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

Peter Bell bell.peter at gmail.com
Fri Nov 18 18:33:00 EST 2011

Hi, Bert

1.110V - so presumably the c-field current is about 4.1mA.



On Sat, Nov 19, 2011 at 6:16 AM,  <EWKehren at aol.com> wrote:
> 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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