Did I mention my Rubidium reference was in need of a fix?
Some time ago I've received them and believed I've had fixed them:
http://wunderkis.de/efratom-frs/Stashed away, and pulled out at a few occasions, I noticed they started to loose lock again and were thermally instable and drifty. One of them even changed its frequency significantly if one loosens or tightens the cover screws
![Shocked :o](https://www.eevblog.com/forum/Smileys/default/xshocked.gif.pagespeed.ic.CrwtrNUjLO.png)
So at first I suspected the pots had gone drifty again. Did a re-adjust and replaced them by fixed resistors (circled green). Worked for a short amount of time, then the same symptoms appeared again. While tinkering and testing, I've noticed one of them was out of adjustment range. This could be fixed by changing the resistor that was intended for this purpose. Apparently there're quite a few components that are intended to be selected while test.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1235624;image)
Anyway, this didn't fix the main issue (loosing lock and drifting). Some more testing showed the sync loop loosing demodulator signal amplitude when the units reach their final thermal equilibrium. By lucky chance, I've noticed the amplitude changing while probing the 60MHz output (marked ASP1) with a 10:1 scope probe. Interestingly there's a space for a capacitor C30 "SIT" unpopulated. Adding a capacitor (one unit took 10pF, the other one 18pF) significantly increased the demodulator signal amplitude.
So I soldered these capacitors (circled red) and now I'm watching them:
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1235626;image)