cpsystem3

Forgotten Worlds repair log

 PCB Repair Logs, Repair Logs  3 Responses »
Jan 072016
 

I received a very nice condition Forgotten Worlds PCB from a friend of mine, intended for my personal collection. The PCB showed up in spectacular shape, there wasn’t even any dust or grime on it. All original stickers and labels in tact. In fact, this is the first board on the CPS-1 platform where I have seen serial numbers on the A and B board that match:
fw1

Sadly, when I fired up the game, the colors were all wrong. Previous experience with the CPS1 told me this was likely a bad custom Capcom IC, or a bad ram chip.
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Due to the nature of the CPS1 hardware, and the fact it is very hard to diagnose the motherboard (the A/B double stacked board makes it very hard to probe), I generally just swap the motherboard from a lesser valued game. In this case however, since the serial numbers match, repair was the only option!

With the assistance of the CPS1 Old Hardware Revision A Board schematic posted right here on Jammarcade, and some critical input from Caius, I was able to determine with high certainty where the fault was. In my case, the ram that controls RED was the prime suspect (at location 2D). I did not have any Sony CXK5814 ram chips on hand, but I did have Toshiba TMM2018 which is equivalent. Without hesitation I swapped the IC.
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And…. SUCCESS!
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If you were wondering, yes I am enjoying this classic with the proper “Rolling Switch” spinner controls.
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Another copy of this wonderful Capcom shooter has been preserved!

Twin Cobra Repair Log

 PCB Repair Logs, Repair Logs  3 Responses »
Apr 212015
 

Recently, I picked up an ‘untested’ Twin Cobra PCB off of eBay. As most of us are aware, when something is advertised as ‘untested’, 9 times out of 10 its completely broken. This Twin Cobra was no exception to this rule.

twin_cobra

 

Out of the box, the game booted up to a flat black screen, zero activity at all. When I see this on unknown condition boards, the first thing I do is give it a thorough visual inspection. I generally look for rust/corrosion, deep scrapes and gouges severing traces, physically damaged capacitors and IC’s, etc. Right off the bat, I noticed the the Koyo 28mhz crystal at X1 was hanging on by a thread! The other three had snapped off at the base of the crystal, so it was time to find a replacement. I was able to find a donor crystal in my parts boards.

 

I desoldered what was left of the original crystal, and then I installed the donor. The donor is slightly faster than the original, but it should be OK until a proper replacement arrives in the mail.

       

 

The rest of the board looked good – no gouges, no other damage so to speak. At this point when I powered it up, the game sprang to life!

twin_cobra4

 

Everything appeared to be working as expected, so I coined up and tried a game. Almost instantly, the next fault presented itself. Some (not all) of the sprite layers were incorrect. When the tanks aimed at towards 7:00, their turret would disappear completely. When larger tanks were destroyed, their remnants would take priority over my helicopter.

 

I knew something was at fault either with the ram or layer priority sections of the video board. From here, I started reading up on the mame driver, as well as the memory map for Twin Cobra posted on Toaplan.org. From what I could tell, it looks like sprite priority is controlled by the bipolar roms present on the TP-011 SUB (graphics) pcb.

 

I gave the 82S123’s a closer look, the bipolar rom marked B30-22 on the parts side of the PCB had a factory defect! There was a small solder bridge connecting pins 5 and 6. I fired up my soldering iron and removed the bridge.

twin_cobra3

 

Fired the board up, all faults were cleared! Game now works 100%. It’s mind-blowing to think that a game manufactured in 1987 has only played properly some 28 years later. This Toaplan masterpiece lives to fight another day! Until next time…

 

 

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