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Thread: An update on that busted VA2.3 with the bad ASIC

  1. #1
    Road Rasher
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    Default An update on that busted VA2.3 with the bad ASIC

    Well as usual I was wrong about assuming it fixed after I had replaced the 315-5786 ASIC. Turns out that was merely the beginning of its troubles... shortly after I had declared it fixed, it started glitching again, although this time it thankfully wasn't crashing anymore.

    Spent days poring over the board, reflowing joints, replacing almost every single IC on the board (the only things to not have been changed at this point were the HD68HC000, YM2612, Z80, Z80 RAM and the 315-5684 amplifier), etc. until I discovered that pressing a part of the board next to the 68HC000 made some of the glitches go away. Reflowed all the 68HC000's pins... and it helped, it made Adventures of Batman and Robin run without glitches, but Aladdin, Jungle Book and rarely Streets of Rage 2 would still occasionally show corrupted sprites and tiles in spots. Having changed almost every other IC, I figured, 'why not try changing the 68HC000'? I did just that, replacing it with a known-good Motorola MC68HC000 taken from a parts motherboard - and that seems to have done the trick, no more corrupted tiles or sprites. But, I'll wait until I subject it to a battery of tests before I declare it 'fixed' this time!!

    My guess was the Hitachi HD68HC000 was very subtly damaged - not the kind of damage that would prevent it from running game code correctly, but the kind of damage that might cause, say, the occasional bad signal on its bus lines. Probably, whatever damaged the 315-5786 also damaged the HD68HC000, albeit in a much less obvious manner.

    Currently it's set up with a Fujitsu MB3514 encoder (slightly oversaturated color aside, I really like its winning combination of near-CXA1645 level sharpness with near-zero pixellation) and I adjusted the 315-5684's filtering and replaced the BA10358 with a BA15218 (stripped from my X'Eye) with active filtering; however due to me being short on parts I have it temporarily using 270pF filtering caps. They'll have to stay until I get some 120pFs.

  2. #2
    Road Rasher
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    Another update. I wound up changing the little resistor arrays next to the VDP and CPU. So far so good, haven't noticed anything 'off'.

    This system would have to have been a lot cheaper than it was, to be worth this kind of headache. That said, any and all VA2/2.3s are worth an attempt at salvage, as they're the best Model 2's by a vast margin - though the VA3 comes pretty close admittedly.

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    Video Game Modder Road Rasher segasonicfan's Avatar
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    this sounds like RAM trouble. I bet replacing the SRAM and VRAM would make it 100%. Ive never seen a 68K get glitchy. thats usually a ram issue, which interfaces with the 68k.

    -Segasonicfan

    p.s. the headache sucks, but hopefully ure learning some cool repair skills! i think of this as EXP points.

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    Yeah, I tried replacing the work RAM and VRAM, but neither helped. The VDP and HD68HC000 were definitely bad, changing those (and their related resistor arrays) made things way better. So far no further problems, after some hours of testing. My supposition is the VDP and 68K were ESD victims, with the 68K being only subtly broken by ESD while the VDP was roached by it.

    Only chips left I didn't change are the Z80, Z80 RAM, YM2612 and 315-5684 (which is not part of the data bus, and therefore is totally unrelated).

    I did learn that Virgin-made games based off the Cool Spot engine (Aladdin, Jungle Book, etc.) are a good test for problems on these Genesis systems as if there's even the slightest issue with the bus, VDP or 68K, you'll get massive graphical glitches and corruption.
    Last edited by Eep386; 10-25-2017 at 07:44 PM.

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