gottlieb system 1 - no displays

scoots

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(i made this from memory, i can't guarantee the wiring here is really correct but it should be good enough for our discussion)

so the transformer IS putting out the correct voltage (70v across the lugs).

i replaced the fuse holder and all the wiring/pins connecting fuse holder, transformer, and the 69v pins on connector A2P1 (i used the schematic, shop manual, and DMM when i did this, not my faulty memory ie i believe this wiring to be correct whether my drawing above is or not). all ground mods have been carried out. the big filter caps on the PS were replaced with beefier parts. the bottlecap transistor on the PS has been replaced (blew it trying to jumper the fuse holder, dumb).

problem(s): 1/4A fuse blows when connector A2P3 is plugged in. if connector A2P1 is unplugged, voltages on the 69V pins is only around 30V each. many diodes on PS showed a reading in both directions on diode test, but i was testing them in circuit.

everything else in the game works properly, just no displays. at first they would flicker in and out a bit but the credit display (42v) was still good. now there is no life any any of them, period.

i am a noob with no real experience trying to attack this as logically as possible but google searches aren't helping me at this point. can someone suggest a plan of action or a logical place to look for problems? i don't have any other known working parts to swap out the displays or PS etc. thanks.
 

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ARe you using a ni-wumpf or the original cpu board?

If original, it might be the U6 or the Z16 or Z17
 
also, not quite sure what those parts you referenced are..?

here is a schematic of the high voltage section of the power supply for reference.
 
Unplug all the displays (with the power off naturally). Now power on to see if that fuse blows. If it does not blow with the displays all disconnected, then you probably have one shorted score display. If it still blows then there's a problem on your power supply board.
 
Unplug all the displays (with the power off naturally). Now power on to see if that fuse blows. If it does not blow with the displays all disconnected, then you probably have one shorted score display. If it still blows then there's a problem on your power supply board.

is there any test i can do on the displays without powering them up to tell if they are shorted? i'm on my last fuse here and can only get them via mail order.
 
i have checked both of those sites many times. i had tried unplugging the displays before but i think the fuse might have been blown without me realizing it. the displays look ok visually so i guess an internal short is what i'm looking for.
 
There could be a shorted IC on one of the display boards. Also check the connectors for each display as there could be a bent terminal touching an adjacent one.
 
ok, got some more 250mA fuses in so was able to look at this again briefly last night.

ALL voltages on A2P3 are correct. i'm satisfied that the transformer and wiring from it to the PS are good. however, even with all displays unplugged, the 1/4a fuse blows when A2P3 is plugged in. did a lot of googling this morning and it is leading me to believe the power supply has to be the culprit. i have a few specific things to check based on others' experiences (shorted bolts on the transistors, and the 1N4004 diodes that make up the rectifier in particular).

can't believe i'm going to have to unsolder that @#%* TO-3 from the mounting bracket again, argh. what an awful design choice.
 
progress.

no longer blowing the 69v fuse. cleaned up an accidental solder bridge, replaced a couple of blown diodes (CR3/CR4) and the four diodes that make up the bridge rectifier on the power supply (CR6-9). now instead of dead displays i get this:

http://www.youtube.com/embed/4j0Atzy2qm4

checking the displays in the test menu shows no improvement. something is "working" because they change while scoring, but nothing usable. plugging/unplugging the individual displays didn't improve anything. all connectors are good and clean and snug.

noticed Z16 and Z17 (the 7448 decoders) on the MPU were screaming hot, made me wonder, though the game and sound proms were also hot and they still seemed to be functioning fine....

just found this regarding someone else's issue, but seems like it could also apply here:

"Since you said the problems show up same on players 1 and 2 and same on
players 3 and 4 - you need to look at digit strobes, not segment drivers.
Digit strobes are driven by Z18-Z21."

Z18-21 are 7408's

wondering if i want to just shotgun this (literally and figuratively ;) ) or try to learn how to use a logic probe.
 
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