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Capcom 1942 Repair Log


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Had @kadeath's original Capcom 1942 board set on the bench this arvo. He'd bought it as working, but when it arrived it didn't, the board ran but all the sprites had lines through them, plus there was screen interference in time with the background sound effects.

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6633.jpg

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6635.jpg

 

It's in very nice condition, aside from the mangled mix of PCB feet that make the board very wobbly even on a flat surface. However when fired up I actually got this instead of sprites, so either the fault had got worse, or another fault had introduced itself into the mix

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6654.jpg

 

I had no issues video issues with the audio but the amp did start to overheat ( due to it starting to oscillate, which is usually due to bad or missing decoupling capacitors on the 12v rail ), and when taken to extremes this can cause video disturbances, it can also destroy the amp chip if it doesn't have thermal protection built in.

 

The initial eyeball check only threw up one issue, the prime candidate for the amp issue, a very sick 470uF capacitor in the amplifier section.

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6636.jpg

 

It came off and a new one went in, this cleaned up the audio a bit and the amp now runs a lot cooler.

 

The sprite fault was almost certainly on the back board as that's where most if not all the gfx processing happens, and to work out what part of the board did what I started introducing faults by shorting out pins on the data bus coming out of the EPROM banks, to see what each one did. The column of 6 ROMs handled the background data as poking around there left the sprites untouched, but really annoyed the background. So assuming the issue was likely to be close to the EPROMs that held the sprites I went looking for likely candidates. I could have dumped the ROMs at this stage, but as they were actual EPROMs I took the punt that they were probably fine. I did reseat them all and check for bent pins but there were more likely causes elsewhere to look at first. Had I made no progress I would have gone back and dumped them but there was no need.

 

Sadly this board is peppered with Fujitsu TTL 74 series logic ICs, which is bad as they are a common failure, they just die randomly, which is why I don't think Kadeath got scammed by the seller, these are quite liable to die in the mail even when well packed. On the flip side they tend to die in pretty obvious ways if you have either a logic probe, or a scope. Poking around I quickly found that the 74LS283 at 8L had no outputs...

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6637.jpg

 

... and a chip with floating outputs is a prime candidate for some piggybacking as the onboard chip is not outputting anything so the piggy-backer drives the show without having to fight the original chip outputs.

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6662.jpg

 

The results tend to be instantaneous...

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6664.jpg

 

... and the original fault was uncovered again - sprites with lines through them. Bad chip comes off...

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6667.jpg

 

...fails off board tests...

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/Bad283_8L.png

 

...and a salvage chip from a very dead Snow Brothers bootleg PCB goes in.

 

The other 74LS283s in the area all looked fine on the scope, so I went a bit further, and found a 74LS163 at 4F with the same issue, all outputs dead and floating.

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6669</p><p> </p><p> Bad chip comes off...</p><p> </p><p> [img]http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6670.jpg

 

...tests as hosed

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/Bad163_4F.png

 

... and another salvaged chip goes in.

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6671.jpg

 

Fixed!

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6673.jpg

 

http://i40.photobucket.com/albums/e230/Womble76/1942/1942_2/IMG_6678.jpg

 

Once on Fujitsu of a certain type has died on a board I try to replace all of them all before releasing the board back into the wild. The failures that plague Fujitsu TTLs seems to be due to the bond wires corroding and breaking internally and it does seem to run in the batch, once one goes the other chips of the same type and the same batch are likely to fail. In this case all 283s and 163s should probably be replaced and I think I have enough 283s on the Snow Bros bootleg to do those, but the 163s may be a problem. If I can replace those it will only leave 27 Fujitsu's on the board ;)

 

Oh - a set of four matching PCB feet have made the board a lot more stable too. :)

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