Yes for the opening of these handles - it can be done non-destructively, with some care taken. And perhaps you can see inside what few is in there to go wrong. (If not a poor ground pin contact, but in general tolerances between the rings and the contacts can be less well fitting to make some clean and strong connection).
When the cartridge was connected mid-air, there was no noise
Could you please try to clarify what you are actually saying here?
Are you saying by "cartridge mid air" that you put clips onto a bare T245 cartrigage (not in any handle whatsoever). And then ran yourself custom wires to the base unit, to test the cartidge alon outside of the handle?
Or are you saying that you took a C245 cartridge in 1 hand, and then the Aixun T245 handle in the other hand, and plugged it in while the handle was removed from the stand, that then it buzzed?
but when I used the handle, the handle was buzzing.
Ok. So again, I am not fully clear. Are you saying that "handle is cradle", that when you leave the handle into the cradle stand. That then it buzzes? Because this could be that when the ground jacket of the cartridge is inserted into the stand. That it then has a poor electrical connectivity through the metal stand itself. That then is supposed to go through that banana wire, which is a return path back to the station's sense pin (stand detect pin).
This could then be like something to give a partial circuit, for example if not plugged into. Or if through the MCU isn't clear path to ground.
I am scratching my head here. But when I measure this stand detect pin on my PCB, it's not connected to ground, it's like mega ohms (or not connected to GND path). However you can see on the PCB that they used a green and yellow ground wire to run as a seperate cable to the back of the station, the 4mm banana jack to the stand detect port. That has it's own dedicated wire.
So if you re-assembled your station back together, and mistook this ground marked wire for ground (that it is not actually). Then perhaps some mis-wiring or something can be possible by error? But then the stand detect function cannot work, that the heater cannot heat up. And it seems like an easy to spot mistake. (To eliminate at least).
Sorry if that line of enquiry is redundant, or unclear. But we do at least know that the stand detect (of the stand itself) can be a bit dodgy sometimes. Within the mechanical assembly of the stand, for the way it is constructed.
I forgot now what was some exact discussions about that. But some mod around the hinge nut or some things like that. During reassembly. That the connection fits against.