iz8dwf | HP-8640B repair. It needs a broken lamp! @iz8dwf | Uploaded July 2023 | Updated October 2024, 15 minutes ago.
Both my HP-8640B's opt-001 oscillators failed in the same way: one lamp
inside the 5080-1718 thermistor assembly goes open circuit. This lamp needs its envelope broken for the circuit to work properly.

Timeline:
00:00 - Welcome
01:07 - opt-001 fault
02:16 - Troubleshooting
04:04 - The faulty part
05:18 - Broken lamp?
10:01 - Failed attempt
11:27 - Rebuilding the thermistor
12:28 - First test
13:47 - Second not so easy one
15:37 - Lamp differences
17:52 - Second one works too

Various HP-8640B manuals:
ko4bb.com/getsimple/index.php?id=manuals&dir=HP_Agilent/HP_8640_Signal_Generator

Filament lamps introduction:
us.rs-online.com/m/d/04fe4bc2ac7e247d70e8e7d88fe48b19.pdf

Music in this video:
No.4 Piano Journey - Esther Abrami