[Elecraft] KPA100 oscillating

[Marcus Busch [DL1EKC]]

Hi Elecrafters,

just returned from my 2005 WAE CW activity in LX, where I operated my K2 
on a camp site in a fieldday-style operation. So far all went fine. Even a 
high power (100 kW +) MW transmitter with really big antennas only 1 mile 
away didn't produce any problems to the K2 RX. And I didn't find any non-
harmonics of this transmitter, great.

The only strange effect was the SWR LEDs of my KAT100 flashing in the 
rhythm of the modulation of this transmitter during RX (!) when I switched 
to my 45ft high 40 m dipole while the K2 was set to 20 m ...  hi

K2 on 40 m with this 40 m dipole was no problem.

So all went fine ...  until just two days before scheduled departure, when 
my KPA100 suddenly started heavy self oscillation. KPA100 started 
oscillating whenenver PTT was activated, even with the K2 being in TEST 
mode (no input power). DC input current was just above 20 A !

At home I figured out that Q6 (ZVN4424) has failed putting D12 and D14 (RX 
path in T-R circuit) in conducting state continuously, even during TX. 
This presents a low resistance feedback path to the amplifier. Having this 
in mind the oscillation is no surprise.

I know these ZVN4424 being good candidates for such failures, so I have 
some in spare (two years ago a ZVN4424 failed in the T-R circuit in my 
basic K2). But the high power resistors R20, R21 and R39 show ugly signs 
of having been heavily overloaded. They still have the right DC 
resistance, but I would like to replace them, just to be sure.

But the technology of these resistors is not stated in the documentation. 
Are these standard metal film resistors, or do I need even lower 
inductance resistors?

Doesn't make much sense sending these few parts around the world, so I'd 
like to buy locally.

What do You think about D11, D12, D14 and C67: Should I replace these, 
too? They look fine and measure ok. D13 should be no problem.

Q1 and Q2 don't heat up during self-oscillation, but R21 heats to painful 
high temperatures within 2 s or so.

Thanks for Your help.

VY 73 de Marcus, DL1EKC