Have you checked all of the wires running from the board to the monitor? Sometimes those little connections can be fickle. I suggest unplugging and plugging back in the wire that runs from your PCB to your amplifier (on both ends, PCB and amplifier). Also check the wires running to the speaker from the amp. Lastly, I'm not sure if this would affect it or not, but ensure that your Z80 was seated properly when you removed your High Score Save Kit and put your Z80 back in. I'm far from an expert and you probably already know a lot of this stuff, but that's where I would start. Surely someone more seasoned in this stuff will be able to help further.