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

[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?

[brownb2]

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

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

[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

[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?

[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.

[brownb2]

Same here - installation pics and sheet don't show it.

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.

[brownb2]

I had ann unstable V1, sent it back to Amigakit and received a V2 back (both without any screws).
How essential are the screws to the correct operation (i.e. with it overclocked is it likely to be more stable)? I've not seen any problems with it running stock speed.

Do you have a picture of the of the ACA with installed screws (to see if I have anything similar lying about)?

You must have an early version of board. As new ones have screws.
This must have been the discussion, to include something like that to screw into ground.
Giving you the extra grounding point.

[brownb2]

I'm not happy with the +12v going through  the ACA's negative that's currently only taking the +5v from the ACA, it still seems a bit dodgy (for me, not having the electronics experience).
I've had a quick Google for step downing +12v to +5v and a car charger for a mobile phone and use the MC34063 mentioned in this thread http://www.instructables.com/answers/How-can-I-convert-12v-down-to-5v/ can probably do the job.

Having said that it may just be too much work and so I'll leave it with an always on fan pressed up against the chip heat sinks I fitted instead (as opposed to the cool shim job mentioned on the last post).

... 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.