[Elecraft] K2/100 #4198 PA Stability Problem?

[Andy Cook, G4PIQ]

Was feeling very smug with myself yesterday having finished the K2/100
in decent time, but having put it on air in a local contest last night
and getting some grim audio reports, I've found that I have a problem!
Since adding the PA, above around 40W on 80m (and maybe just a hint on
160) I get rough AM on the carrier (spreads a long way - probably pulsed
if I looked on a scope) at a few hundred Hz. Exact frequency rises a
little with frequency . Sound like LF instability in the PA. On SSB
makes the audio quite rough - on CW spreads with these sidebands.=20
=20
Before I go round applying the usual tricks of extra C and snubbing R
here and there, any suggestions? Have checked for obviously bad grounds,
wrong components, removing shield, increasing R98 on RF board, extra
1000uF C from Z1 to ground, but none have a significant impact. Any
leads?
=20
73,

Andy, G4PIQ


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