating afterwards,
but the machine still did not work properly. Pulled-out my old Micronta (Radio Shack) logic probe
and found a few more chips that had incorrect logic output for the state of their input pins.
More chips replaced. Player 1 and player 2 score displays now showing all '000000' as expected.
Whew, something bad must have occurred at some point in this machine's past.
Turns-out the issue was the center displays board's three F9368PC display driver chips - were bad.
Ordered, received and replaced (still with sockets). Eureka, 2 out of the 3 middle displays worked afterwards!
However, the 2nd display in the middle still did not work even with the new chip installed.
Quick troubleshooting indicated it did not get its expected +5V arriving on its supply pin,
although all connectors / pins were OK with no cold solder joints.
After further investigation, turns-out the P9 connector on the logic board has a manufacturing/design error,
as the pin 1 label is printed on the wrong side of that connector! Looked at the wiring and connector,
reversed it, powered-up, and whew, the center display finally worked.
potentiometers and the speaker are good, and capacitors tested good with an ESR meter.
Nothing overheating, but the amp chip did not appear to work, i.e. no hum when touching its input pin.
Ordered a TBA820 amp and replaced it. Also had to replace the LM3900 op-amp chip.
Most sounds work now, but not all - will follow-up, probably another chip either on the main logic board
or possibly one of the 741 op-amp on the sound board. In any case, I still need to replace U6 on the main
logic board as a prior tech added a replacement chip on top of the bad one and it looks like a hack - although it works!
Also fixed a prior harness wires cut that were badly respliced, replaced a few bulbs and adjusted a few switches.
Fun project for sure!
Some repairs do take time. Patience, investigation and research are key.
Keep on flippin' (and sometimes repairing
