http://elecraft.85.s1.nabble.com/KPA-1500-SWR-fault-above-700-800-watts-on-160m-tp7664618p7664628.html
I took the Balun designs 4:1 out of line and put a Dx Engineering maxi core 5kw 4:1 in its place. It tunes a little better on 160 but the KPA1500 finally faults out and says cannot find match and shows 9:1.... better than 99:1
So I put the KPA1500 straight to a 2.5kw bird dummy load and it tunes instantly 1:1 in 160M
The only other Balun I have to try is a DX Engineering 6:1 which I will try in the morning.
I’m going to also borrow a antenna analyzer tomorrow and see what gives.
> On Sep 2, 2020, at 6:05 PM, Alan - G4GNX <
[hidden email]> wrote:
>
> Ken, sorry this is rather long, but here goes...............
>
> I had a similar problem in the UK, with a OSCFD antenna, purchased from a UK company. I didn't question the theory and just assembled it according to instructions.
>
> As you would expect the antenna is made from two differing lengths of ruggedised, plastic coated copper wire, which is fed by a 4:1 Guanella balun, rated at 400W. The balun is potted in a weatherproof box and is fed via the customary PL259/SO239 connector with Mil Spec RG58 coaxial cable. About 6' down from the feed point is a common mode choke (sometimes called a sleeve balun) which consists of 8 ferrite inline cores, around the co-ax and held in place with heat shrink sleeving and cable ties.
>
> The rig was just the K3S/100W with internal tuner and all was well for many months, mostly using 40 metres SSB. A while later, I added the KPA500/KAT500 combo which I ran for a couple of very short periods at 400W to test. For a weekly net, I used only 200W and all seemed OK for a while, until suddenly the VSWR started to increase over the period of an 'over'. Over a few weeks this got more frequent and I would have to re-tune part way through an 'over'. At that time, I wan't sure whether it was an antenna issue, or the KPA500/KAT500, although I suspected that the KPA500 was getting too hot, so I posted a question on a forum and was contacted by Jack Brindle of Elecraft, who kindly looked at the KPA500 fault reports and sent me a comprehensive explanation of his findings based on the figures and his experience, which concluded that the antenna was almost certainly the culprit.
>
> I ordered a new 1KW Guanella balun and some much bigger ferrites, with the intention of replacing the RG58 with Westflex 103. In the meantime I soldiered on with the original setup until one day all hell let loose, with the KPA500 and KAT500 fault lights flashing and even the K3S got 'ticked off'. At this point I switched to my HF vertical, to keep me on the air.
>
> When I dismantled the wire antenna, I expected a balun fault, but it was actually OK. On further checking, after disconnecting the co-ax from the balun, the open ended co-ax showed a DC short on a test meter!
>
> I used the Rig Expert 600 to determine exactly where the short was and it turned out to be exactly where the sleeve ferrites had been, BUT there was no discoloration of the outer covering, or signs of melting, so I knew it was unlikely that the ferrite cores had got heated. I cut the section out of the co-ax and started to take it apart. With the outer covering all removed, there was still no sign of discoloration of the screen braid, but as I tried to remove the braid, it was obvious that there had been considerable heating as the center section of the braid has melted into the center conductor insulation, and somewhere in there, it has made it right through to the center conductor.
>
> Unfortunately I haven't yet found the exact point of the short, because I left the partly dismantled piece of inner co-ax on a table and one of my cats seems to have run off with it! :-D
> When I eventually find it, I'll continue investigating.
>
> In the meantime, one theory is that the common mode choke presented a high impedance to RF coming back down the co-ax and this in turn produced a very high voltage causing an arc between inner and outer and as the characteristics changed, in turn a high current node was created, which caused the heating effect. Another theory is that it is just a crappy piece of cable and it just failed.
>
> Unfortunately this is not the sort of fault that can be detected in its development stage by a low power antenna analyser, it needs a fair amount of power.
>
> This long tale may just give you some clues to your possible issue on 160m although on a different power scale..
>
> 73,
>
> Alan. G4GNX
>
>
> ------ Original Message ------
> From: "Ken Ramirez de Arellano" <
[hidden email]>
> To:
[hidden email]
> Sent: 02/09/2020 22:41:30
> Subject: [Elecraft] KPA-1500 SWR fault above 700-800 watts on 160m
>
>> Every bit of antenna hardware ( coax, balun, insulators, etc. have been
>> changed out or proven good. Connections are all solid. My KPA1500 will
>> transmit above 800 watts for about 15-30 seconds and then the SWR jumps up
>> and it faults out for high VSWR. I can run 1500 watts on all the other
>> “Legacy” bands (10,15,20,40,80) with no problems. I’ve forced an STU
>> relearn and still the same issue. The antenna B amp is a Clipperton L and
>> that transmits at a KW out and I see no change in VSWR in the external
>> wattmeter using all of the same cabling. It’s starting to look like
>> something heating up in the KPA1500. Is anyone aware of a known issue for
>> this with the KPA1500 or a known fix? I would rather do a field repair
>> instead of sending to Elecraft from KP4. TIA Ken, KP3MM
>> --
>> Ken Ramirez
>> KP3MM
>>
>
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