Astro Fighter missing video sync

joemagiera

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Got an Sega/Gremlin/Data East Astro Fighter that's having problems. The schematics can be found here: http://arcarc.xmission.com/PDF_Arcade_Manuals_and_Schematics/Astro_Fighter.pdf. I never did board repair work before and since even a fully working boardset for this game isn't worth much, I figured this would be a good place to give a first try.

It's a 4 board stack in a cage with an additional interconnect PCB on the cage. Only 3 boards actually plug into the cage, the 4th board sits on top of one of the boards and is connected to it via a cable. I have a known good boardset to test with, so I can swap boards in and out one by one.

I'm having a hard time identifying matching up the actual boards to the schematics, but the two boards I'm having trouble with are the one with the crystal & dip switches and the one that plugs into the main wiring harness and has 3 adjustable pots on it. Both of these boards display the same problem. The game coins up and plays and controls and sound appear to work, but the display does not have video sync. It's like the vertical or horizontal hold is not working.

If you had any ideas where to start or what might cause a "no video sync" problem, please let me know.
 
It could be any number of things, but usually when I see this it's a blown gate in the chip that drives the sync line. I don't know what causes it, static, shorted wiring, whatever, but I've seen it a couple of times. It's also possible that you have a bad chip in the video timing circuit somewhere. Put a logic probe on the output of the sync lines, see if you're getting anything at all. This game appears to have separate syncs - are they both out, or just one?

-Ian
 
OK, from looking at the schematics, the sync is generated on the CPU board. Vsync comes off the chip at 4L, and hsync appears to come from 6F.

But, first, see wether you have anything at all coming out of the vsync and hsync lines with a logic probe or a 'scope.

-Ian
 
OK, from looking at the schematics, the sync is generated on the CPU board. Vsync comes off the chip at 4L, and hsync appears to come from 6F.

But, first, see wether you have anything at all coming out of the vsync and hsync lines with a logic probe or a 'scope.

-Ian

Thanks for the feedback Ian. Logic probe, o'scope. Yikes. But have to try to learn sometime. I know I have a scope. I know I can borrow a logic probe. Learning to use an entirely different matter, but that's what goggle is for!

Quick question, why would the other board display the same sync looking issue? (I'm assuming both sync chips you referenced are on the same board).
 
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Quick question, why would the other board display the same sync looking issue? (I'm assuming both sync chips you referenced are on the same board).

OK, I thought you had one working set and one bad set. Or, are you referring that two of the boards from the nonworking set produce the same symptom when used individually in the good set?

The only two boards the sync signals go through are the CPU board and the I/O board. They are generated on the CPU board and simply carry through the I/O board. The I/O board has almost nothing on it - save for a couple of pots and very few chips, none of which are connected to the sync circuit. Follow the traces through the I/O board and verify no corrosion or cuts/shorts. You should be able to make that I/O board work with the good CPU set.

-Ian
 
OK, I thought you had one working set and one bad set. Or, are you referring that two of the boards from the nonworking set produce the same symptom when used individually in the good set?

The only two boards the sync signals go through are the CPU board and the I/O board. They are generated on the CPU board and simply carry through the I/O board. The I/O board has almost nothing on it - save for a couple of pots and very few chips, none of which are connected to the sync circuit. Follow the traces through the I/O board and verify no corrosion or cuts/shorts. You should be able to make that I/O board work with the good CPU set.

-Ian

Yes, I do have a fully working set that I've been swapping boards in and out one at a time. That's why I thought it strange that on board (the one with the 3 pots an plugs into the wiring harness), when run with all other boards known good, produces a no-video-sync symptom. And then the other board, the one with the crystal and dip switches, when run with all other boards known good, also produces a no-video-sync symptom.

I figured a synch issue would be isolated to one board type of the PCB set, but this happens on two different board types on the PCB set.
 
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