[time-nuts] troubleshooting a FE-5680A (got two; only one works)

Peter Bell bell.peter at gmail.com
Wed Nov 30 06:09:36 EST 2011

I tend to agree with this - but it looks like it's already been
opened, which could cause problems.

Some things you can quickly check are that the lamp housing and the Rb
cell shielding are getting hot (use a thermometer, not your fingers!)
and that the lamp is lit by checking for a purple glow at the back of
the lamp housing.  I suspect these are all OK, though, since the
current consumption is about the same as the other one.  One of my
dead units had a bad 3.3V regulator (MAX882, on the back of the board
-  input is 5B on pin 5, output is 3.3V on pin 4) - since that 3,3V
line runs the Xilinx PLD that divides down the clock and you have a
low output signal, it sounds possible...

If your supplies are good, try scoping the photocell testpoint - it's
the one above and slightly to the right of the FPC going to the Rb
cell assembly in your "photo 4 of 8" - it should show a signal when
the oscillator frequency sweeps through the Rb resonance frequency -
if you can't see anything at all, then check to make sure that the
VXCO is actually sweeping through 10MHz - if not, you can try
adjusting C217 to centralise the sweep range.  If if is going through
10MHz and there is no signal, check the drive to the snap diode on
that little RF connector - you should see a 60MHz signal mixed with
about 4.3MHz - if that looks good, and the there is still no signal at
the photocell, suspect the physics package.



On Wed, Nov 30, 2011 at 5:26 PM, WB6BNQ <wb6bnq at cox.net> wrote:
> John,
> Instead of trying to fix it you should fault the unit back to seller and get a refund or have him
> send another unit.  I would also attempt to get them to pay for the return shipping as well.  After
> all it is their fault they sent a bad one when all of them are claiming that they are working
> correctly.
> As for troubleshooting, that is way easier said then done.  These newer series of FE-5680's are much
> more digital in nature then the older design, which makes it much harder.  The company that makes
> them is not and will not give out information.
> Bill....WB6BNQ
> John Beale wrote:
>> Just yesterday I received two FE-5680A units from China.  One of them works
>> and one doesn't achieve lock. Both are labelled with FEI P/N
>> 217400-30352-1. Here's a photo of the one that works, the other looks just
>> the same but different serial number.
>> https://picasaweb.google.com/109928236040342205185/FE5680A#5680473663653140162
>> FE-5680A Rb #1:
>> At startup, RF out (pin 7) ramps from 9.999828 to 10.000066 MHz for a few
>> minutes, then locks at 10.000000 MHz and pin 3 drops low.  The RF signal is
>> about 2 Vpp.  Power draw at startup about 25 W, dropping to 10 W.
>> FE-5680A Rb #2:
>> RF out scans from 9.999799 to 9.999994 MHz repeatedly, and does not achieve
>> lock. Pin 3 remains high.  The RF signal is about 1 Vpp (half the level of
>> the working unit).  Power draw similar to unit #1.
>> Any suggestions for what might be done to prod the 2nd unit towards
>> working?   I see some things which might be trimmer caps on the board, and
>> what might be a round heating element(?) soldered to a crystal can in this
>> photo:
>> https://picasaweb.google.com/lh/photo/0WRM10pGG0Kd89Ji80yoa9MTjNZETYmyPJy0liipFm0?feat=directlink
>> thanks for any advice!
>> John Beale
>> N8JUF
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