jonsig hello again,
Temp tests: Very hard to get a better than 5 deg C accuracy. Ambient, warmup, drafts, contact to tip etc are all variables.
A 30...36 ga TC in a drilled hole or welded to the DUT is one solution. Also a thermal compound gives better accuracy.
Coax length : Easy to make a faulty splice on either end of a coax, that is more likely the fault than the length.
But aside from the matching and wavelength at 13.5 MHz, all coax have losses, eg 15 db/100' at a particular freq.
Consult the excellent data and app notes from Belden wire and Steve LAMPEN for details.
My SMPTE 2017 digital audio transmission paper had data on 18 TP and coax cables.
If you can ID the exact coax Metcal uses, then the attenuation at 13.5 MHz with various lengths may explain the differences if a cable is shortened.
Control: I have no info on MX-500, modern units, the 13.5 MHz Metcal nor the control systems, my design was SP-200 in 1990..1993!
The original system forced a constant current thru the cable and induction loop at the tip, and the curie point material maintained a calibrated temp as the tip heated up.
Thus for a calibrated current and particular tip, neither cable length nor power supply/workstation affected tip temp, only the tip model and curie point.
I suspect the same patented principle applies to the modern units and the 13.5 MHz.
Bon journée,
Jon