(Topic ID: 317002)

Twilight Zone no lamp matrix lights

By In99flames

64 days ago



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  • 5 posts
  • 3 Pinsiders participating
  • Latest reply 63 days ago by PinRetail
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#1 64 days ago

Hello all. I have an issue i worked on all day and need some extra heads in on this. It has me puzzled but I think I am on the right track. On my Twilight Zone, I lost my whole lamp matrix lighting. Surrounding playfield GI works fine. Game works perfectly except the lamp matrix. I've done some work to this in the past. Power driver board has been recapped and has all new bridge rectifiers along with a new regulator. GI connectors have been re-pinned and the sockets on the board have been replaced. All this work was done a while BEFORE the issue started. I have a normally working High Speed 2: The Getaway right next to it and started to compare some values when things started to read funny in Twilight.
As of now here is what i found:
Issue is NOT in CPU board. Ive verified this already. All test points for voltages are spot on where they should be. No transistors for the lamp matrix are shorted as Intested them ALL. The anomaly ive found which led me to dig deeper, is in the voltage area where it feeds around 1.4v to the LM339 ICs at u15 and u16. I looked at the 1kΩ resistor at R162. It IS getting 5v from VCC on one side....other side of resistor is nothing. It should be 1.4v(based on schematics and verified on The Getaway)All components tested were done OUT of circuit. I tested the resistor, nothing wrong with it. I replaced it anyways. Still no 1.4v. I did check the logic chips(ls74. 374. 240. Uln2803)in this area with an oscilloscope and found no issues. So why is the 1.4v not getting by this resistor? I dug deeper. Started checking resistances of components in the area and found some odd results. I noticed that C31 that is in line with R162 and D33 and D34 has almost no resistance(like basically none .40Ω) across it to ground. That didnt sound right to me, checked it on The Getaway and we had MegaΩ across that little cap. Tested that cap....no issue. Its not a shorted cap and its value is on spec. Tested D33 and D34, still good. Using a power supply I decided to try to manually inject 1.4v to the other side of the R162 resistor and voila, lamp matrix came back blinking and doing its thing normally. This verified that the logic chips and transistors work as intended along with the cpu board sending the right signals. So it seems theres a short or almost a dead short in the 5v line somewhere. So my thought is there is a component that has failed or is in the process of failing and causing a short to ground. Am I on the right track here? Anybody else encounter this? I may have to look in the 5v origination area and see where it goes awry. Its strange because I am getting 5v to where it needs to be....im just a little burned out after looking all day. Thanks

#2 64 days ago

Still suspect C31.
Try to lift one leg and then measure Vref.

#3 63 days ago

I tried that too still nothing with it clipped. Just started plugging away again.

#4 63 days ago

Well.....found the issue. I pulled the board again to resolder C31. And it barely caught my eye. On the back of U15, i saw this(see pic).....it bridged 4 legs together. Laying right across them.There's my suspected short. Game is working normally again.. This was tiny....i wonder how long it has been this way. That caused a day to find....makes me feel ridiculous.

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#5 63 days ago

Absolutely don't feel ridiculous.

My friend Jack refused to give up on a Gorgar board. He was going to fix it! I forget how many weeks later, he had pulled every chip and every socket off the board and found one tiny bead of solder rolling around underneath a socket.

He was so disgusted with the effort that he didn't work on Gorgar boards for a couple of years after that.

This was a really good find. Great technical work. Good job.

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