[Elecraft] KX3: thermal frequency drift

[Edward R Cole]

I decided to test the frequency stability of the KX3 on 6m under 
key-down conditions one would see using JT65.  Note : I have not 
performed the Temperature Compensation Procedure so not expecting 
frequency to hold as close as after  doing the procedure.

I checked frequency five times with quick transmissions that would 
not result in heating:
50.125000 was the dial frequency measured with 5w output measured on 
a Bird 43 meter.
50.124981  -29 Hz error
50.124950  -50 Hz error
50.124960  -40 Hz error
50.124957  -43 Hz error
50.124957  -43 Hz error

Next I ran ten sequences where I keyed down for 50-seconds and Rx for 
70-seconds to duplicate JT65 transmitting sequences:  I got drift downward of
50, 49, 34, 25, 29, 32, 19, 12, 9, and 12 Hz
Cold frequency at the start of each sequence varied from 979 to 960 to 994 Hz.

It did appear that the radio warmed up over the 20-minutes test and 
high/low frequency deviation narrowed to about 10-Hz downward 
drift.  At the end the radio case was quite warm and the HI TEMP 
warning came on!  From this I would not recommend running JT65 for 
extended time with the stock KX3.

Next I installed the nice external heat sink being made by Fred 
Meier, VE7FMN.  I installed it in place of the original
heat sink plate which took about 20-minutes to install (remove four 
screws that hold the heat plate and install the new heat sink using 
supplied allen screw bolts.  I decided to purchase the plain aluminum 
heat sink vs the black power-coat version.  I used no heat-sink 
compound trusting just the metal surface conduction and two 4-40 
screws that secure the transmit transistor tabs.

Testing at 5w I observed 50.124962 +/- 1 Hz over three 50-second 
cycles.  The heat sink only felt mildly warm (barely 
detectable).  Note: this is without performing the Temperature 
Calibration Procedure!  To say I am impressed it an 
understatement.  I will be interested to see if the initial 38-Hz 
cold-temp frequency error is minimized by the Temp Compensation 
procedure.  Note: I tested my frequency counter accuracy against my 
10-MHz OCXO reference and saw less than a Hz error.

How this will affect temperature related drift using the new 2m 
module remains to be determined as the Tx device on the 2m board 
probably does not use the existing heat sink interface. I'll post the 
raw data on my website later on.

73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
     dubususa at gmail.com