NOTE: Without a schematic, this is a bit of generic info and speculation.
My first suspect would be a shorted op amp on the channel with the pegging meter. On many of these machines, the inputs share op amps. In other words CH1 and CH2 will share a chip. CH3 and CH4 will share a chip, etc. Look for an op amp with supply voltage or DC voltage on its output pins. You can compare the suspect op amp voltages with the working channel(s) op amp(s). If you can't find a schematic, find a datasheet for your IC's. Off the top of my head; I'm not sure, but my recollection is that these Yamaha's use SIP op amps (in case your trying to find them looking at the bottom of the board).
On the power supply: It would be more likely that the supply would be damaged than the 4-track if it wasn't rated for proper current. At any rate, if the device calls for a 600mA supply, you should be using at least a 600mA supply. I doubt that is what caused your problem, but it's still not a supply you should be using.