[Elecraft] K2: MARS Frequency Access?

[Don Wilhelm]

Jon,

How far out of the ham bands you can go with the K2 depends on 3 factors.
First is the range of the VFO tuning.  If you measure the PLL/VFO range 
at the left end of R30, but extend the voltage range  to 1.0 volts to 
7.0 volts. and you will have an indication of how far out of band the 
VFO will tune.  Beyond those points, the VFO indication on the display 
will not be reliable.
The second is the falloff of the bandpass filter.  That can sometimes be 
modified slightly by tuning at a different frequency than is indicated 
in the K2 manual.  For instance, if you need to tune up to 4300 kHz, you 
may be able to shift the bandpass filter tuning by setting the frequency 
to  3900 instead of 3750 when tuning the 80 meter bandpass filter.  The 
response falloff at the edges of the new "BAND" limits should be checked.
The 3rd thing is the Low Pass Filters.  Remember there are 2 sets in the 
K2/100 - one in the base K2 and another in the KPA100.  The bands that 
they cover are not necessarily the same.  For instance, the base K2 LPF 
combines 30 and 20 meters, so if you want to transmit at 12 MHz, that 
one will give you no problem, but the 40/30 meter LPF in the KPA100 is 
designed to cut off the 2nd harmonic of 60 meters, so it has a rather 
steep cutoff after 11 MHz, so attempting to run a 12 MHz signal through 
that filter will likely result in low output and high current drawn by 
the KPA100 and there is a liklyhood of damage.

Bottom line - unless the MARs frequencies are quite close to the ham 
bands, the K2 will not do the job.  It could be modified for the MARs 
frequencies, but it may not cover the entire span of the ham bands 
afterward.  I would discourage such modification, and any mod to the VFO 
Range Select is quite tricky (a change of a few pF makes a big change in 
the range) and not all K2s will respond the same to any given capacitor 
due to component tolerances.

So, yes, the K2 can be modified with a LOT of work, some of it requiring 
redesign of the VFO, Bandpass Filter, and the Low Pass Filters.  When 
you are done, it may or may not be usable on the ham bands.

I believe the K3 (with the KBPF3 option) and the KX3 are better 
candidates for MARS operation.

73,
Don W3FPR

On 9/24/2012 11:34 AM, Jon Perelstein wrote:
> Is the K2 a viable radio for MARS work?  I found some references in
> the archives from back in 2005 suggesting that it won't transmit on
> some of the MARS frequencies even though it appears to be transmitting
> on them -- the VFO display shows the desired frequency, but that far
> out of band it's not transmitting on the frequency it's displaying.
> The issue appeared to be one of how far out of band the PLL will go,
> as opposed to there being a restriction programmed into the
> microprocessor.
>
> I confirmed last night that mine (#6550) will not -- on some of the
> bands -- transmit as far out of band as needed, even though it is
> transmitting.
>
> There was no mention in that exchange of any mods that might be done
> to make the K2 capable of transmitting on the MARS bands, and I
> haven't found any other useful posts on the subject.
>
> Has anyone successfully modified a K2 for MARS operations, and if so
> what did you do.
>