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

[EWKehren at aol.com]

Pete 
again thank you very much
Bert Kehren
 
 
In a message dated 11/18/2011 6:33:13 P.M. Eastern Standard Time,  
bell.peter at gmail.com writes:

Hi,  Bert

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

Regards,

Pete


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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