[time-nuts] FE-5680A ("New" version) fine frequency adjust

[EWKehren at aol.com]

Pete
reading your description of the C field circuit they are not doing any temp 
 compensation or adjustment to the C field like other manufacturers. Seems 
like  they have determined an optimum current and do all tuning with the 
DDS. Makes  sense. It would be nice to find out what the current is. Would you 
mind  measuring the voltage across 270R in the C field circuit. Mine is all 
boxed up  and coming close to 1 week.
Thanks again for all your work.
Bert Kehren
 
 
In a message dated 11/17/2011 9:59:14 P.M. Eastern Standard Time,  
bell.peter at gmail.com writes:

OK, I  just managed to get some spare time to have a look at this, and
here are  some conclusions:

1) The fine frequency adjust pot is not connected to  the c-field
circuit. All the connections to it go across the board to the  MAX1246
ADC chip.  The track is wired between ground and the vref  output on
this chip and the wiper goes to a little pi filter (ceramic cap,  100K,
1uF tant) that feeds channel 1 (pin 3) on the ADC.  As far as I  can
see, it goes nowhere else.
2) The c-field current is fixed.   The circuit is very simple - there
is a 5.1V reference generated using a  zener and a 681R resistor to the
8.8V and this is fed into a potential  divider consisting of a pair of
120K resistors.  The centerpoint of  this divider is connected to pin 5
(+ve input) of a TLC27M4BI - the output  of this amp (pin 7) is
connected via a 270R resistor to pin 2 of the FPC  connector and hence
to the c-field coil.  The return side of the coil  (pin 1 on the FPC
connector) connects to pin 6 (-ve) of the opamp and via a  620R
resistor to the 5.1V reference.

If there is any other input to  this circuit, I have not been able to
find it, despite probing all over the  board.

Conclusion:

This version carried out the fine frequency  adjust digitally by
changing the reload values on the DDS chip - the reason  the pot does
does nothing in this specific model is that the DDS trim is  now being
carried out over the RS-232 port and as a result the pot value  (if
it's being read at all) is not being used for  anything.

Adjusting the c-field still works, though - if you connect a  DC supply
set at about 2.55V via a high value resistor (I used 100K) to pin  5 on
the TLC27M4BI next to the flex cable connector then you can trim  the
frequency by adjusting  it.

Regards,

Pete

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