Hi all.

I need some help with diagnosing a lamp issue on JD. All lamps on column 4 are not working. All of these lamps are on the large lamp board in the bottom centre of the playfield. I have checked all of the bulbs are actually working - they are. I have checked all of the diodes and solder joints on the lamp board - all good. Solid 18v is present at TP8. I have checked for continuity from J137-4 in the backbox to each of the lamps - all good. I have also checked the transistor at Q95, which controls this column - all good. I also did the jumpered lamp socket test (between J133 and J137), and all columns lit the test lamp properly.

I'm not sure where to go with the troubleshooting from here. I suspect that I need to start looking at the ICs on the power driver board, but I do not have a logic probe.

Thanks for any help.

Initially I found a couple of cold solder joints on the lamp board and reflowed some solder onto them. So the lamp board is 100%. I will recheck the connector and the connector on the power board.

I checked the TIP107 two ways. First, I checked for continuity between the tab of the transistor and TP8. There was no continuity. Second, I used diode test to check each of the transistor's legs. They returned similar values to the TIP107 next to it, which was working fine, so I took that to mean the TIP107 was OK.

I will check for the 3V and see what I get. However, I know that the wire from J137 to the lamp board is fine. I will check the row return as well while I'm at it.

If I am understanding you correctly, if the socketed lamp test works, then all lamp matrix components on the power driver board should be working 100%. Is that correct?

Aha! I made some progress today.

I redid the socketed lamp test. Turns out I did it incorrectly the first time. Column 4 does not turn on the lamp. I took a video of the process:

So let me know if this thinking is correct: according to the schematic the problem must be the TIP107, ULN2803, LS374 or LS240. I tested the TIP107 and got good readings, as I did last night. I've attached some photos of the tests.

So how do I go about testing the other components? Which is the most likely culprit?

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I just noticed something strange when testing U18. In diode test, two pins do not return normal values. Looks to me like a short. Could this be the issue?

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It looks to me like that pin relates to Q96. Or am I not reading the schematic properly? The pin from the first image seems to be related to Q98. Please correct me if I'm wrong.

Regardless, I will check the LS240 chip tomorrow. If it also gives me bad readings, should I replace both of them, or just the LS240?

Hi zaza, sorry, I'm not sure how to verify where the short is coming from once I clip the pins. How should I do this?

OK, I think I understand. So when one of the legs is clipped, that should isolate the short to just one pin - is that correct?

OK, understood! I assume it is OK to desolder U18 completely and then test it while out-of-circuit and then test U9 while it is in-circuit? That way I won't damage pins on a potentially good chip.

Yep, that is my plan. I just bought a nice soldering station with a desoldering gun so I'm eager to try it out. I will give this a try tomorrow and report back.

I desoldered U18 from the board. Even with a desoldering gun I lifted a few solder pads. How are you supposed to avoid lifting the pads? I could see light through every hole and desoldered for a good length of time on each pin. I made sure as much solder was off as possible. Is lifting some pads off unavoidable when you take an IC out like this? Very frustrating. I know you would normally clip the IC off the pins but I did not want to ruin a good IC.

But now that U18 is out, all of the pins give good readings. Multiple U9 pins give bad readings on both sides of the IC. Does this just mean U9 is bad, or do I need to test further up the chain?

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Maybe I'm not testing it properly. I'm testing it the same way I tested U18, in diode test with one probe on ground and the other probe on each other pin. Is that the correct way to do it?

OK, it looks I wasn't measuring properly from ground the first time. I get normal readings on all pins except Q3 which reads 0.32 in diode test. So it looks like U9 is bad to me. What do you think?

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Hi Mike,

Absolutely agree. I usually clip the pins and remove the IC first. But this was my first try with a desoldering station; I was confident I could do it cleanly without having to destroy the IC. Turns out I saved the IC and can reuse it like I planned to, but it wasn't worth the damage I caused. I have had good success with just a solder sucker previously. I just thought a desoldering gun might be able to do it in one go. Lesson learned.

Good point. However, I only do these kinds of repairs quite rarely so I can't justify the expense. It definitely would have made things easier, though.

I finally got my replacement ICs and installed them into JD yesterday. Success! All lamps now work properly.

Thank you all for the advice.

An epilogue to this thread.

Today I was repairing a Getaway with a similar issue. No lamps on column 5 were lighting up when they should have been. Instead, they would be lit by lamps in other columns.

Straight away I remembered this thread and the advice I got here 3 years ago. Since that time I've done many repairs with a logic probe and soldering station. So, I managed to narrow down the issue by checking for activity on pin 2 (input) and pin 18 (output) of U9 with my probe. Pin 2 was good, but pin 18 showed no activity at all. Totally blank. So, U9 was bad. I confirmed this when I removed it and tested it with my meter. Moreover, I did the repair much more cleanly this time; no broken traces!

Just goes to show you can learn a bit in 3 years

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