Atari asteroids chrystal
- Thread starter bonehead
- Start date
Battlezone
Active member
Steve,
It's less than 1% difference so my advice would be to drop
in a 12MHz 'can' (as I had to do recently).
Firstly you (or anyone else) won't be able to tell the difference,
secondly the crystals are manufactured to a particular tolerance
anyway so even if you found a 12.096MHz can it will likely
oscillate at a different frequency to the number on
the part and it will also vary with temperature.
The tolerance over temperature is usually pretty good for these
parts (typically better than +/- 100ppm) but there is always some
variation from part to part.
Battlezone2016-09-12 13:59:12
It's less than 1% difference so my advice would be to drop
in a 12MHz 'can' (as I had to do recently).
Firstly you (or anyone else) won't be able to tell the difference,
secondly the crystals are manufactured to a particular tolerance
anyway so even if you found a 12.096MHz can it will likely
oscillate at a different frequency to the number on
the part and it will also vary with temperature.
The tolerance over temperature is usually pretty good for these
parts (typically better than +/- 100ppm) but there is always some
variation from part to part.
Battlezone2016-09-12 13:59:12
I'm seconding John (Battlezone)'s thoughts here as i've put in 12mhz versions into both Asteroids and Missile Command and measured them and the crystal isn't putting out an exact 12mhz anyway, it gives out 12.03 and then as it heats, it settles to around 11.95.
I was curious about this when I repaired a couple of boards so measured it on my scope.
From an engineering point of view, as the games speed and everything else is derived after the crystal via various dividers, the aim was to generate 6mhz, 3mhz and 1.5mhz in most cases. A lot of the circuitry and game code is tied to the vertical sync which runs at 59.97khz and can't fluctuate by a vast amount anyway so the original use of 12.096mhz is likely more "what they had on stock" than anything else.
I was curious about this when I repaired a couple of boards so measured it on my scope.
From an engineering point of view, as the games speed and everything else is derived after the crystal via various dividers, the aim was to generate 6mhz, 3mhz and 1.5mhz in most cases. A lot of the circuitry and game code is tied to the vertical sync which runs at 59.97khz and can't fluctuate by a vast amount anyway so the original use of 12.096mhz is likely more "what they had on stock" than anything else.
Battlezone
Active member
Steve,
You might have to forgo all your Twin Galaxies Asteroids record attempts on your "slow" board.
James,
Exactly. It's interesting to see that variation over temperature in practice.
It's a little more than I was expecting, and then there is the 'scope calibration of course too.
(I notice a slight typo in the (Missile Command I guess) vertical sync units -
I bet that's flicker-free).
You might have to forgo all your Twin Galaxies Asteroids record attempts on your "slow" board.
James,
Exactly. It's interesting to see that variation over temperature in practice.
It's a little more than I was expecting, and then there is the 'scope calibration of course too.
(I notice a slight typo in the (Missile Command I guess) vertical sync units -
I bet that's flicker-free).
Battlezone said:
Could well have been my scope at the time, I wasn't paying that much attention and it could be more like 11.99 or fluctuating around that point. Although i've bought quite a powerful DSO I probably only use about 1/100th of the function on it - I bought the Rygol DS1054Z to use as a poor-man's 4-channel logic analyser and boy has it sped up debugging bad 244's and 245's by looking at input v output based on its gate/direction pins.
I think in all honesty the source clock and the final CPU clock are so far apart that unless you want to be super picky (or fight TG to debunk myth about freeplay modes lol) then there won't be any difference to even the hardened player.
As parts diminish in availability to provide original exact replacements we'll see a new subset of scoreboards that are 100% original vs repaired boards - the super-anal will demand an "original" and "12.0mhz crystal" version of the score table to cling to their record for a little longer.
For the rest of us 99% level players its just game on.
Mitchell Gant
Active member
I've got an Asteroids on my bench right now, and it has had a 12MHz crystal replaced in at some point.
I have an old frequency counter, although it's not calibrated and I thought I'd put on the board. I put it on the half divided 6MHz test point as it was easier to clip on than the 12MHz IC pin.
Freezer spray: 5.997530 MHz
Room temp: 5.997326 MHz
Finger temp: 5.997299 MHz
My 'scope reads it as 5.99738 MHz at room temp.
An original Asteroids with a 12.096MHz crystal reads:
Freezer spray: 6.046593 MHz
Room temp: 6.046705 MHz
Finger temp: 6.046714 MHz
Notice how the original crystal frequency moves in the opposite way to the replacement 12MHz one with temperature changes!
I have an old frequency counter, although it's not calibrated and I thought I'd put on the board. I put it on the half divided 6MHz test point as it was easier to clip on than the 12MHz IC pin.
Freezer spray: 5.997530 MHz
Room temp: 5.997326 MHz
Finger temp: 5.997299 MHz
My 'scope reads it as 5.99738 MHz at room temp.
An original Asteroids with a 12.096MHz crystal reads:
Freezer spray: 6.046593 MHz
Room temp: 6.046705 MHz
Finger temp: 6.046714 MHz
Notice how the original crystal frequency moves in the opposite way to the replacement 12MHz one with temperature changes!
interesting, are all crystals made the same way?
Battlezone
Active member
Interesting. I don't know what spec parts Atari used, but those variations (better than 4 sig fig) are more like the variations was expecting (based on reading a couple of crystal datasheets - probably newer datasheets).Mitchell Gant said:
The bottom line remains the same. Unless anyone has found a source for 12.096MHz then do as I and others have done and drop your new low profile 12.000MHz crystal into your old boards and be damned.
Battlezone
Active member
In case people come across the thread.