(Topic ID: 229485)

Big Guns switch column out...so close to remedy..,maybe

By lpnsocal

12 months ago

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  • 8 posts
  • 2 Pinsiders participating
  • Latest reply 11 months ago by wayout440
  • Topic is favorited by 1 Pinsider


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InkedInkedbg 1J8 schematic_LI 2 (resized).jpg
b6289313c534734fd057552bf1a5358692a79f7e (resized).jpg
Williams_1987_Big_Guns_ pg 51.pdf (PDF preview)
Inkedbg 1J8 schematic_LI (resized).jpg

#1 12 months ago

Ok, I made it pretty far(so I think) without having to call on the experts, but alas here I am. It's column 5(grn-blk) and initially I found two disconnected diodes, repaired them and tested the rest, all good. Still no column switches so I check continuity from switches to connector 1J8 and then to first component past it, all good. There is a small inter connect board but the 1J8 wires go straight to pf. Then I check continuity between all components up to leg 5 on u40, all good. R77 tested good, so Then I check Q48 w/ diode test, red lead middle leg/blk outside legs and got the same results as surrounding transistors, 670 +/-. With options running out/along with my skill set, I wip out my latest lil tool, logic probe! Ok, now I'm testing the connector attached and the U40 legs while in game play mode, with my 3 and 4 player displays out this way seems easiest.. Connector results w strobe all Lo, cept for pin 5 which is Hi. All the corresponding output legs strobed Lo.

This is where the big dumb blank stare happens upon my face..., I figure I'm close but, I don't know.


Williams_1987_Big_Guns_ pg 51.pdfInkedbg 1J8 schematic_LI (resized).jpg
#2 12 months ago

You didn't mention checking the 4.7K pullup resistor SR15. If this is shorted, or resistance is low, you'll be sticking the line high because it will be pulled up to VCC. Check resistance from pin 5 to pin 1. It's in circuit so compare this to a measurement of pin 4 to pin 1, if it is significantly low then replace the pack. If it is close to the same, U40 is suspect at this point.

b6289313c534734fd057552bf1a5358692a79f7e (resized).jpg
#3 12 months ago

Well, this is interesting. I checked the resistance on the SR15 and what I thought was pin#5 was a 4.65k similar to the rest except for the 4th from the end(#7?) and it was 1.65K. ( BTW, There is a dot at one end of SR15 and it seems to be tied into a larger trace so I call that #1.) Normally a small celebration would have ensued but that wasn't where I expected to find the low reading!

So now what? I can change U40 and this component but would have prefered to see the fault for a lesson to be learned.
Thanks for the help/lesson.

#4 12 months ago

Are you certain you didn't misinterpret the part orientation?....almost seems like you might have counted in the wrong direction when flipping the board over. And yes, pin 1 is marked with the dot. Keep in mind that pin #6 is not used.

#5 12 months ago

I tested SR15 w board in so pin#1 is to the far rt. But and it's a big butt..., I now understand how I ended up down the blind alley. When I first looked at schematic and subsequently circled in green what I thought was for 1J8-5(column 5) but I actually circled pin #4 and tested as if. The odd reading is where it's supposed to be! and maybe as important, when Q43 was tested a short was found on the rt leg! So at the very least Q43 is bad and maybe the SR15(?) Still not sure why leg 5 on U40 strobed Hi, but considering my poor vision and schematic following skills...,who knows?

InkedInkedbg 1J8 schematic_LI 2 (resized).jpg
#6 12 months ago

You seem to be on the right track now. Just make sure to retest after changing components before powering up again.

#7 11 months ago

I replaced the Q43 and SR15, tested components and continuity, fired it up and column works like a champ. I'm sorry I can't follow a simple(?) schematic, but your help kept me asking questions until I found my screw up. I hope there comes a day when I feel confident enough to assist others, beyond my "check fuses and continuity" skill set.

Again, Big Thanks!

#8 11 months ago
Quoted from lpnsocal:

Again, Big Thanks!

Your very welcome. This online troubleshooting forum is a long ways from what I can accomplish by helping in the flesh, but with the ability to patiently ask questions back and forth we can help move a troubleshooter into the direction of success. Where would we be without Pinside?

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