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Prehistoric Isle in 1930, power shorted out


Anthony Flack

Question

I have a Prehistoric Isle in 1930 which I found abandoned in a gutted cabinet. I don't have a working monitor to test it with yet, but when I attached it to a JAMMA harness and speaker, it started playing music like it was working.

 

At first.

 

A couple of minutes later, the power supply is ticking and the ground wire on the harness has started melting! I guess the +5v line has shorted to ground. The board won't work any more.

 

I'm not reading an outright short between +5 and ground, resistance is 238ohm. There is a power LED which is simply connects between +5 and ground via a resistor, that won't come on any more. Well, I'm hoping the power is just going to ground and no chips have been damaged. Nothing but the ground wire got hot at least.

 

The first thing the power hits is smoothing capacitors connected to ground. No doubt they are ancient and need replacing anyway. Is one of these likely to be the culprit though?

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There's also a PST518B straddling the +5v and ground near the JAMMA harness. It looks like a transistor but apparently it's a power regulator IC of some sort, so maybe that's what's malfunctioning. Anyway it was cheap to replace so I've ordered one of those too.
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Blam, a capacitor array had broken off the board; soldered on a new set of legs on and reattached it, now it's making all the right noises again. Or almost.

 

The samples aren't playing; they just make a popping noise. But I can already see why that is - next to the sampler chip is a space where its clock crystal is supposed to be and it's not there. So I just have to get one of those now, and then hopefully it should be all functioning on the audio side at least.

 

I'm just talking to myself here but it seems to be working.

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Got a new monitor chassis from Jomac, and got a packet of 640khz ceramic oscillators from the one dude in Germany who still sells them, and duly installed one in the missing spot. The Prehistoric Isle board is now confirmed 100% fully working... yay me.

 

Oddly, it seems to have a level select hack, whereby you can select which level you want to play at the start of each level. Default is "NEXT" but you can also select any other level including the ending. I'm not sure why this is; it seems to be an original SNK board and you can't dip-switch the cheat on and off. I need to check MAME and see if there is some deal with different romsets I guess.

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