Slow output/no output on KPA500 after changing drive from K3

[NH7RO]

Recently I've begun having a very annoying power problem with my K3/KPA500 combo on 40M.  I recently built a 7-foot diameter STL that I am using as my sole antenna for that band.  

The problem started after I had some hard faults at QRO;  I gradually increased the drive to full output but due to my closely mounting the loop's capacitor and element ends (high voltage points) to its wood support stand I might have had some arc-over; the previously perfect 1:1 SWR climbed into the red and the amp would alarm.

I quickly corrected the mounting so that there was more distance between the stand and high-voltage bits and was then able to load the antenna with 500-600 watts without fault thereafter.  That apparently corrected the SWR/excessive dissipation issue entirely, except I am still left with another issue.

The problem is that whenever I key up and or change the drive level via my K3's drive knob the amp's output either woks as intended or not at all---especially beyond the 12-watt level.  Or if I have the drive set at 20 watts and up it to 22 or 25 or anywhere else the amp's output falls or is nil.  SWR is still fine, 1.2:1 or better but that makes no difference whatsoever in the amp's erratic behavior.

I have developed a sort of workaround method in order to chase DX with my newly constructed loop using 400-500 watts SSB:

I hit the TUNE button on the K3 for a moment then immediately key the mic and begin calling---that seems to "goose" the amp into transmitting for a while.  If I don't keep transmitting often then it falls back into the "no output" scenario and I must repeat the TUNE "fix" again to reactivate things.

This has been happening both before and after I updated the firmware in the K3 and the KPA500 the other day.  The updates installed without a hitch, too.

I am sure that the TX Inhibit is OFF and most if not all of the other parameters that could affect this are set to default.

I had the same problem with the power not coming out of the amp back in November of 2011 but that turned out to be my using both the KPA to KE big cable as well as a separate PA keying cable plugged in which shouldn't have been---now there's only the big K3 multi-pin cable hooked up to the amp.

My HV reading is around 75 to 76VDC and it drops to 61VDC in the worst-case scenario (generally about 63VDC or higher key down).

In searching this forum as well as Google before posting this today I ran across a few similar situations posted by others---NOT ONE OF THEM COUGHED UP AN ACTUAL DEFINITIVE SOLUTION to this strange anomaly.  IIRC, Don, W3FPR did say that the problem was in the K3 and not the amp and I tend to agree.  This often happens when I change the drive setting ever so slightly.

Others had cabling issues that were corrected and as I discovered to my dismay that the majority of folks chalked up the issue to an occasional glitch that was of little consequence.

Not to me!   I would LOVE to be able to fix this once and for all and be confident that when I key the mic or trip my VOX that my call is being sent at the power level desired.

I have also heard that this may be a transformer issue or a voltage tap issue; any thoughts or info on these would also be welcome.   I sure hope I can get this sorted before the ARRL Phone contest in a couple weeks!

Thanks for any help that comes my way; I really hope I won't need to ship this gear to Elecraft for diagnosis/repair if I don't absolutely have to.

I can no longer easily open/read the mailing list sent to my Outlook inbox (problem with a recent change in Outlook, I think) so feel free to email me directly as well as reply to the list:  cathrowinternational (at) hot mail (dot) com

73,  Jeff,  NH7RO