Centipede: Slow vertical crawl

texasmame

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Thought this was a monitor issue, but it's not:

http://forums.arcade-museum.com/showthread.php?t=211682

Anyhow, the crawl is slowly to the left or right. All the resistors in the boards video output circuit test within tolerance (R72, 103, 104, 107,108)

If I'm reading the schematic right, A9 and B7 also impact the sync from the board. A9 has already been socketed and replaced but B9 is original. No logic probe here so I guess the next step is to replace those with known good ones.

Help appreciated.
 
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If it's in the board, there is the sync chip at B7. According to the manual it's a 74LS08 chip that controls the vertical and horizontal sync. It handles all of the sync in the circuit.

It's on sheet 2 of the schematics, in the video output circuitry. There is a resistor on it too, and another chip that controls it at A8, it's a 74LS175 chip. PM me if you need that sheet.
I downloaded it from Mike's Arcade years ago.

It is probably something around those two chips that are bad. Check the 3 resistors in that circuit to see if they are good before poking around on those chips. They might be off. But if it's only on one side, it's probably the chip at B7.
 
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If it's in the board, there is the sync chip at B7. According to the manual it's a 74LS08 chip that controls the vertical and horizontal sync. It handles all of the sync in the circuit.

It's on sheet 2 of the schematics, in the video output circuitry. There is a resistor on it too, and another chip that controls it at A8, it's a 74LS175 chip. PM me if you need that sheet.
I downloaded it from Mike's Arcade years ago.

It is probably something around those two chips that are bad. Check the 3 resistors in that circuit to see if they are good before poking around on those chips. They might be off. But if it's only on one side, it's probably the chip at B7.

Ah, okay. I see A8 in the schematic. Is A9 not a potential problem then, too, as that also seems to be in the circuit for the sync? Also, which resistors are you referring to - the R50 set shown on the schem near A8?
 
The sync comes off pins 2 and 15 of the A8 chip and goes through R103 (3.3K) R107 (2.2K), and R108 (1.5K) to pin 3 of the sync chip at B7.

Then it goes thru a diode at A9 (I doubt if that's the issue), then there is another resistor before the lead R72 (220 ohms).

To me, that looks like the complete sync circuit. It works for the most part, just the vertical roll you see (vertical because the monitor is sideways).

One of those is what I think is wrong. If it was my board, I'd replace that Chip at B7 and see if that fixed it.
 
The sync comes off pins 2 and 15 of the A8 chip and goes through R103 (3.3K) R107 (2.2K), and R108 (1.5K) to pin 3 of the sync chip at B7.

Then it goes thru a diode at A9 (I doubt if that's the issue), then there is another resistor before the lead R72 (220 ohms).

To me, that looks like the complete sync circuit. It works for the most part, just the vertical roll you see (vertical because the monitor is sideways).

One of those is what I think is wrong. If it was my board, I'd replace that Chip at B7 and see if that fixed it.

I tested all those Rs in circuit and they all showed the sameish ratings for their counterparts on the board with no sync issues, so I would think those are fine.

Haven't gotten into any swapping of parts just yet as I'm waiting to hear from the guy I got the board from first.
 
I was wrong about that schematic. That's not a diode at A9, it's some kind of chip. It's a "type 7407 integrated circuit". That could be it too.

Sorry, I was looking at that and it had pin 5 and 6 marked on it.

I'm not sure what that is without a board in front of me. That's the last chip before it goes to the lead. It's the main video chip where red,green,blue, and ground comes out in that same view of the schematic. Hell, it could be that one too.

There's another one at A/B 10 according to the manual.
 
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Guys... He bought this board from me. I have sent the OP another and once I get this one back I'll troubleshoot and post back my findings for future reference. At first I thought it maybe a revision issue as his original pcb has no resistor at R73 and the one I sent did, but even with that resistor removed it still doesn't sync. The replacement pcb has never had a resistor there so hopefully this one works out. I'll keep everyone updated.

Thanks,
Brian
 
Guys... He bought this board from me. I have sent the OP another and once I get this one back I'll troubleshoot and post back my findings for future reference. At first I thought it maybe a revision issue as his original pcb has no resistor at R73 and the one I sent did, but even with that resistor removed it still doesn't sync. The replacement pcb has never had a resistor there so hopefully this one works out. I'll keep everyone updated.

Thanks,
Brian

Yeah, all good here. Workin' it out, folks.

Nothing to see here, move along. ;)
 
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