Topic: Another Golden Axe Repair Log  (Read 3119 times)

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Offline Womble

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Another Golden Axe Repair Log
« on: August 19, 2012, 09:11:20 AM »
Had Dragonlee's Golden Axe board on the bench yesterday...



... was running but the characters and parts of the title screen were messed up.





Classic symptoms of a RAM or RAM logic fault.

The board itself was in good nick, fairly clean and had been de-suicided, actually probably converted from another game as all the ROMs on the board were non Sega ones.It had also been converted from System 16 pinout to JAMMA on the edge connector...



...not a fan of this personally, I much prefer boards to be kept as original as possible.

Anyway - on boards with a test function you might as well use it but on Sega boards you need a combination of the Service, Test and 1P start buttons to get into that mode and whoever did the edge connector mod hadn't connected these up. The wiring installed for the conversion was pretty tight and felt fairly brittle so to avoid having to find the end of a cut track under the wiring I just poked around with a grounded probe on the input buffer chips...



... until I found the right input for the missing controls. This got me into test mode and I fired up the RAM test. Stepping through the memory tests got me this





Sounds like progress but its not. What Sega call Scratch RAM is actually system RAM and if that really was bad then the system would not boot at all. What this is a symptom of is a board that has been desuicided, Sega used two protection methods on some ROM sets, firstly the FD1089 encrypted CPU and secondly a separate protection chip.



Some versions of Golden Axe needed this and others didn't, if you desuicide a board that used it with ROMs from a version that didn't then this chip somehow breaks the RAM test without affecting the games ability to run. This makes no sense but it seems to be the case. My own Golden Axe doesn't have this chip, it passes its scratch RAM test and runs perfectly, if I install this chip in the empty socket then the board still runs but the RAM test fails if you run it. Basically this chip is no longer needed so it came of the board, its not doing any harm there but it could easily waste a future repairer time trying to find a fault with the system RAM that doesn't actually exist.

The ROM fail tests were a result of a poorly socketed GAL on the ROM board, once that was re-seated all the tests passed, and I still had the original fault.

This is because the RAM test doesn't test all the RAM on the board :) Specifically it misses these...



... sprite and sprite palette RAM, which are highly likely to be the cause of the problem. They are also Sony CXK5814s which are known common failures. I went over them with the oscilloscope but they all seemed to have healthy outputs, but a RAM chip can be outputting healthy crap so all that proved was I couldn't narrow down which chip or chips were the cause.

In cases like this I would advise you start from the PCB edge, chips on the edge of boards seem disproportionately more likely to be bad than ones further in. Probably this is because of static laden fingers grabbing the boards at the edge.

So I de-soldered the pair of chips on the edge



... threw them in my tester and...



...the edge one was bad. The inner one of the pair was ok.

As they operate in pairs I soldered in a pair of equivalent chips, MCM2018-45s and fired the board back up.







Fixed  :D

The other one has serious issues, I also washed it in the kitchen sink earlier as it was filthy. Looks great now, just need to fix the bugger!
Sic Transit Gloria Atari

Offline AndyRCM

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Re: Another Golden Axe Repair Log
« Reply #1 on: August 19, 2012, 11:06:32 AM »
What a great read (as always) . . .  great game too! :)

"I could see the faces of those who led pissing themselves laughing" - Funeral Pyre by The Jam

Offline Womble

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Re: Another Golden Axe Repair Log
« Reply #2 on: August 20, 2012, 04:25:38 AM »
Wait for the next one, that board was so filthy threw it in the sink and I gave it a scrub with a brush :) Should be an interesting fix, assuming its fixable at all.  :D

It sure looks purdy this morning tho, needs a few days parked over the heating vent to ensure its nice and dry.
« Last Edit: August 20, 2012, 07:03:58 AM by Womble »
Sic Transit Gloria Atari