Overclocking A3640

[Sandman]

I have checked the polarity of all caps on the A3640 and they are all correct.  BTW, they were all just replaced.  

Also checked the +5v on the A3640 @ the caps and it is 5.02v.

The A3000 MB chips are as follows:
Ramsey is -4
SuperBuster -11
SuperDMAC -02
WD -08 (chip was just upgraded from -04)

CPU is XC68040-40 and has a heat-sink and fan.  The GAL's on the A3640 are also heat-sinked.

No zorro cards installed for test, only a CF card through a ACARD scsi-ide and ide-cf adapter.

Still boots fine @25mhz (w/ 50.0 crystal)  but no go with a 60.0 crystal oscillator.

I'm thinking this A3640 just can't do it.  

[SpeedGeek]

@Sandman

Thanks for the updated info. There were some early versions of Ramsey and SuperDMAC which had problems with the A3640 even at 25 MHz. But I have the same versions as you in my A3000 and managed to get a boot at 30 MHz. The heat sinks are needed for reliable operation when the chips start to warm up but are not needed for a cold boot.

Check the ROM speed jumpers to make sure both are set to the 25 MHz position. (It's still possible that some masked ROM's could be to slow to run @ 30 MHz). You could also try connecting the CLK90 jumper back to the 10 ns tap just to see what happens. Also, please describe what happens when the power is first switched on. (i.e. power LED cycles dim/bright or HD led stuck on or none of the above).

[mechy]

I don't know if it matters or is the case here, but i have found 3.1 marked boards with early pals as a 3.0 board would have.

also would upgrading u209 help ?

Of the 8 of these i have overclocked in the years, i have found 1 that would not tolerate over 28mhz even with the delay line hacks. I never did figure out what was the cause.. i have to assume its a pal or timing problem.

mech

[SpeedGeek]

If U209 was the problem here than the A3640 would not even work @ 25 MHz on an A3000. But just in case this is one those A3640's which does not like to run above 28 MHz maybe it would be good idea to try it with an A4000 @ 30-33 MHz.

[Sandman]

Well, I discoved a few things!

With SCSI disconnected, A3000 will boot from floppy @ both 30mhz and 31mhz but will lock up (power and HD leds both stuck on) @ both those speeds with SCSI->CF connected.  Still boots fine to CF @ 25mhz.  

I tried different CF cards and sizes and I get the same results.

I've heard of the A3640 breaking the A3000 SCSI but why only @ overclock speeds?
 
Do I really need a DMAC -04?

[SpeedGeek]

If you can find a rev. 4 SuperDMAC it would certainly be worth a try. You could also try another rev. 2 SuperDMAC as some work better than others. Another thing you might want try is replacing the 74F08 @ U103 with a 74HC08.

[Sandman]

Well, I relplaced the 74F08 @ U103 with a 74HC08 and the machine managed to boot once @ 31mhz but the machine hanged and the graphics corrrupted.  Put back in the 25mhz Oscillator and everything works fine.....

I wish that I didn't have to give up here but I have no idea what else to try.

[mechy]

Well, I relplaced the 74F08 @ U103 with a 74HC08 and the machine managed to boot once @ 31mhz but the machine hanged and the graphics corrrupted.  Put back in the 25mhz Oscillator and everything works fine.....

I wish that I didn't have to give up here but I have no idea what else to try.

Grab a warpengine 040/40 or some 060 card, it will run rings around that 3640.
I know its not what you want to hear and it costs more,but its worth it,if not for only the 128MB ram you can install. The warp engine scsi also rocks.

Mech

[SpeedGeek]

I never could get my A3000 mobo to run reliably past 30 MHz until I hacked SuperDMAC to run @ 1/2 CPU clock. The reason I didn't suggest this before was because DMA performance will get much slower. However, Bebek on Amibay says his A3000 is working @ 33 MHz with the SMD 030 so I'm waiting for more info on this.