Thank you for your response!!
The RAM upgrade has pins on it that push into the MMU socket. It's not soldered. In an attempt to restore the machine to stock, I used a PLCC extractor I removed the MMU from the RAM upgrade and put it back into the motherboard socket. Same with the shifter chip - I removed the upgrade from that socket and put the shifter back in place. This is what gives me a black screen when powering on the machine. With the RAM upgrade installed I usually get bombs across the screen but sometimes the machine will work for several minutes before bombing.
I don't see any snips near the RAM on the top side of the motherboard. I will check again. I was told the machine has 2.5MB of ram. Attached is a photo of one of the ram chips on the expansion board. Searching for TC511000P does bring up this page which says it is 1mb. Since there are 18 chips on the board that seems to be 2MB on the expansion.
Also below is a photo of the first RAM chip on the motherboard which has a piggyback socket installed. Is that factory installed? Looking at this photo of a rev D board it doesn't have a socket like that.
Hhhm, if it is failing after warming up that could be a number of things. How is the power supply for this, has it been recapped? If it is the original external power supply it could be getting a bit long in the tooth now, those caps will be on their last legs probably. They could be failing which could perhaps cause your problems with the bombs.
The memory upgrade pushes into the socket between the MMU and the socket then? This might have caused the pins in the socket to fail to make proper connection when you have plopped the MMU back in. You could try gently trying to bend the pins in the socket out a bit. You did definitely put all the chips and connectors back the right way when you reinstalled didn't you? I can only assume that something has gone amiss when you reinstalled, I'd check the fitting of the MMU bit first as it can be difficult to line it up when you can't see it very well.
2.5mb makes sense, so presumably the spare slots could take you up to 4mb if you disable the onboard memory.
It's not entirely unusual for STs to have some in factory alterations, which may have been simply to make the ST work, although that socket is a bit unusual and probably not done at that stage. Soldering the socket on top would have caused a lot of heat to be transferred to the memory chip underneath potentially, which could cause it to fail over time. I can't see what purpose it could actually have really in its current state, unless the original owner was going to try to do a piggy back memory upgrade, gave up after soldering the first socket and brought that memory upgrade instead. If that chip has failed you could try and disable the onboard memory (this would reduce you to 2mb), or replacing that chip if your soldering is good. Again that could be the source of your crashes.