[time-nuts] FE-5680A ("New" version) fine frequency adjust
Peter Bell
bell.peter at gmail.com
Fri Nov 18 23:33:00 UTC 2011
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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