[time-nuts] Adding adjustment pot to 5680

[Arthur Dent]

 I resisted the suggestion to use Twinkies for insulation (and ate them!),
plus tossed out all the other 'good' suggestion for modifications like 
using a robotic arm to move a magnet to trim the frequency, and used 
a very simple modification that I just completed a few minutes ago; and 
it seems to be working well. On the inside of the case where the DB-9  
pins go through the circuit board, I mounted a 7805 fixed regulator. I 
cut a piece out of pin 4 and connected the output of the 7805 to the 
board side of the pin with a tantalum cap for bypass. I decided not to 
use a 78L05 because at the elevated temperature I felt it would be 
cutting it too close to the 100MA current limit. I then soldered a wire 
to the connector side of pin 4 and ran it through the center heatsink to 
pin 5 of the IC next to the ribbon connector (in series with a 100K ohm
resistor) as described by Bill Riches on Thu Dec 22 22:00:40 UTC 2011. 
Pin 4 now 'floats' at 2.5V and can be connected to an external 10-turn 
pot adjustable from 0-5V to trim the frequency, or to a DAC for EFC. 

 So the 5680 I have can now be adjusted externally by a 0-5V signal 
and it requires just the single 15V supply. And for the penny-pinchers,
the added parts cost well under $3. ;-)  A photo of the regulator before 
I connected the wire to pin 4 on the connector is at the link below. 

http://farm8.staticflickr.com/7013/6668466093_90782cf7e9_b.jpg

-Arthur