the two jumpers under the CPU card need to bot be set to "external". thats the only configuration you should have to do so the mainboard clock signal comes from the CPU clock. but as you have an 040 board, this is probably set already. is it the commodore 040, or something like a warp engine?
when you say black screen, does the keyboard capslock light flash? the floppy click?
take the simms out and only have the mainboard memory see what that does. ok it'll be dog slow, but it'll show if you have bad ram on board.
while the simms are off, are there any burn marks around the mach logic chips between the simm slots?
if you have the card out, give it a sniff, does anything smell cooked? have a look at the card. its a 6 layer board so if its warped some of the traces inside the card may be broken, does it look bent? i remember mine have seating problems (and was slightly crooked), so i ended up making my own mainboard standoffs to mount the thing with. no more problems having to reseat the card on the CPU slot.
pop the PPC heat sink off. is there any discolouration on the chip itself? does it smell "hot"?
another thing, with the machine starting from cold (and i mean temperature wise) and with the simms off, and that you have grounded yourself, do any of the chips on the board heat up REALLY quickly? if so, this could be a sign that the chip has gone byebye. this also works with memory aswell. sure all the chips on the board run warm, but i mean finger burningly hot after a few seconds.
and keep that PPC cool. i ended up mounting a Socket7 headsink and fan on there with heatsink paste, and 1mm studding. make a world of stability problems go away.
check its seated properly. mine occasionally squeezed itself slightly off the slot when first put in.
