Why does a simple fix turn into a hair pulling exercise?

So... the joke starts like this. I decided to swap #3 & 4 numeric displays because # 3 was no bueno and included #4 because I wanted the 2 matching/working ones from a Space Shuttle that I'm going to upgrade. I power it up, and happy to see all displays working, but unhappy to see the #4 10s zero staying on and actually getting brighter when the display presents all the digits lit! So now my new problem exists where there was no problem before. Let the hair pulling begin....So I swapped connections and lit zero moves to #3 display. I swapped complete ribbon cables and it stays on #4. At this point I'm thinking it's in the display board so I take my limited testing skills and check everything my Fluke will let me, which is only resistors since there are no diodes there. They all test good. I tried to read the schematic from a downloaded manual and it was a challenge due to questionable clarity and the display circuit is a tougher animal to decipher. I did look for any previous Forum topics with similar display issue(s) but no luck. Maybe I missed one? Anyway, my crazy guess would be a short somewhere but I don't know where to look at this point. I'm not even going to ask why it happened with the replacement of the new displays, kinda moot right now. If anyone knows of a forum topic that has some good info, please share. If anyone knows the easy or not so easy fix, please share. Or...at least a finger pointing in the right direction.
Much Obliged..!

Peter thanks for reply and direction. The Road Kings schematic was helpful as well.

I did attempt an in board check of U2 against the values of the other 3(U1,5,6) by placing my red lead on pin 1(left of notch) and checking down all the legs. All values were the same on the all the left sides and pin 10(bottom rt from notch). However, on U2 rt side legs/pins I got 119k ohms against U1=133k, U5=135k and U6=137k. Interesting note, pin 10(bottom rt from notch) measured 117-119k on all 4 chips and pin 11 consistently measured 2-3k less than other legs on rt side(pins 12-18.)e

I would not have thought a 10-12% resistance variation would be evidence of a short, but since there is an obvious difference and U2 has some responsibility for #4 display then replacing that chip is in order. I'll have to order a couple online and see what happens. Since I am placing an order can you think of anything else I should buy as insurance or preventative maintenance on this board( or the CPU)?

Thanks again!

I found I had a UDN 6118A, checked it with the multi meter and interestingly it would not hold a steady reading on the rt side legs. Regardless I installed it and checked readings again and oddly they were similar to the readings of the one I removed. I put the board in the machine, flipped the switch and no more rogue zero! Good Call on the U2!
My machine already has the fused BR's and remote batteries, but I did read about the voltage reducing diodes and think it's a great idea.

I'm down to my last issue on this machine, special solenoid #6 top pf flash lamps don't light. PF circuit is good as I tested it using a different solenoid # and the lamps lit during test. I also have connector continuity to the nearest component Q79(TIP122). I checked the Q79 and Q78(2N4401) values against others in the board and got very similar readings. Also local resistors checked out fine too. My guess is to replace both Q78,79 and see what happens, after I order them.

Thanks again for all the help and direction and feel free to chime in on my solenoid problem/solution!

Ok...still without my flash lamps. First, the board is an 11A, not sure it matters. Before I put in the new TIP122 and 2N4401 I swapped U45 and 50 as they were in sockets. No change to problem so assumed they're good. With new parts in and the problem still persisting, I double checked all the components I could(in board) related to Special Solenoid 6 in circuit with Q78, 79. All tested fine. I also tested continuity between them, also fine. I double checked continuity from TIP122 through connector all the way to 1st bulb socket, also good.

I can only think that maybe the relay K1 has a bad switch in it. Also, I have no idea what the fat striped line labelled 2D1 does/goes to?

Thoughts?

And, Ive not checked/changed power supply caps, another good idea, once my hair grows back.

Grumpy, Thanks for chiming in! I see that on the schematic now, it's been a chore reading them without a manual. I've been trying to find the clearest copies on IPDB where they still have some clarity after I blow up to 250%. For some reason, I thought the SS were handled by K1, nevertheless I went looking for the source of the org/blu at 1J18, pin 9. Traced to the big black connector and that's where it ended. No corresponding wire on female side of connector. I have a parts PF and it is missing it also. I verified correct wire from 1J18 to connector with my meter, so I'm stumped. I would have thought that there would have been something like a K1 relay somewhere else. All other solenoids(from coil test) do their job. This machine is new to me so I'm learning the good , the bad and the ugly, albeit without the appropriate music.

I'm assuming you mean the wire to nowhere(pin at 1J18 pin 9), so I'll give it a quickie tomorrow.

Thanks again.

Thanks for that direction, they lit. I guess that should confirm my board circuit is good but without 1J18 ,9 coming from a switch it'll never flash. I guess that's my next step, finding the missing org/blu wire source/route. You don't have a HS close by do you?

I thank you in advance!

Ok, all those switches check out(in switch test with appropriate # showing on display), tho I don't see the relation to SS#6.

Further confusing me is a conversation with a HS owner. He says his 1j18-9 org/blu ends in the connector as well, but his flashers work!

Thanks for confirming the wire to pf does nothing. Test function for SS#6 in coil test failed. I have a socket at my U49. Is there a way to check it with a multi meter when the chip is out?

I gotta buy a logic probe but, I finally found an analog MM and I grounded the other lead to one of the board mounting screws. I checked U-49 pin12(14 is rt of notch,so 3rd back) I got a solid reading, with no needle flutter. Which now means.......?

I'm sorry, yes. went into tests/coil test/ #22 upper pf flashers(#6 special.) Tho I had no way of knowing if it was pulsing, like all the other coils when they fire. I guess the needle would have indicated as much?

I should also say that the MM was set on the lowest scale for VAC, 10 I believe, and the needle was pegged near 10.

Let the learning curve continue!!! Ok, grounded to same place, needle pegged. 4.2+/-

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Interesting.., pin 13 had same results. changed coil test to sol 20 and checked pin10, same result, no needle movement as pop bumper was firing. The photo could be used for every test made.

I ordered up one of the Pinsider recommended Logic probes from Amazon, should be here by wkend. They look simple enough to use(famous last words!) I'll run through the same tests on the pins and maybe our culprit will be revealed.

I received my new tool/toy and read terryb's tutorial on logic probes and optimistic with a we bit of knowledge and my probe, I proceeded to duplicate the previous MM tests.

I garnered from the tutorial I was dealing with a TTL situation(5v and 7407), so I found a 5v source and verified w/ MM and proceeded with tests. Both pin 12 and 13 registered high, no pulse. Repeated test on sol #20 pin10 and same results. The photo could be used for any of the tests.

It's hard for me to make any conclusions other than this chip seems to be ok. Insofar as a working solenoid and non-working one produce the same test results and the trouble should be down stream. However, the down stream components were tested or replaced/swapped(U45/50.) My limited experience is probably shining brightly thru my thought process, but that's all I got

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Ha.....What a bonehead I am! Here I mentioned in post #21 how I should be counting the pins(using the notch) and I failed to do it correctly in every test I made, repeatedly doing it wrong! Thank you for the BIG chalk board reminder, we'll see if I can't get some real results!

15 min later....New results......sorta. Pin 12 and 13 looked just like the previous photo. However, our working #20(pin10) was pulsing and by that I mean the hi is lit up until the low is lit when the coil fires. So I'm very excited to see a difference. but I'm thinking that means we have to go to the big U54 now?

I set up the logic probe the same way and left it on TTL. I checked pin 39(one down from notch on rt side) and the reading matched others as Hi. Am I to assume that this chip is the source of the pulsing that is supposed to be occurring?

Based on the manual diagram that I referenced, U54 is on upper rt side of the board installed under the Q78 and Q79 that I previously replaced. I did not check U51 which is actually closer to U49. So I'm fairly confident, despite my past mental lapses, that I checked the correct chip.

I know it's a relatively unimportant flasher and I did consider throwing in the towel after the Q78,79 replacement, but I think it would be a shame not to finish after such a great lesson. If it's just a matter of careful removal of old chip and installation of a socket and new chip, I think I can handle it. I do have a Hako desoldering gun and Weller soldering station to assist me plus parts on hand!

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Let me first say that removing the U54(40 pin) chip and carefully cleaning old solder and leg remnants is quite the chore. Putting in the new socket was comparatively easier albeit requiring some delicate soldering. Having the right tools gave me additional confidence too. After the socket was in I tested all the traces to make sure I had good continuity to the next components. I thought I had screwed up when I was getting a no resistance tone between #3 and 4pin sockets, but was relieved to see the other 6821s had the same reading at those pins. Now for the bad news, after I put the new 6821 into the socket and then reinstalled the board in game, I proceeded with the coil test. Working through the coils one by one making sure they're all working I get to #21(upper left bumper) and it doesn't work..., WTH!!! Then on to the reason for all this fun #22(top flashers) and...nothing. My heart sank..., what the heck could I have done? From 1 solenoid not working to 2...., I was so careful going painfully slow and methodical in my work. I needed to walk away. I had a couple cocktails and forgot about the machine for the rest of the evening. New day and new confidence in my work. What if it wasn't anything I did or didn't do? I pulled the 6821 from the socket and put in a different one. I fired up the machine and proceeded thru the coil test, fingers crossed! #21 fired..,and then #22 flashed! Now there's the Good News I needed! I played a couple games and all appears to be fine.

GRUMPY and Inkochnito Thanks so much for the the direction and help with both my issues, lessons well learned .

Cheers..., Internet beers for everyone!!!

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