[Elecraft] KPA-100 oscillation problem

[rt_clay at bellsouth.net]

I've been fighting an oscillation problem with one of my K2/100's for a few years now. My other K2/100 does not have the problem at all. Here is what happens:

Specifically on 7 MHz and when connected to an ICE model 419 bandpass filter, the KPA-100 has some kind of oscillation. I can tell it is there because I hear a little "tick" after letting up on the key. When sending cw, it often causes the K2 to reset and/or show the hi-swr warning. Twice I have managed to blow the microprocessor in the KIO2 attached to this K2!

Other observations:

1) it only occurs with the filter in line. If I switch the filter offline the problem goes away completely.
2) it is not a defective filter. I have two of the 419's, and both cause the same problem. I have tried tuning them, etc.
3) it is not a rf-in-shack problem. It occurs with just the radio, filter, and a dummy load in line
4) I notice the tick in the keying as soon as the power hits 11W and the KPA is switched in line. Even when set at 11W it can cause the K2 to reset. 
5) it only occurs on 7 MHz
6) it only occurs with one of my two K2/100's

I have put in the KPA upgrade with no change. I just tried the C31 mod, no change either.

Any ideas where to look in the KPA circuitry? For contesting I really need to be able to use bandpass filters with the K2.

Tor
N4OGW/5