[time-nuts] DIY FE-5680A lobotomy (disable temp compensation)

[Scott Newell]

Bert asked me to send an update on the FE-5680 tempco mod progress.

It appears that the FE-5680A temperature signal (or maybe it's really 
a current sense signal?) can be disabled by removing a single 10k 
0805 surface mount resistor.

Using Elio Corbolante's terrific high-res scans, I've noted the 
resistor location: http://www.n5tnl.com/time/fe-5680a/lobotomy.png

Why would you want to disable temperature compensation? As we've 
seen, the unit's firmware will adjust the DDS frequency as the 
temperature signal changes. If you're using the '5680 inside a 
control loop, it's likely to conflict. By removing the resistor, that 
channel of the 12 bit ADC will be tied to ground through an existing 
2.21k resistor. The unit will see a constant 0 counts from the ADC 
and assume it's really cold.

I modified one unit and monitored it for a few hours over a range of 
temps, running it nice and hot with no heatsink, then blasting it 
with a fan and placing it on an ice-cold heatsink. I observed no 
change in the DDS tuning words.

It's a really easy mod--remove four screws, set aside the insulator 
sheet, and apply your hot leucotome/soldering iron.


I've also found a simple mod to replace the temperature signal with 
the output of the unused trimpot. This allows you to simulate any 
temperature you want. If there's any interest, I'll set up a test and 
monitor the DDS tuning words as the unit's firmware tries to adjust 
to the fake temp signal.


-- 
newell  N5TNL