Rubidium Standard repair

[veedub565]

Hi just looking for some pointers on fixing up a rubidium standard.

Quartzlock A10-LPRO. the VCO does not lock, I can see the voltage ramping up and down (on the VCO monitor pin), checking with a frequency counter shows the frequency ramps up and down through 10MHz. So this appears to be functioning normally.

The module gets hot, too hot to hold my hand on it, so it looks like the heaters are working.

Lamp monitor voltage is 5.07v and this is where I suspect the problem lies. From what I've read healthy lamp voltage is 6-9v.

The BIT lamp does not illuminate, so I assume the rubidium is not locking and that iswhy the VCO won't lock either. I did look at this in the dark with the cover off,  and I couldn't see any sort of glow from the unit.