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Effratom rubidium strange issue

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gossamer:
I was buying a frequency counter on the ebay and I got Effratom 1003018 as an easter egg inside the enclosure. Someone tried to connect it internally to the counter but gave up/failed or whatever other reason. Wires were never connected, it was just bolted to the chassis.
So I got working freq counter with crappy oscillator and rubidium standard.

I have no experience with rubidium standards so I dont know what is normal behaviour. What I am seeing with this unit is obviously not in spec.
I would assume this unit was not running for awhile and it is an old unit(9515 date code), so not sure if I need to leave it running for several days or ?

Unit seems to lock as I am getting lock signal correctly, lamp voltage is around 8.7V which also seems to be in spec, but the frequency output is all over the shop. We are talking about +- 100Hz. It drifts between 9.999902-10.000090. It is very slow drift. It toggles within 2Hz for awhile and then jumps on the next value, then its stable for few mins and start toggling again. Goes to one range and then slowly to other range ...

Is there anything that can be done with the unit or is it a rubbish ?
 

thinkfat:
Congratulations on the "inadverted" purchase of an atomic clock :-)

A rubidium clock typically locks steadily after maybe 10 minutes max, typically within a few minutes, depending on the model. The behavior you describe sounds like a unit that is not locking, or just marginally locking and losing the lock again.

The good news is that you can usually repair Efratom brand rubidium oscillators, as long as none of the unobtainable parts are broken. There is a lot of repair information available on the internet, but in order to help you out it'd be helpful if posted a couple of pictures of the outer appearance and maybe of the inside of the device.

gossamer:
Thanks,

I did fast disassemble of the unit and attached images from all sides.

capt bullshot:
Looks like an FRS.

I've got two of them, and finally found the issue they had:

https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg3605897/?topicseen#msg3605897

Too low a RF level on the input to the physics package causes instable frequency and loss of lock. Fixed by finding an appropriate capacitor that restored the resonant properties of the output driver circuit. Typical behaviour was: The unit locks after about 5 min warm up time, then gets more and more instable as it warms up more and loses lock. Can be diagnoses and fixed only if the unit has warmed up for some time.

Former repair attempts of the same units here, including some measurements and how to adjust them:
http://wunderkis.de/efratom-frs/

thinkfat:
That's an Efratom FRS-C or FRS-N.

Service manual here: http://www.ko4bb.com/getsimple/index.php?id=manuals

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