(Topic ID: 218078)

Taxi display segments stuck on after unrelated mpu repair, help please


By Oldschool77

1 year ago



Topic Stats

  • 6 posts
  • 3 Pinsiders participating
  • Latest reply 10 months ago by InfiniteLives
  • Topic is favorited by 1 Pinsider

You

Linked Games

  • Taxi Williams, 1988

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There have been 3 images uploaded to this topic. (View topic image gallery).

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#1 1 year ago

So I've always noticed the my upper two pops on my Taxi fire slowly compared to the lower pop (they take a second before they are ready to pop again, it's not instantaneous like they should be), and compared to my last taxi. I finally took the time to dig into it, and found those two transistors that have been replaced on my MPU by a previous owner were the wrong value. As you can see from the pics the transistors prior must have burnt up BAD, because the board is just ate up around them....It looks hacky, but i did the cleanest job i could with what i had to work with (The before was WAY worse then my "hack" job, and the jumper was a precaution, the trace looked questionable). I replaced the transistors and drive transistors, did a general inspection of things, and put the board back in the game. Done this a million times on other system11s, never had an issue.

NOW....i have segment "d" (bottom segment on my bottom display) constantly lit all the way across the display at all times, correct brightness, not dim or in and out. I haven't had any display issues ever with this game up 'till now (had it a little over a year now). The BAD thing is, i didn't have my multi meter with me at home this weekend, so all i could really do is check for stray solder that may be shorting,and trace the path from the display back to the CPU....everything looks fine. I'll start tracing back to nite to see if i can find where the ground is.

My question is......i find it hard to believe that this is a huge coincidence that i had no issues, did an unrelated repair, and now have an issue. For the gurus out there, are there any traces around my repair that i could of some how shorted that would be causing this? I looked closely all over, and although it's rough looking, everything appears to be in tact, and no solder overflow...but if i DID overflow and create a ground, where would you recommend i look in my repair areas?? FYI the board has no current or previous Acid damage, everything is clean except around the previously blown transistors. And a new ribbon cable is on order, but I've never seen a ribbon cable turn on an entire row at once. but i guess anything is possible.

Thanks!

Incidentally my top two bumpers are still slow to recover, the proper value transistors made no change, I'll get back to tackling that after i get this display segment issue resolved.

Fyi I cannot see or post on pinside while at work so I apologize for any delay in replies.

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#2 1 year ago

Most likely bad MPU to display ribbon cable.

#3 1 year ago

Ok, using my Rollergames as a tester, I've determined that the display and ribbon cable are good. So I disconnected every plug from the mpu except the ones supplying power so it can boot and the ones to the display and no change. So I'm pretty sure I ruled out anything external to the cpu causing the segments to light.

Then I went to display test, put all "8's" on the lower display and I had 4.5 vdc on the corresponding legs of U51 that make up the "8" EXCEPT leg 13 which corresponds too "d", on that leg I had 1.28 vdc.

I don't have a logic probe, and I'm not sure why a lower voltage would cause that segment to lock on....so I'm hoping the pros can step in and help me at this point. I still think I shorted something replacing those transistors....I just don't see any evidence of that on the board.

#4 1 year ago
Quoted from Oldschool77:

\
Then I went to display test, put all "8's" on the lower display and I had 4.5 vdc on the corresponding legs of U51 that make up the "8" EXCEPT leg 13 which corresponds too "d", on that leg I had 1.28 vdc.

Did you mean U41?

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#5 1 year ago

Ok, latest update (sorry for the delay, been really busy)...

Here's everything i have confirmed is NOT the issue (I have a Rolelrgames now, so even though i don't have a logic probe, i feel like having a good system 11 to go by voltage wise, i can rule most of this out just by comparison)...

It is not the following...

Solder flecks on the back of the board
Ribbon Cable (brand new)
Display or Master display board (swapped with Rollergames, and the issues stayed with the Taxi)
Wiring from MPU to the display (checked continuity on all wires)
Power supply (rebuilt by me a while back, and all voltages are spot on)
My repairs (I know the issue didn't start until after my repairs, but i tried removing all 4 transistors and leaving them off the board this time, cleaned up the traces really well so i can visually see there is no shorting with any of the traces, verified that with a meter, put the board back in the game with all 4 transistors out, and no change)

Here's what i THINK it is...But i need someone to confirm I'm reading the schematics correctly please....

The segment in question is " d' " (lower display, bottom segment). Tracing it backwards from the display board it goes like this...

Pin 5 Jack 3 of display board to pin 5 Jack 3 of MPU

Pin 5 Jack 3 of MPU to pin 5 of SRC9

Pin 5 of SRC 9 to pin 13 of U51

And then that's where i don't know where to go next....

But i do know this....On SRC 9 the pins are all 4.7Kohm except pin 5 which is around 4.0 Kohm (new one on hand....not sure that's my issue, i feel like something upstream is pulling that pin down, not the SRC), this was done with J3 unplugged.

And when the game is on, and in display test with all segments lit, I'm getting around 1.9VDC on pin 13 of U51, the rest have 4.8'ish VDC.

So to me that says that i have a potential U51 issue....and i have a new one and socket on hand, but I'm not ready to start replacing it until i reach out to you guys a bit more to see if I'm on the right track here.

On a side note, during normal operations U51 in my Taxi is significantly hotter then the other chips, and significantly hotter then U51 in my Rollergames

5 months later
#6 10 months ago
Quoted from Oldschool77:

Ok, latest update (sorry for the delay, been really busy)...
Here's everything i have confirmed is NOT the issue (I have a Rolelrgames now, so even though i don't have a logic probe, i feel like having a good system 11 to go by voltage wise, i can rule most of this out just by comparison)...
It is not the following...
Solder flecks on the back of the board
Ribbon Cable (brand new)
Display or Master display board (swapped with Rollergames, and the issues stayed with the Taxi)
Wiring from MPU to the display (checked continuity on all wires)
Power supply (rebuilt by me a while back, and all voltages are spot on)
My repairs (I know the issue didn't start until after my repairs, but i tried removing all 4 transistors and leaving them off the board this time, cleaned up the traces really well so i can visually see there is no shorting with any of the traces, verified that with a meter, put the board back in the game with all 4 transistors out, and no change)
Here's what i THINK it is...But i need someone to confirm I'm reading the schematics correctly please....
The segment in question is " d' " (lower display, bottom segment). Tracing it backwards from the display board it goes like this...
Pin 5 Jack 3 of display board to pin 5 Jack 3 of MPU
Pin 5 Jack 3 of MPU to pin 5 of SRC9
Pin 5 of SRC 9 to pin 13 of U51
And then that's where i don't know where to go next....
But i do know this....On SRC 9 the pins are all 4.7Kohm except pin 5 which is around 4.0 Kohm (new one on hand....not sure that's my issue, i feel like something upstream is pulling that pin down, not the SRC), this was done with J3 unplugged.
And when the game is on, and in display test with all segments lit, I'm getting around 1.9VDC on pin 13 of U51, the rest have 4.8'ish VDC.
So to me that says that i have a potential U51 issue....and i have a new one and socket on hand, but I'm not ready to start replacing it until i reach out to you guys a bit more to see if I'm on the right track here.
On a side note, during normal operations U51 in my Taxi is significantly hotter then the other chips, and significantly hotter then U51 in my Rollergames

did you ever figure this out? im having some segments always lit on in my f14 trying to figure out where to go next

new display, new power supply, working ribbon cable, re-did connectors

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