As has been mentioned by Pete and others the majority of the inexpensive
5680 units can only be adjusted for frequency by inputting rs232 info.  I am
experimenting with a simple mod to add a fx adjust pot to my unit.  I
removed the unconnected "dummy" pot and glued a miniature 100 k pot upside
down in the area where the original pot was.  Its connections are on the top
of the pot.  Cold side of pot went to ground, hot side of pot went to output
of 8 volt regulator through 1 k resistor. Wiper goes through a 100k resistor
to pin 5 of the  chip next to the ribbon connector. (To find pin 5 have unit
positioned with db9 connector at upper left, ic area will be in lower right.
Pins 1 through 7 are between the chip and the ribbon connector - pin 1 is at
the extreme right next to the marking on the chip - count left to pin 5!  I
got my 8 volts from the 8 volt regulator that is the first xsistor of the
two that are mounted on the vertical heatsink. The third pin is 8 volts
output.  Pins are counting down from db 9 connector area.   

Three hours after doing this mod my unit is about 5x10-12 high. The voltage
on pin 5 is 3.52 volts.  I have my thunderbolt 10 mhz output triggering my
465, the 10mhz from 5680 going to vertical input, time base set to .1 usec
and the x10 mult on giving me .01 usec per div.  With a stopwatch I time the
time it takes for the trace to move one division. I divide the seconds
measured and divide that into 1 to the minus 8th and get the offset.  in
this case 1 to the minus 8th divided by 195 seconds gives me about 5 parts
in 10 to the 11th.  The trace is moving to the left which means the osc is

I would appreciate any of the 5680 gurus to go over my math and what I have
done and let me know if I am all wet!!  Thanks to all of the threads on this
unit that have been on this list.


Bill Riches, Wa2dvu
Cape May, NJ


-----Original Message-----
From: [] On
Behalf Of
Sent: Thursday, December 22, 2011 1:39 PM
Subject: Re: [time-nuts] Using Thunderbolt to Discipline FE 5680A?

Hi there,
maybe this has already been discussed here.. There is a small hole on the  
side of the unit with a small trim-pot visible through the hole. Does anyone

 know if this is an analog frequency adjustment?
If it is, maybe that trimpot connection can be used as disciplining  input..
In a message dated 12/22/2011 09:20:57 Pacific Standard Time, writes:

I think  the FE-5680A has to course of adjustment to be used in the
"fast" control  loop inside the t-bolt

Chris Albertson
Redondo Beach,  California

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