I just finished a complete teardown rebuild of my Earthshaker- super happy with it; plays like new!

I have one outstanding issue with display 2: The "b" segment on display two is always lit (see picture.)

b segment on display 2 always lit

This is the troubleshooting I've done so far:
-Verified continuity for this segment from J5 pin 8 on the display board all the way back to U51 pin 11 on the CPU board so I don't believe it's a bad wire/connection or broken trace.
-This b segment stays lit even if I completely unplug J5 from the display board- This leads me to believe it's an issue on the display board itself.

Any suggestions from this point? I believe this segment passes through U20 on the display board but I'm unsure of how to test that. (I do have a logic probe on the way if that would help.)

Thanks in advance!
-Mike

The only other thing I noticed was that the leg (4) for segment b was pressed up against resistor 68. Wouldn't think that would cause a short but adjusted it away just in case. No change.

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Thanks mbaumle - I just went through the procedures on the link you provided testing the UDN chips in diode mode but they seem to be fine.

One other thing I just noticed- R19 and U5 appear to have been previously replaced. I hadn't even been looking on that side of the board. They appear to test ok... wondering if they could be somehow related though.

Am I making the right assumption that if the segment is still on even when the related cable/connector is removed from the display board that it's definitely an issue with the display board and not something in the CPU board?

GRUMPY - R-28 is measuring at 1.3k. The other in that same bank are ~17k. I'm color blind and don't trust my eyes to decipher the color bands on the resistors but I'm assuming R-28 is supposed to measure 17k as well? I just checked pin 14 and 15 on U-11 and neither is pulsing while in display test (compared to pins 3 and 2 for the a segment which both had pulses.)

PinballManiac40 Didn't notice any obvious corrosion on the MPU; just tried taking a picture with cell phone but quality not good enough; will need to wait until I have my nice camera. This particular segment doesn't go through the ribbon cable between the MPU and display board, it's on a regular harness (1J3 from the MPU)

Also, first time using a logic probe! woot!

Thanks all for the assistance!

I measured with J-5 disconnected last night, but today when I double checked with all cables removed from the board today both R-28 and R-29 measures ~12k ohms. I'll replace both of those.

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I replaced R-28 and R-29 this morning... however there appears to be something else going on. Once I removed R-28 and R-29 both resistors tested fine (~18k ohms) out of circuit. I replaced them with new since they were already out but the readings in-circuit on R-28 and R-29 are both still low- 12k ohms. I'm guessing this means there's a short somewhere? I don't see anything amiss following the traces from both resistors- is it possible the short/problem is in the U-11 chip itself?

Thanks GRUMPY - For the U-11 chip I believe it is this 4050 chip https://www.greatplainselectronics.com/proddetail.asp?prod=4050. Not nearly as easy to replace....

Display 1 runs through a different chip than U-11- On the equivalent display 1 chip I get pulses with a logic probe- On U11 I'm not getting any pulses for the b segment on U-11. Pretty sure that means my problem is higher than the UDN7180.... wouldn't say 100% sure though.

-Mike

Turns out they are 4049s. Had some 4050s on hand but will need to order the 4049s. Are all 4049s interchangeable? I.E. can I replace the TC4049BP that's currently on the board with a CD4049UBE?

My 4050s came from GPE . . They're closed until the New Year. Based on some research I did earlier today it seems that 4049s are interchangeable; ordered some today from another source that will hopefully be here this weekend.

Success! Replacing the 4049 at U11 resolved the issue. Thanks to everyone who responded, especially GRUMPY for the guidance!

The original chip at U11 was a TC4049B. I replaced it with a CD4049UBE, which I can now confirm is interchangeable.

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