So I was trying the hack to 40 MHz for the Q605 and related models which is described here: http://homepage.mac.com/schrier/q605_40.html and I can not get it to work.
First, I switched the resistors to the 33MHz configuration as instructed and that worked fine. Then I lifted pin 9 of U17 and tied it to 5V through a 4.7K resistor. At that point I could get the start-up bong but no video or hard drive activity. Same battery as works at 33 MHz.
So I replaced the MC88920 with a MC88916DW80 and still had the same problem.
Thinking that the MC88916DW80 has been sitting around for the better part of ten years, I tested it by putting U17 back to stock configuration and the machine once again booted fine at 33 MHz. So that should confirm that the MC88916DW80 is installed and functioning, since the machine couldn't boot without it.
BTW, I have replaced the 68LC040/25 with a full 68040/33. So there should be no issues with the CPU at 40 MHz.
Anyway, I suspect one of three possible problems.
1) This Q605 has a 343S0161 chip instead of a 343S1135 at U17. It could be that this differently numbered chip is actually different in function.
2) The pin number given in the instructions could be wrong. Typically, one counts chip pins by starting at pin 1, which is marked (usually with a dot or indentation) and counting counter-clockwise from there. So the chip at U17 would be numbered like this:
But it is possible that the author of the article counted pins in some other manner, perhaps meaning pin 16 when he wrote pin 9. This is the main reason I'm writing to the list. If someone else has successfully implemented this speedup, then that would eliminate the second possibility.
3) There could just be something wacky about my hardware.
I have three other Q605 boards on hand, so when I'm not so exhausted I may try again, but I'd like to read the collected wisdom of the group first.