[Elecraft] KPA 100 Low power on 160M

[Don Wilhelm]

Andrew,

OK, sorry for the 'dumb' response, but I had to check it out so there 
would be no 'tail chasing' in a non-problem area.
Yes, it would seem that the input to the KPA100 is nowhere near 50 ohms 
on 160m.
I don't know the vintage of your KPA100, but if it does not have blue 
toroids at RFC1 and L16, now would be a good time to install 
KPA100UPKT.  That change was mostly to the T/R switch area, but still 
can impact the input circuit response.

You can check out the KPA100 LPF easily with an antenna analyzer - put 
the base K2 into CW TEST just to be safe, and set the power less than 11 
watts.  Pull the input coax from the RF Board and insert a 51 ohm 
resistor.  Switch to 160 meters and connect the analyzer to the SO-239 
connector.  The K2 and KPA100 must be powered (and the ribbon cable 
connected) because the KPA100 relays are not latching types.  The SWR 
should be less than 1.5 over the entire 160 meter band.

You could try running the antenna analyzer into the KPA100 input 
(connect the output to a dummy load since some power may be developed at 
the KPA100 output).  If you set the K2 power to less than 11 watts, you 
will be reading only the KPA100 LPF, but when the power is set at 
greater than 11 watts, you should read the input impedance of the 
KPA100.  I have never actually done this measurement, so I don't know 
what is normal, but you could compare with the results on 80 and 40 
meters as a sanity check.

73,
Don W3FPR

a at lenton.org wrote:
> Hi Don,
>
> I am sorry I should have said:
>
> The K2 with the  KPA100 removed behaves normally, current
> does not exceed 2.5A on any band at full power (between
> 10-17 Watts depending on band 40M I get the most power)
>
> I have retuned 80 and 160M and I have checked all the
> components for correct value C11,12,13,14,15 + 16, L34 +5,
> 160M board checked C1,2 and 3 and L1 and L2 for 21 turns.
>
> Now UK band is 3.5 to 3.8Mhz 300 KHZ wide 80 Metre gives
> full power where ever I centre tune 3.6, 3.65, 3.7 or 3.75
> MHz, now top band I get 15 Watts if I tune at 1.9 MHz drops
> off a bit at 1970 upwards, and also 1840 downwards, however,
> in dB terms lest than 1.5dB across the band, looks worse in
> watts when you see 15 drop to 10! I have compromised top
> band to give the max output between 1900 and 2000. It is
> better than it was.
>
> With your help, I am beginning to think that the input match
> to the KPA 100 is not 50 OHMS on top band, hence the high
> current warning( High return loss). I will replace the
> co-ax, as a start, and measure S11 and S12 form my network
> analyser, and check the match into the KPA100 and report
> back, it is odd the other bands seem happy with the match. I
> must of damaged something somewhere removing the KPA100.
>
> 73's
>
> Andrew
>
>
>
>   
>