Hello from Binary Dinosaurs!

[MbeeTech]

I'm guessing that the 'bee can't tolerate a single RAM chip failure so if one goes they won't boot at all.  

Cheers!

It depends on where in the memory space it is.  If it somewhere in the middle, basic will still boot, but if for example it is where basic stores
its scratch pad / system variables etc., it may not.  Also, If it is at the start of ram, it probably wont work but should at least initialize the screen and
throw a wobbly.

The RAM chips are all 6116 aren't they? I have spares of those so a quick poke with the scope might find some dead ones.

Yes, the ROM based bees use 6116 chips for their main memory, unless you have a late production one that may have 4 x 6264's instead.

I read on the forums last night that the 6545 is also prone to failure so that'a another thing to check.

Simple chip swap from dead machine into your working one will verify whether the 6545 is working o.k.