In the last few years, I've obtained 3 FRK's and all 3 had the same issue. The C-field voltage was so far off from where it needed to be that the servo loop wouldn't lock up. In all 3 cases, I was able to get them working by changing out resistors R17 and R20 in the C-field circuit on the power supply board. Other resistors in the same area, R16, 18, and 19 might come in to play too. In at least one of the units there was also a wire that needed to be moved and possibly a jumper that had to be removed or added. This is all very easy to do as it's on the top side of the power supply board, and the resistors are mounted on terminals to make them easy to change.
My guess is that the FRK's were installed in some larger system that used the external EFC input and when the FRK's were pulled they were not configured properly to be used standalone as I am using them. I probably took some notes on exactly what I did to get them working. Let me know if you need any details and I'll see if I can find (or remember) exactly what I did. Good luck, Rodger -----Original Message----- From: ed breya <e...@telight.com> Sent: Wednesday, September 22, 2021 2:15 AM To: time-nuts@lists.febo.com Subject: [time-nuts] Re: Some assistance with Ball-Efratom FRK-L Rb Osc. Needed If you look at the schematics of the C-field adjustment circuits, you'll find where the tweaking is available for fine tuning the Rb system. I think that's the only "frequency adjustment" spot to worry about. The other is the tweak for the crystal oscillator to accommodate long term drift there, if its tuning range hits a limit. This an internal, seldom needed adjustment, so the C-field is likely the only one that would be useful externally, to (very) fine tune the output frequency. I have some similar M-100 units, which have access to the C-field pot through a hole in the back end heat sink and case. An external adjustment port would be either a substitute for the internal pot, or something to add to or subtract some current from the C-field coil, that's mostly controlled by the internal pot. So, look for an internal pot. If there is one, then the external stuff likely just modifies what it does. If there isn't one at all, then the external circuit may have to provide the proper current, so it may be a little more complicated. It could also be that the C-field is set up to some nominal value, with fixed resistors, without any internal pot, and an external one just moves it around a bit. If you just run it, and it locks, it should be good to go, and you can figure out these fine tuning issues later. Ed _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.