I have this VO-9850 Umatic deck that I've been lugging around for the past few years and it has a really funky problem. Originally it ran perfectly fine after a re-belting but then one day when I needed to use it I discovered it would not record. :tears:
If I feed it a pre-recorded tape I can playback video and audio cleanly with timecode but if I record to a tape I get clean audio and timecode but no video. It totally screams a problem with the record heads or circuit but a cleaning with a Q-tip and rubbing alcohol at least didn't fix it. I do have the service manual but I have no idea where I should start looking. It's a very specific fault.

IIRC the older units like that were similar to older cassette decks in having a large MULTI-contact record/play mode switch. If yours has one try cleaning the contacts. Most video recorders use the play heads (plus an erase head) to record so if there is playback, but no recording the heads are not likely the problem.

It's probably a relay somewhere as the front buttons are only directly coupled to microswitches.

delete

It wouldn't be recording timecode or audio if it was a plug/sense switch issue.

Didn't catch that before... never mind.

Sorry to suggest it... but maybe the input select switch is in DUB. Audio connectors work the same for both LINE and DUB inputs. If this is not the case, check the video level meter. If it reads in the blue zone but you don´t get the video recorded, there is something wrong with the video modulator or record amplifier circuit.
The Sony 5000, 7000, 9000 series UMatics employ solid-state mode switching. Maybe a faulty transistor is keeping the recording signal path grounded. Time to get a service manual.