Have you hold your finger on all the chips? Any of them getting extremely hot? Warm is okey, you should be able to hold your finger on them. If it is burning it is a sign that is is shorting. MMU or perhaps memory chips hot?
Otherwise you need to start checking if you got clock signal on all chips and if you got signal on address lines and data lines. If anything seems strange or a missing signal. Or checking cold solder joint or cracks. Anything suspicious really.
When does it start bombing by the way? At boot screen? Directly after power on? Does it have time to start accessing the floppy drive?
When booting with diag cart in you could connect to another computer via a null modem cable and see if anything appears on the other computer with a communication program on it. 9600 baud 8N1 (8 bit char, No parity, 1 stop bit).
RAM seems to look as they do on factory Mega 1. I got 256 in my Mega 1 as well.