Dr_Righteous wrote:
I've got mine clocked to 33.3MHz (66.666MHz osc.), and an increase of 8.3MHz seems a speedier to me. Sysinfo thinks so too. I tried pushing it to 37.5MHz (75MHz osc.), but it wasn't stable. 40MHz wouldn't boot at all. I'd like to try it with a 33MHz rated 040 sometime, just to see what happens.
Nothing will happen. As a matter of fact, and contrary to some previous posts in this thread, the problem is not with the CPU getting too hot. A 25Mhz '040 will clock to 33Mhz without a problem (but will require active cooling else it will crash eventually).
The real problems with overclocking the A3640 are two:
1) Everything runs hot on the card. That can eventually damage it as well as dry out the crappy SMD caps which are also placed backwards from the factory (unless you've swapped them)
2) Everything runs hot. Not on just the card, but also the motherboard. What you're actually doing by changing the crystal is overclocking the non-classic chips. This actually does give a much needed performance boost. At 36Mhz Doom is nothing like at 25Mhz. It's actually playable! However, I HIGHLY recommend you either actively cool the Amiga chips (SuperBuster, Ramsey, Alice, etc.) or at least ad some small heatsinks with the proper heatsink compound. Otherwise your Amiga will die a quicker death. And unlike the '040, those chips aren't usually socketed!
Right now I've been able to achieve 36Mhz (with active cooling since I'm using a 25Mhz '040) which I believe is the highest A3640 stable overclock ever (*) (without a Peltier and other fancy stuff), but I believe I can push it a little more. 40Mhz won't happen (of course I tried with a 40Mhz '040 just to eliminate any CPU overheating issues) - the custom chips can't handle it. Once I max it out I'll post a little hint on how to get a more stable overclock, as well as some benchmarks.
Last note: all this overclocking should be done after you've had a good software setup (patches and things like Oxyron/CyberPatch/HSMathLib* installed, otherwise you're still wasting cycles).
(*) correction: I now know of two claims of A3640s running at 40Mhz, however I've not seen it myself. But I still believe it's possible but will depend on their motherboard revision, custom chips and the A3640s themselves. At least that's my belief right now.
