[time-nuts] Schematic for the 'analog' section of an FE-5680A?

[Jamieson (Jim) Rowe]

Hi,

I’m an elderly time-nut newby, and I’ve already struck trouble with a used FE-5680 Rb-vapour reference
I bought via eBay. It runs (from +16V and +5V), and the current drain starts at 1.75A and then drops over
5 minutes or so down to about 650mA. But it won’t lock, even if I leave it powered up for a few hours (with a fan
to keep it from getting too hot).

The output frequency just keeps switching up and down between about 9999850Hz and 10000080Hz, with a bit of ‘lingering’
near each end. And of course the ‘lock’ output at pin 3 stays stubbornly at about 4.35V.

To my newby brain, this sounds like the problem is either in the Rb lamp (too dark, perhaps), or
else in the photodetector and buffer, etc, forming the ‘dip detector’ part of the feedback loop.

Could one of you much-more-experienced time-nutters tell me if my diagnosis sounds right?

Also, I’ve found a schematic for the digital sections of the FE-5680A in your archives, kindly drawn up
by one of your very experienced members, but has anyone done a similar schematic for the ‘analog’
sections?  I suspect I’m going to have to work out for myself  where to test for a fault in these
sections. There’s also a bit of a mystery (in my mind, at least) regarding that little 2-pin SIL header
just near the Rb lamp, on the top of the PCB. Anyone know what that’s for?

All the best in anticipation, folks.

Jim Rowe