Julien I don't think I would read a lot into the failed R just replace it. They are in a nasty hot environment and simply can age and die. Get at leat 2 resistors just incase there really is a shorted transistor or something else going on. Regards Paul
On Fri, Feb 4, 2022 at 3:50 AM Julien Goodwin <[email protected]> wrote: > Well that was unexpectedly easy (to diagnose). > > Thanks all for the replies, much appreciated, you have once again > confirmed my appreciation for this group. > > I sat down this evening to work through your various suggestions, and, > by chance, picked up the unit without my lab power supply (and more > importantly, its noisy fans) being on, where I heard a little rattle I > must have missed before. > > Removed the bottom panel, which I hadn't done yet, and out dropped the > remnants of a Vishay RW80U 20-ohm resistor. Based on the circuit this > pretty much has to be R2 on the lamp board (following the December 73 > circuit I have, which has that at 10 ohms), which is directly in-circuit > for the lamp, so would explain exactly what I've seen. > > The lamp board in general seems to have a few holes missing components, > so I'm going to have to get it out which looks like a project in itself. > > If anyone has any pointers to decent guides on optimum process on > further disassembly, otherwise I guess it's copious photos, notes, and > hoping I don't stuff up anything too much. > > I'd also appreciate suggestions for replacement foam to use in the lamp > cavity as the original stuff has long since degraded. > > Paul, Peter Robert, > Thanks for reinforcing what I'd suspected with the lamp. > > Matthias, > I hadn't bothered with an actual temperature meter, luckily on the FRT > it's actually easy to confirm lamp temperature in use, and it certainly > wasn't anywhere near 100c. I think what heat there was likely leaked > through from the crystal oven, which was getting pretty hot. > > Bob, > Fortunately the onboard crystal does seem likely to still be close > enough, based on what I was seeing on a counter, so I feel like I have a > hope of recovering the unit if I do manage to get the lamp going again. > > On 1/2/22 8:17 pm, Julien Goodwin wrote: > > I picked up an Efratom FRT a while ago, and it finally arrived the other > > day, sadly it doesn't lock. > > > > The oscillator inside is an older Efratom FRK, and it's also clear > > someone had been inside already, although it does appear to be complete. > > > > The FRK manual does include a key description I wish I'd noticed a few > > days ago before full disassembly: > > > > "Terminal pin 7 provides an indication of proper operation of the > > rubidium lamp. For nominal operation the signal is 6 to 12V. An > > inoperative lamp is indicated by an signal of approximately 3V. The > > internal resistance of this circuit is approximately 6K." > > > > ... indeed I'm seeing 3v on that pin. > > > > Based on the circuit this appears to simply be the output of the first > > stage op-amp from the photo-cell, so is this effectively an indication > > that the (electrical part of the) lamp is dead? Certainly it wasn't > > anywhere near the temperature it should have been when I removed the > > bulb after running for over an hour. > > > > Just for completeness, the unit does draw up to ~0.8A when started up > > cold with 24V, dropping down to about ~0.5A after a while. The sealed > > sub-section of board A4 seems be where all the heat is, and I'm assuming > > that's an ovenised part of the crystal oscillator. > > > > I also popped the rubidium bulb out and confirmed there's no obvious > > issues there. > > > > I do have another FRK here, but as it's an FRK-LN, and it's also my > > working house standard I'm somewhat loath to open it up lest I end up > > with two dead units and zero working. > > > > Any suggestions? > > _______________________________________________ > > time-nuts mailing list -- [email protected] -- To unsubscribe > > send an email to [email protected] > > To unsubscribe, go to and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send > an email to [email protected] > To unsubscribe, go to and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
