Asteroids plays about 2x normal speed.

Arcadenut

Well-known member
Joined
Jun 12, 2003
Messages
9,355
Reaction score
4,887
Location
Glendale, Arizona
Just picked up an Asteroids and the game plays at about 2x normal speed.

It has PROMS on it, so it's not an upgrade (unless the factory had a speed up hack). I looked for hardware hacks and didn't see anything. So at this point I'm thinking it's a PCB fault.

It appears that it's been fixed before, so maybe one of the components that they replaced, failed again...

I replaced C4 (it was one of the chips that had been replaced prior) and it didn't change. I'll probably swap B4 out as well and see if anything changes.

Any other Ideas on where to look? Could C5 cause this behavior?

Thanks!
 
Yes, it could be C5 (NMI counter) or anything in the clock circuit - B5, C4, B4. There are a few ways to speed the game up.

More than likely it's a subtle mod like a pin or track that has been snipped on joined to another in connection with one of those chips. Have a really close look with a magnifying glass and a good light, both the solder side and component side around those ICs.
 
I had one like this:

Symptom: Game plays faster than normal
Check C4 (NMI timer). It is likely that the clock input is being held high. Check to see pin 5 is not connected to ground.
 
Check to see pin 5 is not connected to ground.

It seems to me...

Grounding pin 5 of the LS161 (vice pulling it up) would cause it to pre-set the counter to 0 (vice 4), so the 3kHz signal would end up being divided by 16 (vice 12), which should make the NMI frequency lower (188Hz vice 250Hz), which should slow the game down... not speed it up.

Unless I'm mis-understanding the NMI counter circuit.

To double the NMI frequency, you'd want to make the counter divide by 6 (instead of 12), which could be accomplished by pre-loading a value of 10 (vice 4) into the counter. This would be b1010. To do that, hold pins 4 & 6 high, pins 3 & 5 low.
 
It would be nice if someone would document a little dip switch "kit" hack that would allow one to incrementally change the speed of the game. (ie: 100%, 110%, 125%, 150%, 200%).

Darren Harris
Staten Island, New York.
 
Interesting concept.

Seems to me that by using a 4-position DIP switch on the 4 input pins of C5, you could achieve the following speed factors:

75%, 80%, 86%, 92%, 100%, 109%, 120%, 133%, 150%, 171%, 200%, 240%, 300%, 400%, 600%, and 1200%.

It'd be a pretty ugly looking hack, though.

Also, at some point, the NMI will occur before the code is ready for it, then who knows what'll happen.
And there may be other interesting concequences of too much speed, too...

Oh, and it wouldn't be just as simple as adding the DIP switch... you'd need 4 pull-up resistors between it and the counter, too.
 
Last edited:
Ok, solved the issue. As Solder mentioned above, it was the NMI chip with two clipped legs.

Desoldered the chip, put in a socket, slapped in a new 74LS161 and all was normal again :D


AsteroidsClippedLegs.jpg
 
Back
Top Bottom