Sega Hang On Fails Ram Check (IC 74) but seems to play fine...

mclemore

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So I'm working on a Sega Hang On. On the self-test, it fails RAM check IC 74. If it really is that IC, it seems to be a Fujitsu MB8484-15L 8kx8 RAM.

And of course it's soldered on the board.

Now the game seems to play great, and my time is pretty limited now...

I don't know if that RAM location points to some video memory or something where I wouldn't notice an error on one bit or what...

I'm curious...

What's the chance that IC 74 is really bad (as opposed to the cap next to it or some other part).

And if the game plays great, do I really even need to worry about it?
 
Is the error consistent? If so, is the IC74 at location 5J on the control board (lower board of the large stack)? If so, and by looking at the schematics and this post, it will likely be related to text generation, so it may not be immediately obvious if it is bad.

I would try piggybacking it or try and see if a voltage adjustment clears the error.

p
 
Mine does the same thing. When I bought it, it had an intermittent glitch where half the road would fold over itself . Seemed to happen more intensely during warmer weather. The game was still playable but annoying. Just before I was going to bite the bullet and attempt repair, I found some bad connections coming from the 5v that was hacked in. After fixing that the problem hasn't resurfaced but IC 74 still shows as bad. Hadn't tried piggy backing mine yet .
 
Depending on the test routine, only 1 bit has to fail to mark the RAM bad. If that's in a section that's not likey used (e.g. the top end of it's address range) then it's not likely to cause an issue with the game but is still technically bad.

And if the game plays great, do I really even need to worry about it?

I'd look for a replacement and sit on it personally.
 
Could be the 8464 but if you notice these both share lines with 8128s also. Its easy enough to socket and test 8464. If its not that, could be other RAM or any ttl touching address or data lines. Jumping ICs usually pans out best if this is all you can do. Its more of a chance of an open line on an IC than a short. In my experience, issue has been with something else non RAM touching it.
 
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