Rebonjour cher Jason and Luca:
1/ The issues of U800 are discussed at length in many threads, see especially the fine notes of Chuck Harris.
use TEKscopes2 as he was banned from Tekscopes at groups.io.
2/ In U800, The original TEK designers used NO heatsink, the dissipation is via the mounting screws and PCB.
The tension on the IC presses it to the PCB. Set by special washers and torque spec on nuts.
DO NOT attempt to tightne the nuts, or remove them or add any HS.
If you already made such useless mods DO NOT touch as further fiddling may kill the custim TEK IC
3/ Later SN 2465B used a substutue U800 as TEK stopped cistom Hybrid prodction.
Those IC are more pronee to fail
4/ BOTH mgt screws on U800 are NOT at ground but -80 (?) V.
If any hardware, nuts, or HS on the screws touches the housing, instant short of the PSU and possible kill of U800.
5/ The need for HS is a myth, them is fine and within rating IF you JUST LEAVE IT ALONE!!
Some use an external fan blowing at U800 if running 2465B with case removed.
Internal fan gives sufficient air with case properly in plave and NO HS or mods to U800.
6/ I have measured the U800 temo at the screws and center of IC, very normal Trise, no HS.
I have many 2465/B/7 that have run for decades with no U800 failure and no HS.
Just the experience of an old retired EE (1968).
Bon Chance !
HAVE AN ABSOLUTELY FANTASTIC DAY!
Jon
Jean-Paul