This will be quick, time is short...here's some quick steps to run your 660av at 40mhz. Note this is using the unreleased 33mhz timings, so running at 40mhz is overclocking the RAM and VRAM. This changes the gestalt id to 79 instead of 60. I found appletalk to still work at 40mhz. None of this is not strenuously tested, so YMMV.
(After)
There's no way to use the 40mhz timings without ROM modifications, only 840AV has those timings and the lack of Nubus controller prevents that configuration from booting.
- Get a 20mhz clock by doing one of the following
- Replace 12.5mhz oscillator with 20mhz oscillator
- Short pins 1 and 2 of J36 to disable onboard osc, and piggyback a new 20mhz on top
- Short pins 1 and 2 of J36 (GND) and inject a 20mhz clock signal on pin 3 of J36
- Replace MC88920 with MC88916
- MC88916 should be at least 70mhz and preferably 80mhz rating
- Move R140 to R142
- Move R143 to R148
- Add a heatsink to your 68040
- (For system 7.1) Modify System enabler 088 gbly resource as below to add the gestalt ID for 660av-33mhz to the list of supported machines. Note these gestalts have an offset for 6 for some unknown reason.
- Note: You'll have to modify the system file for newer OSes, I leave that as an exercise for the reader
(After)
There's no way to use the 40mhz timings without ROM modifications, only 840AV has those timings and the lack of Nubus controller prevents that configuration from booting.
