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What would cause Mortal Kombat video to look like this?
#312777 - 08/13/13 06:48 AM Attachment: IMAG0526.jpg 1072 KB (0 downloads)
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Blue with vertical lines.
[ATTACHED IMAGE - CLICK FOR FULL SIZE]
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italie |
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MAME owes italie many thank yous, hah
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Re: What would cause Mortal Kombat video to look like this?
[Re: PokeMAME]
#312779 - 08/13/13 07:12 AM
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> Blue with vertical lines.
RAM or memory latch would be my first guesses, but I don't have schematics in front of me for that board...
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> RAM or memory latch would be my first guesses, but I don't have schematics in front
> of me for that board...
Ah, according to CPU Sheet 14 of 16, U2 & U3 feed out to RED GUN and part of GREEN GUN.
U20 & U21 feed out to the rest of GREEN GUN and BLUE GUN.
So, I guess, the problem might be between U2 & U3.
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MAME owes italie many thank yous, hah
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Re: What would cause Mortal Kombat video to look like this?
[Re: PokeMAME]
#312833 - 08/14/13 04:14 AM
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> > RAM or memory latch would be my first guesses, but I don't have schematics in front
> > of me for that board...
>
> Ah, according to CPU Sheet 14 of 16, U2 & U3 feed out to RED GUN and part of GREEN
> GUN.
> U20 & U21 feed out to the rest of GREEN GUN and BLUE GUN.
>
> So, I guess, the problem might be between U2 & U3.
U5 would be my first look, then down the line to U3, U2. Logic probe or gropunding out those chips might tell you something.
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Re: What would cause Mortal Kombat video to look like this?
[Re: PokeMAME]
#312863 - 08/14/13 02:38 PM
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> Blue with vertical lines.
Looks like a video RAM fault, perhaps with two or more of the 4461 VRAMs that hold palette data (U11-15, U29)
All video RAM is tested as part of the CPU board test though whether you can make out the results is another matter 
This Smash TV repair log might be useful as it describes a similar fault on an identical PCB:
http://wiki.aussiearcade.com.au/index.php/PCB_Repair_Logs_Smash_TV
Edited by Phil Bennett (08/14/13 06:25 PM)
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I shorted the TEST SW to ground and got into the Utilities menu and ran the CPU test.
It was a bit hard to read the menu so I followed along with what was in the manual.
So now I have a starting point for replacing the RAM chips.
This board was sold as "working" but there were popcorns in the box which probably created the static that blew the chips.
[ATTACHED IMAGE - CLICK FOR FULL SIZE]
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Re: Progress!
[Re: PokeMAME]
#312930 - 08/15/13 05:53 PM
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> I shorted the TEST SW to ground and got into the Utilities menu and ran the CPU test.
> It was a bit hard to read the menu so I followed along with what was in the manual.
> So now I have a starting point for replacing the RAM chips.
> This board was sold as "working" but there were popcorns in the box which probably
> created the static that blew the chips.
The suspect chips share a common output enable signal (/TROE.C1X). I'd check that out first: it originates at pin 17 of the 74ALS541 @ U43, which has a 100 ohm resistor on its output.
Otherwise, here's a source of VRAM chips if you need them:
http://cgi.ebay.com/?viewitem&item=390601177814
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Re: Progress!
[Re: Phil Bennett]
#312989 - 08/16/13 05:06 PM
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> The suspect chips share a common output enable signal (/TROE.C1X). I'd check that out
> first: it originates at pin 17 of the 74ALS541 @ U43, which has a 100 ohm resistor on
> its output.
>
> Otherwise, here's a source of VRAM chips if you need them:
>
> http://cgi.ebay.com/?viewitem&item=390601177814
OK, here's a weird thing. When I first power the board up, the video appears mostly blue-green. But when I touch U5-pin27 to ground just for a second, the screen blinks and snaps into more of a full-color mode still fuzzy with lines only the background appears black and the lines are white. It stays like this until I power down.
When I touch and hold U43-pin17 to ground, the screen goes almost completely white for a second or two until the graphics refresh and the white lines seem to lighten up but the screen still looks very static-y. When I run the CPU test while grounding U43-pin17 it still fails but this time it shows some of the ROMs to be bad and stops, leaving the RAM untested. Without grounding anything it tested the RAM and then stopped. The original results remain the same when not grounding anything.
That VRAM source is a good price and he combines shipping! But the speed on the original RAM chips was -10 and the ones on eBay are -15 (slower). Would that make a difference?
Overall, I am putting it on the back burner for now. I told the guy who bought it he has 3 choices if he doesn't want me to try and fix it: contact the seller and complain, try to sell for himself as partially working or hang on the wall as art. I'm sure the seller will say he isn't responsible for a 20+ year old board and that it was working when he had it.
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Re: Progress!
[Re: PokeMAME]
#312993 - 08/16/13 05:47 PM
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> OK, here's a weird thing. When I first power the board up, the video appears mostly
> blue-green. But when I touch U5-pin27 to ground just for a second, the screen blinks
> and snaps into more of a full-color mode still fuzzy with lines only the background
> appears black and the lines are white. It stays like this until I power down.
>
> When I touch and hold U43-pin17 to ground, the screen goes almost completely white
> for a second or two until the graphics refresh and the white lines seem to lighten up
> but the screen still looks very static-y. When I run the CPU test while grounding
> U43-pin17 it still fails but this time it shows some of the ROMs to be bad and stops,
> leaving the RAM untested. Without grounding anything it tested the RAM and then
> stopped. The original results remain the same when not grounding anything.
Sounds more likely to be dodgy VRAM then.
> That VRAM source is a good price and he combines shipping! But the speed on the
> original RAM chips was -10 and the ones on eBay are -15 (slower). Would that make a
> difference?
Not in this case. The 150ns parts are well within the timing limits of the video hardware so they'll work.
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