Galaga / Ms. Pac Reunion PCB - Plays Blind, Burnt Video Ground

DogP

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Galaga / Ms. Pac Reunion PCB - Plays Blind, Burnt Video Ground

A fellow KLOV'r brought his Galaga / Ms. Pac reunion to the repair party today. It was playing blind, and had no monitor installed. We installed a monitor, which we tested, but it still played blind. Pulling the JAMMA Reunion PCB, I noticed the video ground had a blown/burnt trace.

I reconnected the video ground (which connects to the regular PCB ground), but the video still doesn't work. I'm almost positive that I've read about this problem before, but I can't find any info on it now. Anyone know the solution to this? I see there's an HC574, which I think is driving the video... that'd be my first guess, that whatever blew the ground probably also took out that chip. But I'm hoping someone could tell me I'm not crazy and that this is a common problem and that there's a common solution.

Thanks,
DogP
 
Okay... I got it fixed. There must have been a catastrophic monitor failure or something. I checked the 8 SMT resistors on the output of the HC574 (R8-R15), and they were ALL open (though they looked perfectly fine). I replaced them, and it works 100% now. I'm amazed that the GND blew open, burned every resistor in the video output, but somehow didn't blow the chips driving the video (especially the large QFP driving the sync, with no series resistor).

DogP
 
I saw the same thing on a 60-in-1

I wonder if someone hooked that board of yours up to a monitor that requires an ISO transformer but didn't have one in there...
 
Yep, that's what happened -- no ISO will find the shortest path to ground, and that will always be through the video connector to ground.
 
BTW, I saw the same problem on another board. I think you guys are right... I don't think these cabs come with an isolation transformer, so when the original monitor dies and someone throws in a replacement which needs an iso, and doesn't bother connecting the ground strap to the monitor... BOOM goes the game PCB.

Anyway, this one wasn't quite as lucky as the last. After fixing the video ground and resistors, I hooked it up, and only red was working, and the 74HC574 got really hot. So this one had burnt the video ground and the resistors (and these resistors you could actually see were burnt if you looked closely)... and also killed the 74HC574. I replaced that chip (SOIC package, P/N SN74HC574DWR, $0.51 from Digikey), and it came back to life and worked perfectly.

Burnt resistors:
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The original resistors are 0805, though I replaced them with 0603s, since that's what I had around. R8, R10, and R13 are 220 ohm; R9, R11, and R14 are 470 ohm; R12 and R15 are 1k ohm. To fix the video ground, I scraped away the solder mask on the ground where it originally connected, put a piece of kapton tape over the edge finger (to keep the solder from flowing down the finger, which could bend the JAMMA harness pin out of shape), and soldered a small jumper wire. I used hot air to remove the 74HC574, though you could also use a rasor blade or flush cutters to CAREFULLY cut the legs (without damaging the board under the chip), and desolder the legs individually.

Fixed:
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Success!
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DogP
 

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Nice fix :)

Those are a small resistor ladder, kind of a poor man's DAC on the outputs. You see those on Neo Geo, multicade, and other game boards. Those suffer the same damage from monitors run without ISOs.

Those resistors show the hallmark signs of damage - a small pinpoint burn in the center. That's one of the things I look for when examining boards for damage.
 
Nice fix :)

Those are a small resistor ladder, kind of a poor man's DAC on the outputs. You see those on Neo Geo, multicade, and other game boards. Those suffer the same damage from monitors run without ISOs.

Those resistors show the hallmark signs of damage - a small pinpoint burn in the center. That's one of the things I look for when examining boards for damage.
Yep... exactly.

I always find it funny how 8-bit color has one less bit on blue, but nobody ever notices. Of course 16-bit color has an extra green bit, but the difference between the LSb of 6 bits isn't much... but it seems like people would notice only 3 blues, vs 7 reds and greens, but I guess not.

DogP
 
Hey, I need a 33 ohm resistor of that size. Where did you get your replacements?
 
Jameco, Mouser, Digi-Key, Newark, etc... Everyone should have 'em.

I keep an assortment of resistor values and physical sizes here.
 
I thought I'd bump this old post to add my similar results.

I bought a reunion PCB that played blind on the off chance that it was the same problem(s) as mentioned in this thread. I first verified that all the edge connector traces were good, including the video ground. I then checked that the resistors mentioned above measured good as well.

I went ahead and ordered two of the surface mount 74HC574 chips at .55 each from DigiKey, one extra in case I messed up the surface mount soldering. They arrived today, so I replaced the chip and sure enough that fixed the video!
 
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