Masayuki found that the JAC heater driver shuts itself down as soon as it is turned on. 

Daniel traced this back to that the heater elements are grounded somewhere. Turns out that the DB15-DB9-DCPD to DB25 in-vac tri-cable (D2500336) has a defect. One pin for Heater B Terminal D is shorted to the shield. Since the shield is ultimately connected to the chassis ground of the heater driver, and since the driver is differential, this effectively short-circuits the output of one power amplifier to the ground. Masayuki has a spare, we'll check it in the lab and will replace it.

From outside of the chamber, using DB25 breakout board on D4-F10, when everything is connected in chamber, in-air pin 13 (shield of the cable inside the chamber) is shorted to pin 8, 9 and 22 with only 2 or 3 Ohm. These are all connected to Heater B Terminal D, which is one leg of the Heater B.  In addition, pin13 and pin 10 and 23 show 54 Ohm, these are Heater B Terminal C, so this is expected (nominal resistance of the heater is 50 Ohm). Nothing is shorted to the chamber gound, which is a good news.

When the DB15 connector (that goes to the two heater elements and a thermistor) was disconnected in chamber, the only short-circuit that was observed from outside the chamber was pin 13 to pin 22 (still 3 Ohm).

When the DB25 connector was disconnected at the cable bracket, nothing shorts to anything.

The conclusion is that something is wrong with the DB15-DB25 part of the tri-cable.

Oh, D2500119-v2 says that the JAC control is on D4-F9 of HAM1, but it's on D4-F10 at LHO. This is already captured in D1002872-v10.