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

EWKehren at aol.com EWKehren at aol.com
Fri Nov 18 08:09:18 EST 2011


Pete
Thank you for doing all the work. This is great info and will help all of  
us that want to discipline the beast. One more question, since you have done 
so  much testing is there one pin on the connector that has nothing 
connected to  it?
Thanks again   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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