Well today was "crack open PSUs and fish about inside" day. The Gigabit Ethernet PSU showed nothing on a voltmeter so that proved the PSU was definitely shot. On closer inspection there's evidence of some blown part on the circuit board. Just not sure what it is yet.
The MDD one had a clearly blown thermistor. From what I've been reading on pages like this one this was probably what was causing the fuse to blow. We tried replacing it with a similar-looking one from a spare PC PSU but while it didn't pop like before, it did still blow the fuse in the plug. After some googling around and finding a picture of the actual part (it had blown where the part number was marked), the rating on the one we put in was different (5ohm/7A vs 4ohm/8A on the original).
So I'm a little wiser but still have two dead G4s. I'll take another look at the PSUs and see if I can identify the parts that need replacing (there's a few threads about these on badcaps and a post in one suggests that the same parts tended to fail on all these Samsung ones) and have a crack at getting them going.
The MDD one had a clearly blown thermistor. From what I've been reading on pages like this one this was probably what was causing the fuse to blow. We tried replacing it with a similar-looking one from a spare PC PSU but while it didn't pop like before, it did still blow the fuse in the plug. After some googling around and finding a picture of the actual part (it had blown where the part number was marked), the rating on the one we put in was different (5ohm/7A vs 4ohm/8A on the original).
So I'm a little wiser but still have two dead G4s. I'll take another look at the PSUs and see if I can identify the parts that need replacing (there's a few threads about these on badcaps and a post in one suggests that the same parts tended to fail on all these Samsung ones) and have a crack at getting them going.