[time-nuts] FE 5680a 3pay 36.89 includes shipping. Finally ordered one

[John Beale]

On 1/3/2012 8:20 AM, Robert Benward wrote:
> I got mine for around $25 in a regular bidding affair. Mine had trouble
> locking, It only locked a handful of times, and that took forever, but most
> of the time it just sat there and cooked. The seller replaced it and the
> new locks up in 2 or 3 minutes. I did not have to ship the old one back.
> The Lock output signal doesn't seem to have enough drive to turn on an LED.

One of the three FE-5680A units I got behaved exactly that way. Turns out 
some part of a VCXO circuit had drifted over time (I assume) so the 
free-running frequency never quite reached up to 10.000 MHz enabling lock 
to the signal from the physics package. Assuming you do have a 10 MHz 
output signal, put a counter on it while it is warming up and look at the 
frequency. After I fixed it, the frequency vs time from startup plot looked 
like this:
https://picasaweb.google.com/lh/photo/7E22rkh_YivyesrnTog6atMTjNZETYmyPJy0liipFm0?feat=directlink

The fix was to take off case top and bottom and tweak the trimmer cap 
marked C217 slightly, it is near Y200 (crystal with round PTC thermistor 
attached). Here's a photo of that part of the board: 
https://picasaweb.google.com/109928236040342205185/FE5680A?authkey=djqVhWs9LkM#5680683008490223330

Of course there may be some completely different problem with your unit, 
but it's something to try, if you want to open it up and play with it.

-John Beale