is the MMU adaptor board on the correct way round? - does it the atari work if you remove the marpet? - to check it - remove the MMU adaptor board - and disconnect the video shifter lead from the marpet x-ram board - and then test it - does it work? - also when you put on the MMU board it pushes the pins away from the chip SO when you remove it they may not make contact - you can easily push them back using a needle or pin (i had this once) push the needle down between the MMU socket and pin - CAREFULLY! what were the wires that you soldered? - were they on the MMU or video shifter?
