[time-nuts] FE 5680A Warning

Peter Bell bell.peter at gmail.com
Sun Nov 13 04:36:42 EST 2011

I can let you know what I've found out about these units - I got given
a couple of dead ones and did a bit of poking around trying to get
them working.  I didn't get that far, since one of the units was
easily fixed (the MAX882 3.3V regulator had failed) and the other
seems to have a faulty Rb absorption cell (it works if you swap the
cell from the other unit into it).

The first is, as you noticed, that they have 3 voltage regulators -
but apparently couldn't spare one to generate the +5V line - they seem
to run (1) The Rb lamp / heater (the one nearest the connector) (2)
the cell heater (the middle one) and (3) the analog electronics @8.8V
(the last one).

The quickest way to verify this is to apply +15V / GND to pins 1 and 2
of the d-type and then check the voltage on the VCC pin (20) of the
74ACT240 buffer chip next to the connector - there is no 5V there.
There is also no 5V on pin 5 of the MAX882 - and hence no 3.3V line.

There is a connection between these points and pin 4 of the d-type -
and applying an external +5V to this pin will bring up the 3.3V line
and allow the unit to lock.  When locked, pin 3 on the d-type goes
low, and will sink enough current to operate an LED.

Pin 5 is also connected to GND, and was (on the original PCB) also
connected to the ground plane along with pin 2.  It doesn't seem to be
actually necessary, as the unit will operate with a single ground
connection, but would it would seem to be good practice to use this
pin, too.

It's been alleged that pin 6 is a 1PPS output - but I can't see
anything on it at all except a constant logic 1 - I tried changing the
'ACT240 in case the buffer was damaged, but this had no effect, and
the pin is still a constant 1.  I suppose it's possible that there is
an extremely narrow -ve going pulse on it, but if there is it's too
narrow for the trigger circuit on my (100MHz) scope to see it.

Pin 7 outputs a 10MHz sine wave - about 800mV p-p unloaded.  I haven't
put it into an analyzer, but it looks pretty good.

Pin 8 and 9 are connected to the MAX3232 on the board, and from them
onto the serial port on the DS80C323 CPU.  Pin 8 is RXD, pin 9 is TXD
(from the unit's point of view). This serial port accepts the commands
described in the FEI document that's floating around the net for
trimming the oscillator.  They appear to operate as described.

Internally, the main VXCO runs at 60MHz - it's buffered and fed into
the Xilinx XC9572XL PLD on the back of the board.  This chip appears
to contain mostly frequency dividers - it generates the 10MHz output,
20MHz for the MCLK to the DDS chip (although with a slightly nasty
40:60 duty cycle) a 416.6666Hz drive for the fselect pin on the DDS
chip, a 833.33333Hz drive for the lock in amp.  There are also a bunch
of signals that go to the test connector, and are presumably used for
factory programming.

The output of the DDS is about 5.3MHz with FM on it at a 416.666Hz
rate - this is mixed in with the 60MHz output from the VCXO to give
the drive signal to the snap diode in the physics package.

There are 3 pots on the board - the one that's normally accessible via
a hole in the case seems to be intended as a C-field adjust, but does
nothing on the units I have - the other one near it appears to trim
something in the photocell amplifier circuit.  The one on the other
side of the board (near the VCXO) sets the bias on the snap diode

That's all I've got, since I then worked out that I didn't have an
electronics problem - hope it helps.


Pete Bell

On Sun, Nov 13, 2011 at 1:46 AM, Don Latham <djl at montana.com> wrote:
> I just bought a couple of 5680A's that have the same FEI p/n. They are
> pop-riveted to a heavy double-sided board., obviously cut off from a
> longer board. I've removed one of these from it's board/heatsink. There
> are two pots on the board, only one of them can be trimmed with the case
> on. The sinewave forming filter and maybe countdown chips are not there,
> and never were. There are three identical voltage regulators on the
> heatsink spine.
> I'll be doing some more reverse engineering on these units; they were so
> cheap I had to have a couple! If only FEI was more helpful! hey must
> have moved on from these, and could put one manual out there?
> Oh Well!
> Don
> EWKehren at aol.com
>> The FE 5680A's advertised as NEW are not NEW!  A  friend  that  made me
>> aware of the sale did notice clear signs of use, so I did a  closer
>> inspection
>> under a Microscope and found clear indications that the unit  has been
>> bolted down in the past. Does work fine and I am looking at ways to go
>> directly
>> to the C field to disciplin the Rb. Does any one have info that would
>> help
>> to do so.
>> Bert Kehren  Miami
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> --
> "Neither the voice of authority nor the weight of reason and argument
> are as significant as experiment, for thence comes quiet to the mind."
> R. Bacon
> "If you don't know what it is, don't poke it."
> Ghost in the Shell
> Dr. Don Latham AJ7LL
> Six Mile Systems LLP
> 17850 Six Mile Road
> POB 134
> Huson, MT, 59846
> VOX 406-626-4304
> www.lightningforensics.com
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