[Elecraft] K2 KNB2 Issue

[Don Wilhelm]

Nidge,

The only components the KNB2 adds to the receive path are in the 
bandpass filter.  If you have checked capacitors C3 through C7 for 
correct values (and are all in the right places) and well soldered, the 
next on the suspect list is L1 and L2.  You say you have checked the 
number of turns, so look carefully at the connections on the solder 
side.  If you see a ring in the solder around any lead, that indicates 
the lead was not well stripped and tinned.  Remove the inductor and 
re-tin the lead.

Check to be certain R8 is 120 ohms and R9 is 300 ohms - swapping them 
will significantly reduce the signal.

If all seems well with the bandpass filter proper, do a test after 
removing capacitors C16 and C8.  With these capacitors removed, the NB 
will not function, but it assures that the only components in the signal 
path are the bandpass filter components.  If you have come this far and 
it still reduces the signal, I would conclude you have a bad capacitor 
in the bandpass filter - obtain replacements for capacitors C3 through 
C7 on the premise that one is bad.  It is difficult to tell which one 
unless you remove them and measure with a capacitance meter.

73,
Don W3FPR

On 8/21/2011 2:14 PM, Nigel (Nidge) Smith wrote:
> Hi All
>
> I have an elderly K2 which had the original KNB2 installed, the one with the two trimming capacitors in the
> bandpass filter circuit.  I ordered the revised board and this was built yesterday.  The issue is that with
> the new board installed the sensitivity is very much reduced which is confirmed using the XG3 at 50uV
> output, with the original board the S-meter indicates S9 where as with the revised board the S-meter
> indicates S3.  I have checked that all the components are in all the right places paying particular
> attention to the capacitor values, that all the inductors have the correct number of windings and that
> there is continuity between each end of the inductionr (0.04Ohm).
>
> Is there anything I can further check?
>
> Test equipment available:
>
> XG3
> 50MHz Oscilloscope
> MFJ Antenna Analyser
> Fluke 77 multimeter with RF probe.
>
> As an added bonus today I was using the K2 at a special event where we had multiple
> stations running.  One of the other stations came on my frequency and transmitted.
> The result (with the KPA100) installed is that I get a constant high current reading
> on the display and SWR of 9.9, I’m presuming that the diodes D16 and D17 in the
> KPA100 will have been fried (?)
>
> All recommendations greatly received.
>
> Regards
>
> Nidge (G0NIG)
>