First a general recap of all electrolytic capacitors was on order for both of them, and while I was at it, I also replaced the tantalums.
The power supplies had a couple obviously bad capacitors. I didn't bother shotgun replacing them. The owner wanted to outright replace the PSUs with modern ones.
The Macintosh II line however puts +3..6.5V from the internal batteries onto the power supply's /PFW line. Once the power supply started up, the circuitry keeps this line high with power from the supply.
Seems a bit convoluted, but it boils down to having to invert the /PFW line using the standby power provided by the ATX supply.
Seems a bit convoluted, but it boils down to having to invert the /PFW line using the standby power provided by the ATX supply.
A simple bipolar transistor will do the trick. I chose a BC548C NPN, simply because I have a lot of them.
Some napkin math lead me to pick 10k for the base resistor and 470 Ohms for the collector. The +5V come from the standby power.
Some napkin math lead me to pick 10k for the base resistor and 470 Ohms for the collector. The +5V come from the standby power.
Ok I think I need some help. The idea was to use the HDD from the IIfx to boot and test the other macs as those came without any hard drives. I kinda misplaced the ROM module of the IIsi, so IIvx it is for now. It doesn't want to boot this HDD. Can you even do that? Is there a floppy image I can write to test these machines?
Ok I'm a bit of a dum dum. This is a 32bit CPU and needs all four RAM Simms populated. Boots now.
The onboard graphics however seemed to be bad. With a pattern that changed if I swapped the VRAM modules around. The ICs on there are ordinary DRAM.