Hi Doktor,
converting a CTM-640 to a CTM-644 would also be possible, but it's not something I'd advise for a "Hardware-beginner" to try. If you are confident enough to do the conversion, I can put together a description of what needs to be done, but be warned, it involves building a small circuit and soldering inside the monitor on the power circuitry. Due to the the fact that CRTs carry very high voltages ( >1000V ) and lots of capacitors that need to be dis-charged before working on the circuit, I really don't advise this, if you're not sure of what you're doing.
Bryce.