Overclock ACA620 to 25mhz

[brownb2]

Reading around this is just a job of soldering a wire somewhere to change the clock divider from 3 to 2 and ensuring the CPU cooling is adequate.

Does anyone have a guide on how best to do this (I can't believe nobody has done this yet...)?

[brownb2]

Found the answer http://www.a1k.org/forum/showthread.php?t=43453

[paul1981]

Found the answer http://www.a1k.org/forum/showthread.php?t=43453

Thanks! If I get the urge one day I might give this a go myself. :hammer:

[brownb2]

Tried it today - took a switch from an old Amiga external floppy* (why would you want the floppy off?) and repurposed it so it slightly protrudes out of one of the front underside keyboard vents nearest the left foot, hot glued it in place inside, looks like its meant to be there
Soldered up the wires from the switch to the relevant accelerator points, closed the two points to be "left open" (Jen's PCB/overclock warning) et voilà!
Now I can switch it on the fly when I need more grunt, I get exactly one Amiga more power in over clock mode (0.8 mips) and I have a heatsink on order just in case.
Jen's "economy accelerator" is looking like a sheep in wolf's clothing, I wonder how long the CPU will last lol.

*No external floppies were harmed in the making, I resoldered the drive permanently on.

On a related note anybody recommend some good 020/030 ecs demos to stress test?

[paul1981]

0.8 mips more? I'd of thought it should be higher than that?

[Oldsmobile_Mike]

An overclock from 16.67MHz to 25MHz, .8 sounds about right.  Great job and write-up, bonus points for saving the floppy drive!  

[brownb2]

My poor maths... 0.98, not much different.

0.8 mips more? I'd of thought it should be higher than that?

[AmmoJammo]

I'd probably be running it at stock speed till you have a heatsink fitted

[brownb2]

How did you know? adoom without heatsink will kill it - I let it cool for a couple of mins and reached safe temp and its back to life again. I don't think I'll push it again!

I'd probably be running it at stock speed till you have a heatsink fitted

[paul1981]

My poor maths... 0.98, not much different.

Almost x2 standard Amiga's faster.

[brownb2]

I installed the heatsink onto the ACA620 (in the small space top side) and ADOOM demo mode with DOOM 2 wad lasts about 5 minutes longer before the A600 freezes.
Unlike previously, an immediate reboot works (without heatsink the A600 needed a minute to cool).

Needless to say, 25mhz for Workbench tasks it's fine just to use a heatsink, but for pushing the limits (i.e. ADOOM), a fan is a definite must.

Not knowing enough about circuit design, if I put a small fan in and attach it to +12V FDD connector and the "ground" on the switch (which is attached via cable to the ACA ground/negative), would this cause any damage?

I'll admit I've already briefly tested it (little knowledge, dangerous thing, also fan was connected to +5V rail) and it seems that the fan (and ACA) works and comes on when I flip the ACA switch and put it into 25mhz mode, but I don't know if this setup is electrically safe as the +12V is going to the ACA negative/ground - can anyone comment?

[F0LLETT]

Not knowing enough about circuit design, if I put a small fan in and attach it to +12V FDD connector and the "ground" on the switch (which is attached via cable to the ACA ground/negative), would this cause any damage?

Just stick to floppy connector. All connections needed are there, +12v and Ground. So when machine is turned on, fan comes on.
Also might be worth noting, if you do not have good grounding (via the ground cable that comes with ACA) to the ACA620, then it will cause freezing / crashing issues.

[AmmoJammo]

The ACA620 appears to have been designed with the idea that you can't overclock it unless there's a heatsink fitted over the cpu, which connects the two heatsink mounting holes together, allowing the overclocking to be enabled.

One mounting hole is simply connected to ground, so you can use any ground point for this, meaning, if you want to use the same switch to overclock the cpu, and enable the fan, make sure the ground "input" to the switch is solid, and use the output from the switch to go to the fan as you suggested, shouldn't be an issue.

[brownb2]

I never received the extra grounding cable that Jen's mentioned in A1K post, I've not needed it as afaik it was only required for some mainboard revisions. Is this cable required for all mainboards?

Just stick to floppy connector. All connections needed are there,
+12v and Ground. So when machine is turned on, fan comes on.
Also might be worth noting, if you do not have good grounding (via the ground cable that comes with ACA) to the ACA620, then it will cause freezing / crashing issues.

[paul1981]

I bought my ACA620 when they first came out and nor did I receive a grounding cable. Am I supposed to be using one? I had to send mine back for a firmware update to v2, but I wasn't aware of a grounding issue.