Really funny not to use the 5 V directly from the X43 or from wiring to the power supply.
Does your serial interface work? I don't use the tape anymore and replaced the drive by an self made extended keyboard
to emulate the G66, INP-FWD and INP-REV by a single key stroke.
By pushing the serial link to 9600 Bd instead of 300 Bd transferring programs is a bit faster now.
Has your C5 still the unreasonable 110/300 Baud rate?
By changing one EPROM you can go without problems to 1200 Bd, above (up to 9600) it is a bit more tricky, one must insert 10 ms line delay
by using a terminal program and the real transfer rate PC to C5 is then about 5000-6000 Bd, from C5 to PC there is no restriction.