Effratom rubidium strange issue - Page 1
Forum section: electronics / testgear

author: gossamer
author_rank: Contributor
timestamp: September 28, 2021, 03:35:32 pm
content: 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 ?

author: thinkfat
author_rank: Unknown
timestamp: September 28, 2021, 03:47:52 pm
content: 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.

author: gossamer
author_rank: Contributor
timestamp: September 28, 2021, 04:45:03 pm
content: Thanks,I did fast disassemble of the unit and attached images from all sides.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 28, 2021, 04:52:53 pm
content: 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#msg3605897Too 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/

author: thinkfat
author_rank: Unknown
timestamp: September 28, 2021, 04:54:56 pm
content: That's an Efratom FRS-C or FRS-N.Service manual here:http://www.ko4bb.com/getsimple/index.php?id=manuals

author: gossamer
author_rank: Contributor
timestamp: October 01, 2021, 06:55:59 pm
content: I was looking at that paper and did some measurements. All seemed ok. Was still locking but freq would drift a bit. Then I disassembled the whole thing and measured those resistors on the lamp board. They were ok.While whole thing was in bits I said I will turn it on and poke around with oscilloscope, but then the rubidium locked and it was spot on stable. So I assembled the whole thing again and turned it on and it was drifting again. With several screws out of assembly it is locking fine, but as soon as I put everything together its out. Obviously some trace or solder joint is causing the issue. No clue how will I find the causeI suppose extra screws in this case might not be that bad ?

author: thinkfat
author_rank: Unknown
timestamp: October 01, 2021, 07:08:47 pm
content: I found that spurs from the exciter oscillator can have a quite serious impact on the stability. Try tuning it slightly, see if the problem goes away. Spurs should be visible on the servo test point if you have a scope connected set to slow sweep and untriggered roll.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 02, 2021, 07:35:12 am
content: The symptoms look quite similar to mine.Check the 60MHz level at this end of the coax cable, and the 125Hz output of the servo loop while e.g. fastening the screws / warming up the unit.If this drifts while handling / warming up, test some small 2pF ... 20pF capacitors near the output transformer. If it is the same as with mine, placing a capacitor here should increase the RF level and the 125Hz signal and keep the unit stable when assembled and warmed up.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 02, 2021, 04:55:26 pm
content: Check to see if someone mixed up the long and short mounting screws. Maybe a long screw is shorting something out when you screw it down. I don't know if there are long and short screws, but Murphy just loves to use tricks like that to drive you crazy!

author: JoeyDeng
author_rank: Regular Contributor
timestamp: October 24, 2024, 01:24:19 am
content: I meet the same issue with my Efratom FRS. However, the problem wasn't actually caused by the uninstalled capacitor. I spent day and night trying to figure out what was happening with my clock. I attempted to add a capacitor to C30, but it would still lose lock when the frequency generator board warmed up. So, I think a capacitor on the board may be aging. I tried measuring all the capacitors on the board with an LCR meter, but the problem is still not resolvedI lifted the frequency generator board away from the oven to maintain the lock. I'm using a heat source (a wooden stick heated with a 100°C hot air gun) to touch the components on the board. I found that the lock is lost immediately only when I touch Q5. I then thought maybe the Q5 NPN RF transistor is aging. After tearing it down and measuring it, I found that the Rds of Q5 is almost shorted to GNDI currently don't have the components for replacement, but I will provide updates once they arrive.