[Elecraft] KPA runaway issue

[W1GO (Joe)]

Elecraft faithful,

Any guidance with trouble shooting a KPA500 / K3 issue would be helpful.

Problem: KPA500, primarily in SSB, “redlines” (600W+ LED) after a few seconds of transmission / speech.  This seems to be independent of band.  With the KPA in standby, the K3 output power holds rock steady as indicated in the KPA.

Conditions:
-  Only about 20W of drive in SSB can result in the “redlining”  … To eliminate the redlining, I have to turn drive down to about 18W. … about 20W in CW provides about 500W fairly solid.
-  The K3 is powered by a 70A supply that does not sag much upon K3 keydown (14V to ~13.7V indicated on K3)
-  KPA is powered off 240 … I tried two different KPA transformer taps as I noticed with yellow tap the supply voltage right about 60V and occasionally a little below on key down.  Supply voltage stays well above 60V with red tap but same “redlining” of the KPA.
-  Have tried various mic gain compression settings using dynamic mikes.  Without amp in-line I do fine with gain 30, compression 20 (10db).  I’ve gone as low as gain 15 / compression 15 (~5db) with no impact.
- I’ve tried various TXG VCE rating from -1.5 to 1.5 … no impact.
-  ALC on … ext ALC off …. varied these to see if there would be an impact.  None.
-  SWR (to different antennas) is <1.7 w/o KAT500 and <1.3 w/ KAT500


What I’ve tried / done:
-  Gain / compression changes as above
-  Checked K3 supply voltage and no significant voltage drop on key down
-  Changed power supply to a 35A powering K3 with no change 
-  Varied KPA transformer taps to ensure supply above 60V
-  Changed coax between K3 and KPA and KPA and KAT500
-  Tried with two different antennas / coax runs and had same result
-  Varied TXG VCE from -1.5 to 1.5
-  Performed gain calibration on K3



I’ve had the KPA for about 9 months but primarily operated CW so not sure if this is a recent occurrence.  Any other suggestions would be appreciated.

Many thanks, all, and have a wonderful weekend.



Kindest regards,

Joe
W1GO