Mail from Doug Little back in the day
I take that back!
I just replaced all of the capacitors on the Falcon PSU with low ESR versions and slightly higher values (68uF vs 33uF for the 400V, and 1500uF vs 1000uF for the output filter caps).
The Afterburner is now burning running at 48MHz for the first time and is able to complete a Nembench test... I should probably now look at a PicoPSU to replace it with.
One of the things that I have been wondering about on the Afterburner is the lack of bypass capacitors on the board. I can only see a single SMT ceramic near the cpu. There might be more under the CPU - I don't remember looking when I swapped the chips. But there are 9 big ICs on there and it seems to be relying on ground/power plane capacitance or the Falcon mainboard, and that's on the other side of the expansion port.
Anyway I'm thinking of adding a tantalum & ceramic cap on the underside of the AB at the expansion port where the power comes through, to see if it helps the board at higher frequencies.
I'll also replace the mainboard 4700uF cap while I have access to the board.
Will report back if I find out anything useful.
One of the chips on the Afterburner board is 'MC88915 FN 70' which is a PLL / clock doubler intended for driving the PCLK of a 68040 CPU. The '70' signifies a maximum operating frequency of 70MHz (or 35MHz x 2) so it's only just rated for a 32MHz 68040. It probably doesn't cope well as the BCLK approaches 50MHz (PCLK >= 100MHz). I have also noticed this chip gets warm at BCLK == 40MHz. It is probably the limiting factor on the board.