@Wintermute_BBS I realised it was an unusual machine, but it got me to wondering if they had dropped the 12v and -5v completely. IIRC it's only the RAM that needed the -5v and RAM and video needed 12v. A cursory glance at the board suggests the clone is quite similar.
@Wintermute_BBS Is that the lower and upper RAM? The original Spectrum had 4116s for the lower 16k and 4532s (or equivalent) in the upper RAM. The 4116s are especially vulnerable and require three voltages (-5v, +5v and 12v). The others can fail too, but maybe your machine does away with the 4116s and therefore has a simplified power supply?
@Wintermute_BBS I don't know that particular machine, but assuming it's similar to a normal Spectrum, I would start with the RAM. Make sure you have all the voltage rails and then test the 4116s. They have a tendency to fail, particularly if the voltage rails go squiffy.