You do have the jumper cable on the back panel from the oven ref out to the external reference in?
Download both the operation and service manual from Keysight's web page. It's a bit tedious, but you can march through the debug procedures.
Sometimes, due to age, the internal oscillator drifts out of adjustment and can't lock to the external reference. Tweaking the adjustments often helps. You could try and see if this is the problem if you use a good signal generator (with 1 Hz resolution) at 10 MHz as the reference and adjust it a few Hz up or down.