[time-nuts] FE 5680A Warning

EWKehren at aol.com EWKehren at aol.com
Sun Nov 13 06:40:55 EST 2011

Thank you for your information. I agree with your findings and as I pointed 
 out previously it support the math that the AD 9832 is used for tuning. 
That  allows them to set the Rb at the optimum operating point based on their 
gas  mixture. What  I like to know is how they control the C field. Again I  
think they picked an optimum point and only vary it slightly for 
temperature  compensation. I would like to use the C field to fine tune using a 
modified  Shera controller. Latest voltage measurements on the C field across the  
coil  are 2.5110 and 2.5238 down from 2.5134 and 2.5256.
Bert Kehren
In a message dated 11/13/2011 4:38:09 A.M. Eastern Standard Time,  
bell.peter at gmail.com writes:

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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> Ghost  in the Shell
> Dr. Don Latham AJ7LL
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