[Elecraft] K2 - 40m chirp

Tue Jul 2 06:06:03 2002

I've recently built the KPA-100 PA for my K2 #1509.  Everything went
together without too many problems and I soon had the KPA-100 bolted
into the K2 and producing 100W of RF.

On 40m, I started getting very bad keying reports - chirp and leading
edge clicks.  I took a listen on another rig and sure enough, the K2
sounded pretty awful.  I thought at first that it was something to do
with the KPA-100, but it wasn't!  The basic K2 was chirpy and nasty, but
only on 40m.  It sounded fine everywhere else, perhaps with the
exception of 24 MHz where I noticed some very slight chirp.

Further checks showed that the K2 sounded OK on about 6.8 MHz and
absolutely dreadful on 7.3 MHz.  The chirp and "ploppy" click on the CW
signal at 7.3 MHz was beyond belief.  What could be wrong?  

Well, a quick check of the VCO voltages at the left hand end of R30
revealed the following:

7.000 MHz = 3.91 volts and 7.300 MHz = 7.2 volts

on 80m (3.525 MHz), where the CW keying sounded perfect, the VCO
voltages were as follows:

3.500 MHz = 1.854 volts and 4.000 MHz 6.0 volts

The K2 was within specification for the basic VCO alignment (P. 58 in
the K2 manual) but sounded awful on 40m CW.  

I consulted Gary Surrency at Elecraft support - and got some very useful
help and advice.  The problem isn't unheard of in a few K2s.  If the VCO
voltage on 40m is too high, or is much more than 5 volts, the VCO may
require too long to settle during T-R switching or Split VFO operation.
This can cause chirp on the transmitted signal, especially above 7.2
MHz.  In my case, the chirp was present at 7.025 MHz and was especially
objectionable.

The cure is to lower the VCO voltage to 5 volts or less.  This was
achieved by removing C70 (4.7pF) which is only used on 40m and in my
case by padding C72, with a 10pF capacitor in parallel - soldered to the
underside of the PCB.

As Gary Surrency says "I prefer to see VCO voltages towards the lower
side on all bands, so there is no chirp on 40m at the high end of the
band.  Adding more capacitance to C72 should accomplish this, since the
combined L of T5/L30 will then need to be set lower and that will lower
the VCO voltage on all other bands.  I check and set up all rigs I work
on this way, and shoot for 5 volts or less on 7.300 MHz on 40m."    

Here are the VCO voltages on my K2 BEFORE and AFTER I removed C70 and
added 10pF across C72.  T30 has be realigned to provide the 6.0 volts
reference on 4.0 MHz.

         BEFORE
         ------
80m 3.5  = 1.85     4.0 = 6.00
40m 7.0  = 3.91     7.3 = 7.20
30m 10.0 = 3.87   10.15 = 4.56
20m 14.0 = 3.31   15.00 = 5.46
17m 18.0 = 4.41   18.20 = 4.68
15m 21.0 = 3.14   21.45 = 4.62
12m 24.8 = 5.39   25.00 = 5.88
10m 28.0 = 4.64   28.80 = 5.80

         AFTER
         -----
80m 3.5  = 1.78     4.0 = 6.00
40m 7.0  = 1.73     7.3 = 3.43
30m 10.0 = 2.92   10.15 = 3.47
20m 14.0 = 2.77   15.00 = 4.67
17m 18.0 = 3.90   18.20 = 4.16
15m 21.0 = 2.46   21.45 = 3.69
12m 24.8 = 4.61   25.00 = 5.06
10m 28.0 = 4.12   28.80 = 5.21

Gary Surrency's "Close to Ideal Values"
---------------------------------------

 80m 3.5  = 2.67     4.0 = 6.00
 40m 7.0  = 2.22     7.3 = 4.25
 30m 10.0 = 2.89   10.15 = 3.42
 20m 14.0 = 2.60   15.00 = 4.33
 17m 18.0 = 3.48   18.20 = 3.70
 15m 21.0 = 2.33   21.45 = 3.46
 12m 24.8 = 4.28   25.00 = 4.68
 10m 28.0 = 3.68   28.80 = 4.61

Since I did the modification, I have had good reports on 40m CW with the
K2 and KPA-100 with no chirp or click reported.  I especially like the
RS-232 interface on the new KPA-100, which I've managed to interface to
my Turbolog logging program.  

You might like to take a close listen to the keying on your K2.  I have
to say that the jury is out on the keying characteristics of the K2.
Some people think it is good, but I have heard some bad reports too.
Maybe those K2s were a bit like mine?

73

Steve Wilson G3VMW
Bramham, Wetherby, West Yorkshire
Email: [email protected]
Web page: http://www.dxtech.co.uk