[Elecraft] KPA100 going into oscillation...help!

[Don Wilhelm]

Douglas,

If your 'latest spec' KPA100 was shipped from Aptos after January 2007, 
then indeed you do have the latest - check all your construction work.

If it was shipped before January 2007, check the value of C31.  The new 
design requires a 0.22 uF capacitor.

Are you certain it is an oscillation?  If so, can you determine the 
frequency?  There is a difference between an oscillation and simply 
going to the maximum power output, and the direction for attacking your 
problem depends on the correct answer to that question.
Do you have any mods to the KSB2 board - in particular the KI6WX 
increased RF gain mod.  If that is present with the KPA100, the SSB ALC 
can be overdriven (but there is an easy fix to the SSB ALC circuit).

73,
Don W3FPR

Douglas Maxwell wrote:
> Hi All,
>     My K2 is serial no. 4688. I have just purchased and built the latest spec KPA100 unit for it. The manual for the KPA100 suggests modifications are required for K2's with serial nos 4059 or lower. As my serial number is higher, I assumed all would be well. Firstly the KPA100 performs well on all bands using CW mode. Recently I tried SSB and found that the PA is going into oscillation when on 40m.
>
> Setting the power out to 20W and speaking into the microphone produces 100W output peaks accompanied with a low buzzing sound in the speaker on audio peaks. Varying the power control brings it back into line only to start misbehaving again after a few more words are spoken or if the mode is cycled back round to LSB or you switch off then on again. This happens when connected to a dummy load with no other connections other than the mike and power supply. I changed out the power supply, but the same problem exists.
>
> I have done an extensive search of the internet to try and find solutions:
> 1) I have ensured gnd continuity from the KPA100 to every part of the K2 chassis.
> 2) I covered the hole in the shield near the pl259 with copper tape and ensured gnd continuity.
> 3) I saw the article about installing the 270 ohm metal film resistor to increase Zin, but my K2 circuit is newer than the authors and I haven't seen an updated version.
> 4) I have ensured the 2 lugs from the antenna socket to the shield are good.
> 5) I have removed the KPA100 from the K2 entirely and the same problem exists.
>
> Does anyone know the solution or recognise these symptoms?
>
> Douglas Maxwell
> GM0ELP
>
>