Can you use a different transformer model other than the 42TL004 and does it have to have a specific turns ratio? I can’t find that one in stock for a reasonable price.
I tried gauging the datasheet for that transformer and I can’t tell if this is a primary step down transformer. Do you have a circuit schematic?
Mouser has thousands in stock for about 2.49 USD each https://www.mouser.com/ProductDetail/?qs=%2bLh6ltJumVRX87vpOY3zCA%3D%3D
$18 to get it delivered here though
Transformers take an input voltage over the primary winding and over a secondary winding (two separate coils of wire) creates a different voltage proportional to the ratio of turns of the primary-secondary. This works because they’re wrapped around a certain type of metal core. If you buy a transformer with too many or too few turns all you need to do is unwrap or coil on more wire to change the output voltage. This voltage is the most important property the transformer provides to the circuit, the size of the wire does matter some too because the size determines the maximum current that can pass through before the wire gets hot and things melt down. I don’t think the MK312 has many high current components internally so modifying your transformer to meet the voltage spec of the factory one called for would likely work but I cant say for sure unless it’s tried. If you could get a transformer insanely cheap, like free, I myself might consider it but your risking the board and other components getting damaged if something goes wrong. Is saving $25 worth risking the $100+ other parts? It’s your choice but I would say just spend the money, you only need to do it once and don’t take the risk.
Since I have now the problem of sourcing two 42TL004-RC from xicon myself, I stumbled upon Your posting here. It is generally right, but You seem to be talking about a transformer connected to mains for powering a device.
Have You looked up the schematic of the MK-312BT? The two transformers are sitting in the output stage each just in front of the output channel. So they are powered from the battery and generating a higher and isolated output voltage. Current is not the main concern here, voltage and impedance and resistance is.