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Wombat (650, 800) board overclocking limitations


Well-known member
Oh using the jumper that way is very cute. I'll probably give that a go next time my board is out to add some flexibility and easily swap between stock and whatever customized configs I have in the other configuration.

For now I've been hacking the LC 475 ROM directly to futz with even more customizing timings. A few discoveries:

1) Booting the machine with (non-bump) 40MHz timings seems to pull RAM performance down by 11-13%, per Norton SysInfo. Overall system performance is about the same otherwise. Not the worst impact, and really isn't bad for a 21% clock increase.
2) The LC 475 ROM doesn't seem to have the bump config tables at all, only the regular configs, or at least not in the same places. Which might explain why it applies non-bump (faster) timings than a stock ROM would on 80ns machines (C650/Q650). So simply swapping in the LC 475 ROM serves as a minor overclock for memory.
2) The custom ROM I'm using has 55ns chips, which means you can push the ROM Speed a little, even at faster clocks. Lower is better. Stock Q800 runs at a speed of 3, stock Q650 at 4, and the respective 40MHz configs at 4 and 5. Even running at 44MHz I can push that down to 2 and, presumably, get faster ROM reads, so toolbox calls should run faster. At 1 the machine gets very unstable, and I'm guessing at 50MHz you're gonna need 3+, even with fast ROM chips. But that adds another dimension to overclocking. Not sure there's any ROM read test out there, might be worth creating to benchmark/verify checksums to ensure it's running stable.


Well-known member
Oh and something very low level in 8.1 haaaaaaaates clocks over 42Mhz or so, even with the updated memory timings. I need to dig through in MacsBug but it seems like it's waiting on hardware that never answers back (no longer ignores non-functional serial?)


Well-known member
I've been messing with a Q800 board at 50MHz using the 40MHz timings in the ROM.

I'm having the same issues where about half the time it just stops booting, but the mouse still moves.

It is extremely sensitive to RAM selection. It isn't entirely down to outright speed as a 50ns 8MB SIMM won't work, but some 60ns SIMMs do.

It doesn't like if I add multiple SIMMs, even if it is happy with both individually. I might remove the remaining onboard RAM.

Regarding RAM timings, it does feel like the 40MHz ones are either a bit beta, or they hit the limits of the hardware somewhere. Has anyone compare the timings to those in the 840av ROM (same ROM used in the 660av)?

This machine has a ROM slot fitted, so is more open to experimentstion.



Active member
Hello guys
Awesome thread
I have some 650/800 boards spare, at 33Mhz stock
I would like to mod these to 40Mhz
Unfortunaly i am a bit confused, can I please ask for a little help?:)

Is all i need to do for 40Mhz:

- replace oscillator with 20Mhz one
- add 300R resistor to R151 if not there
- add 300R resistor to R152
- change existing 1.2K resistor to 1.5K at R152
- add jumper or 0 ohm resistor to the spot between R148 and R233