[Elecraft] KPA runaway issue

[W1GO (Joe)]

All,

One additional factoid of interest … when in ESSB mode, the runaway issue doesn’t occur; only in standard bandwidth mode.  I’ve used standard bandwidth recently with out the KPA500 on and I’ve had no power out issues of poor audio reports.  Hence, it seems that somewhere after the K3 there are oscillations resulting (or at least only noticeable) from the narrower bandwidth signal.  Interesting.

Take care, all.  If and when i figure it out I’ll share.  BTW, I did switch miss from a boom with foot pedal to a short cable desk mic just to rule that out.

Joe

> On May 6, 2017, at 12:49 PM, W1GO (Joe) <w1go at icloud.com> wrote:
> 
> 
> 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
> 
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