Hey,
Wow, mystery part no more

, I have ordered a few HMC720s !!
I've received an ultra high bandwidth scope last week and will absolutely probe everything possible on that motherboard

So, on a systematic approach:
1. Check Oven and signal path up to the synth.
2. Check VCO
3. Check Synth and Buffer
4. if signal (+/-) is present at Buffers output (both outputs) then:
Check SPI comms.
5. Check FPGA/ASIC
Last week the same story repeated, one other MSOS804 broken with the same problem, and also a few weeks ago another MSOS404 same problem.
I wonder if Keysight uploads some artificial errors (functions) with updates on such scopes
