(Topic ID: 270033)

Taxi CPU Voltage Issue

By pmurphe

3 years ago


Topic Heartbeat

Topic Stats

  • 33 posts
  • 6 Pinsiders participating
  • Latest reply 3 years ago by pmurphe
  • Topic is favorited by 2 Pinsiders

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  • Taxi Williams, 1988

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2020-06-17 20_00_32-Taxi CPU Voltage Issue _ Tech_ Alpha-numeric _ Pinside.com (resized).png
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Taxi MPU battery corrosion (resized).png
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TAXI POWER SUPPLY 1 (resized).png
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#5 3 years ago

With the MPU board connected...
Meter set to DC volts
Measure relative to ground, pins 4 and 5 of the LM723 5V regulator.
If the measurements aren't (almost) identical, then the regulator has failed.
LMK.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact ... for board repairs
http://www.PinWiki.com - The Place to go for Pinball Repair Info

#7 3 years ago
Quoted from pmurphe:

Failed regulator presents itself under MPU load? And can look good under no load? Guess next step is to replace LM723?

Correct and correct. Have at it. Don't forget to pick up a socket.
--
Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/contact
http://www.PinWiki.com - The new place for pinball repair info

1 week later
#11 3 years ago

Well, that isn't what I suspected.
If this is happening while the sound board is connected (which is providing a load as mentioned above) and only manifests after connecting the MPU board, then it is possible that something on the MPU board has shorted the 5V.

Any corrosion on the MPU?

Power off.
Remove 1J17 (power connector) from the MOU board.
Meter in diode test.
Red on ground.
Black on 5V test point.
Reading should be about .300 or so.
What is your reading?
--
Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/contact
http://www.PinWiki.com - The new place for pinball repair info

#14 3 years ago

OK. That reading is in no mans land.
Let’s see some pics of the corrosion.

Chris Hibler - CARGPB #31
Http://chrishiblerpinball.com/contact
http://www.PinWiki.com/ - The new place for pinball repair info

#19 3 years ago

I measured one of my S11 boards the way that I asked you to.
It measured about the same...so you are out of no man's land and back on the island.

There is some corrosion, but I don't see how that can be the cause for dragging down the 5V. We really need to isolate between the MPU and the PS.

Let's pull that PS and diode test the bridge.
Meter on diode check.
Black on the "oddball" leg.
Red on the flanking legs. Should read about .5 plus or minus.
Now move the red probe catercorner from the oddball leg.
Black on flanking legs. Should read about .5 plus or minus.

As PINTEC mentioned, you may have a partially open bridge.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact ... for board repairs
http://www.PinWiki.com - The Place to go for Pinball Repair Info

#23 3 years ago
Quoted from pmurphe:

Bingo! Replaced BR1. 5.04V at the CPU test point. pintec and chrishibler you know your stuff. Thanks a million. You guys are awesome.
Should I do anything preemptive about that corrosion? Or wait to see if issues develop?

Yay! Glad this one has been vanquished!
I couldn't see the corrosion very well in the pics. If there is just a small amount, you can sand it off, or use a fiberglass pencil to remove the corrosion. If it's more pervasive, then the methods will be more involved.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact ... for board repairs
http://www.PinWiki.com - The Place to go for Pinball Repair Info

#27 3 years ago
Quoted from PinballManiac40:

Posts 17 and 18 didn't catch your eye?

I looked at them but it was a bit blurry. Corrosion is always worse than it appears in pictures.

Chris Hibler - CARGPB #31
Http://chrishiblerpinball.com/contact
http://www.PinWiki.com/ - The new place for pinball repair info

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