[Elecraft] NOT the feedline (was KPA-1500 faulting on 6m; SWR issues)

[Peter Dougherty]

Many here suggested that it was most likely a feedline issue, and I figured
this was indeed a distinct possibility. 

So this afternoon I replaced the feedline. I now have about 80 feet of brand
new LMR-400, new 83-SP1 connectors (and an Amphenol adaptor at the balun
since I don't have any type-N connectors here). The AA-230 ZOOM analyzer
shows about 1.2:1 at 50.313, with resonance just below the band. I'll fix
that tomorrow, but 1.2 or 1.3:1 should be nothing to the KPA. 

The TDR screen looks very clean, just a tiny bump where the feedline goes
through the (new) bulkhead connector to get into the house. See:
https://www.dropbox.com/s/izcwvece7nt5f42/2020-05-31%2019.32.44.jpg?dl=0

The SWR chart between 50.000 and 50.500:
https://www.dropbox.com/s/y2xh4l1lihy2ujr/2020-05-31%2021.23.29.jpg?dl=0

Again, I know the antenna is resonant a bit low, but this should NOT be a
factor. So here's what's now going on.

1) Into a Cantenna dummy load, with 25W of drive from the M3s, the KPA-1500
is delivering 1075W and showing 1.4:1. No problem delivering power and no
faults.

2) With input power increased, 35W of K3s drive gives 1375W at 1.5 into the
Cantenna. Again, no problem delivering power to the purely resistive load.
Why a 1.5:1, I don't know, but fine, whatever. I can TX for 15-20 second
periods without issue.

3) When I switch over to the live antenna, with25 Watts of drive from the
K3s into the actual antenna and the amp faults out instantly in bypass. When
the amplifier is in bypass and the tuner is in bypass, and when the K3s is
delivering about 90W in FT8 mode, the front panel on the KPA-1500 says the
SWR is 1.9:1. When I press the TUNE button on the radio, it's showing an SWR
of 1.4:1. 

4) Amp in bypass, tuner inline, press TUNE button on the K3s and the front
panel of the KPA-1500 is showing a perfect 1.0:1 match. It SHOULD work fine
like this, right?

5) Pressing ONLY the OPER/STBY button to put the amp in operate (KPA tuner
in and untouched since step 4), the amp SOMETIMES delivers power, sometimes
faults out, shows an SWR of 1.7:1. Pressing the OPER/STBY button again to
use only exciter power, and touching NOTHING ELSE, it's back to 1.1:1 with
90W of K3s drive.

6) There is basically no rhyme or reason why this is happening and it's
*somewhat* repeatable. The incredibly frustrating part is if I see a needed
station come up, I'll flick the amp's tuner in line and it says it's tuned.
Once WSJT-X keys the radio, 90 out of 100 times the amp will hard-fault. If
it doesn't fault on the first transmission, it absolutely WILL on the second
transmission. 

So I'm now at a complete loss to understand what's going on here. Amp is
fine into a resistive load. Brand-new antenna, brand new LMR-400 feedline,
brand new connectors, the KPA-1500 is faulting out the same as it did on the
10+ year old 6M5 with a patchwork of feedlines. I can't see how this is
anything but a problem in the KPA-1500 (S/N 0398).

 - pjd

-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On
Behalf Of Jim Brown
Sent: Sunday, May 31, 2020 8:22 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] KPA-1500 faulting on 6m; SWR issues

On 5/31/2020 9:54 AM, Jim Miller wrote:
> Have you vetted your feedline at 1500w? Many times something might be 
> good at low power and fail at 1500.

This is good advice. Guys who do lots of portable setup say to always check
the coax first, which mostly means check for a connector that's bad or badly
installed.

In general, if it doesn't say Amphenol or isn't stamped with a MIL spec
number, it's very likely to be poor quality. This includes barrels,
adapters, etc.  Also make sure that every connector is" wrench tight."

The advice to run it straight to a dummy load is right on -- if you have one
big enough. :)

73, Jim K9YC
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