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.
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.