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

[EWKehren at aol.com]

Pete
Thank you, I will look at its some time in the future, right now I let it  
cook for a couple of weeks undisturbed and than I may cut the trace that 
seems  not to do any thing. If you have a 100 MHz scope it will trigger on any 
pulse in  this unit unless your scope is defective. I think there are 
features hardware  wise in this unit that can be activated at manufacturing 
depending on price and  demand. I am sure you are aware of instruments that a 
hardware wise the same and  cost any where from $15 000 to $40 000 the only 
difference is the software  imbedded and many times you can buy an upgrade.
Bert
 
 message dated 11/18/2011 10:26:02 A.M. Eastern Standard Time,  
bell.peter at gmail.com writes:
 
All the  pins have things connected to them - but pin 6 seems a little
strange -  it's connected to the 'ACT240 (in fact, it goes through 3
sections of the  chip in series!) and finally to the xilinx chip - but
none of these signals  ever appear to do anything and the output is
just constant high.  It's  possible that this the pulse is just
extremely short - the fastest scope I  have at home is only 100Mhz -
but I would have expected to be able to  tirgger on it at least.

Regards,

Pete


On Fri, Nov 18,  2011 at 9:09 PM,  <EWKehren at aol.com> wrote:
> 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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