[time-nuts] FE-5680A ("New" version) fine frequency adjust
bell.peter at gmail.com
Fri Nov 18 02:59:02 UTC 2011
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.
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.
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