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(Topic ID: 270033)

Taxi CPU Voltage Issue


By pmurphe

4 months ago



Topic Stats

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

You

Linked Games

  • Taxi Williams, 1988

Topic Gallery

There have been 9 images uploaded to this topic. (View topic image gallery).

IMG_9606.jpg
2020-06-17 20_00_32-Taxi CPU Voltage Issue _ Tech_ Alpha-numeric _ Pinside.com (resized).png
U42 (resized).png
Taxi MPU battery corrosion (resized).png
TAXI POWER SUPPLY (resized).png
TAXI POWER SUPPLY 1 (resized).png
IMG_9602.jpg
IMG_9595.jpg
IMG_9594.jpg

#1 4 months ago

Having an issue with my Taxi. New to me 9 months ago. Was completely working but after a night of being left on would not work or boot. When power turned on, a couple solenoids loudly buzzed and the bell rang weakly and continuously. Testing has led me to a CPU voltage issue.

Items checked:
Voltage from Transformer:
White-White= 92.6Vac
Grey-GreyWhite = 10.5Vac
GreyGreen-GreyWhite = 10.4Vac

Cable to 3J3 good, minimal resistance, minimal voltage drop
Cable from 3J1 to 1J17 good, minimal resistance, minimal voltage drop

Disconnected all connectors from CPU except 1J17
CPU voltage at TP2 = 1.5V

Power Board Voltages With CPU Attached:
TP1 = 1.5V
TP3 = 2.78V
TP4 = -13.68V

Power Board Voltages without CPU Attached:
TP1 = 4.97V
TP3 = 13.78V
TP4 = -14.00V
Assume Power Board is good. (?)

Kind of a newbie, but coming up to speed quickly and actually having fun researching and learning. Engineer by trade, no problems getting my hands dirty. Over past couple of weeks, have studied manual thoroughly, read as many related posts as I could find here, on PinWiki and System 11 Repair Website but at a loss as to what on the CPU could be drawing down my Power Board voltages. Should I assume Power Board is good? What should I test next? Any help would be much appreciated.

#2 4 months ago

If it hasn't been done already, it wouldn't hurt to try replacing C8 (47uf/50v) and C10 (18,000 uf/20v) on the power supply board and reflowing the solder for the header pins on the power supply. Those caps are for the 5+12v supplies.

I agree it seems like the cpu board is "loading down" the 5 and 12v supplies. Not really sure what else to recommend besides the caps etc on the power supply though. Hopefully someone more knowledgeable will chime in.

Good luck!

#3 4 months ago

Whats the voltage at the connector that plugs into j17 at the cpu with the cpu unplugged. J17 pins 4,5,6.

#4 4 months ago
Quoted from frunch:

If it hasn't been done already, it wouldn't hurt to try replacing C8 (47uf/50v) and C10 (18,000 uf/20v) on the power supply board and reflowing the solder for the header pins on the power supply. Those caps are for the 5+12v supplies.
I agree it seems like the cpu board is "loading down" the 5 and 12v supplies. Not really sure what else to recommend besides the caps etc on the power supply though. Hopefully someone more knowledgeable will chime in.
Good luck!

When I thought it was solely a Power Board issue, I ordered these parts and plan to install. Will reflow 3J1 header. Thanks.

Quoted from newtoit:

Whats the voltage at the connector that plugs into j17 at the cpu with the cpu unplugged. J17 pins 4,5,6.

4.97V for pins 4,5,6. 1J17 header looks good, but will reflow to remove any uncertainty there.

#5 4 months 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

#6 4 months ago
Quoted from ChrisHibler:

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

Thanks for the help.

With MPU attached:
Pin 4 = 1.68V
Pin 5 = 2.63V

Without MPU attached:
Pins 4,5 = 4.97V

Failed regulator presents itself under MPU load? And can look good under no load?

Guess next step is to replace LM723?

#7 4 months 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

#8 4 months ago

Awesome. Thanks for the socket reminder. Would have forgotten. Will replace and report back. Really appreciate the help.

1 week later
#9 4 months ago

Took a while to get the parts. Installed new LM723 (and new socket). Pretty much same result:

With MPU attached:
Pin 4 = 1.32V
Pin 5 = 2.68V

Without MPU attached:
Pin 4 and 5 = 5.07V

Thought on what to check next?

#10 4 months ago

Low Voltage drop at TP1 5V
and TP2 12V while cpu connected to Power supply:

Isolate Power supply from cpu:

Make a dummy load:
Connect a 6V pinball lamp to 5V output on power supply, Normal power will light bulb 60%
while connected check for voltage drop at test pins 1 and 3
No Light? Suspect bad Cap as mentioned C10 and or faulty BR1 bridge rectifier.

#11 4 months 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

#12 4 months ago
Quoted from PINTEC:

Low Voltage drop at TP1 5V
and TP2 12V while cpu connected to Power supply:
Isolate Power supply from cpu:
Make a dummy load:
Connect a 6V pinball lamp to 5V output on power supply, Normal power will light bulb 60%
while connected check for voltage drop at test pins 1 and 3
No Light? Suspect bad Cap as mentioned C10 and or faulty BR1 bridge rectifier.

Will rig up and try.

#13 4 months ago
Quoted from ChrisHibler:

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

Not sure if prior battery leakage issue. Looks pretty clean to me, but I have an untrained eye. Could be as there appears to be a fairly new battery holder. I've since move batteries off board. Can send pics if that helps.

Reading is 0.256.

#14 4 months 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

#15 4 months ago
Quoted from ChrisHibler:

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

Yikes. Never like being in no mans land. Here are the pics...

IMG_9594.jpgIMG_9595.jpgIMG_9602.jpg
#16 4 months ago

Check Fuses 4 and 5 they supply AC to the Bridge rectifier which divides -12V and +12V
Suspect a Bad Rectifier with may have partially shorted internal diodes on the + 12v side
5V cannot be made if less than 12v from BR1 under load is going into the regulator (which also splits off for 12V+ unregulated output) isn't there (Garbage in Garbage out)

TAXI POWER SUPPLY (resized).pngTAXI POWER SUPPLY 1 (resized).png
#17 4 months ago

The board definitely has corrosion within some traces as you can see how dark they are and U41 has several corroded legs. Once you remove the battery holder, you'll see how far it moved along the traces. I would be worried it even got up the legs of SRC2. Taxi MPU battery corrosion (resized).png Fumes even hit some pins on U42. U42 (resized).png

#18 4 months ago

I can even see the corrosion from this angle.

2020-06-17 20_00_32-Taxi CPU Voltage Issue Tech_ Alpha-numeric Pinside.com (resized).png
#19 4 months 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

#20 4 months ago
Quoted from PinballManiac40:

I can even see the corrosion from this angle.
[quoted image]

Thanks for pointing out the areas. Will have to address these areas, if possible. (?) My eye is becoming a bit more trained. I will remove the battery holder and see what lies underneath. Always planned on installing NVRAM vice off board batteries.

#21 4 months ago
Quoted from PINTEC:

Check Fuses 4 and 5 they supply AC to the Bridge rectifier which divides -12V and +12V
Suspect a Bad Rectifier with may have partially shorted internal diodes on the + 12v side
5V cannot be made if less than 12v from BR1 under load is going into the regulator (which also splits off for 12V+ unregulated output) isn't there (Garbage in Garbage out)
[quoted image][quoted image]

Quoted from ChrisHibler:

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

Thanks for the pointers and direction.

OK back to the PS. Rechecked fuses 4 and 5. All good. (replaced anyway) I think we are getting somewhere. Of the 4 test positions, only 1 measured 0.55. The other 3 were open loop. I have a new BR1 on order. Will install and retest.

I am converting a spare PC power supply into a bench power supply to try to see if I get the same behavior. If I get good power on the MPU test points, will further confirm PS issue.

#22 4 months ago

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?

#23 4 months 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

#24 4 months ago
Quoted from ChrisHibler:

I couldn't see the corrosion very well in the pics.

Posts 17 and 18 didn't catch your eye?

#25 4 months ago

Glad to hear it is running, but I am interested in hearing what 12v and -12v now measure on the power supply.

#26 4 months ago
Quoted from PinballManiac40:

Glad to hear it is running, but I am interested in hearing what 12v and -12v now measure on the power supply.

PS TP1 = 5.11V
TP3 = 12.31V
TP4 = -14.09V

Quoted from ChrisHibler:

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

Thanks. Will look to clean those up a bit. There is some on U41 and a couple of U42 pins.

#27 4 months 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

#28 4 months ago
Quoted from pmurphe:

PS TP1 = 5.11V
TP3 = 12.31V
TP4 = -14.09V

Those are pretty solid voltages. Did you already change all the capacitors on the power supply?

#29 4 months ago

I didn't, but looks like they have been replaced previously.

#30 4 months ago
Quoted from pmurphe:

I didn't, but looks like they have been replaced previously.

Picture?

#31 4 months ago

You bet.

Bulletproofing?

IMG_9606.jpg
#32 4 months ago
Quoted from pmurphe:

I didn't, but looks like they have been replaced previously.

Agreed. Congrats.

Just srubbing a board with alcohol or vinegar will not magically get rid of the battery corrosion though. I sure would hate to see this board get any worse.

1 week later
#33 3 months ago

Postscript: Still had some lingering power issues. Zero ohm resistor W6 on the Auxiliary Driver Board was completely smoked. (Gotta love those zero ohm resistors.) Replaced with a metal jumper. F7 was also blown. Fully operational now. Likely cascading effects of Power Board BR1 failure. Thanks again all.

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