Re: KPA 1500 ATU Issue
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What is the SWR with the amp on but in the SSB part of the band? Does the SWR jump there too, or does it provide a good match? I don't know if the TH6 uses traps but it could be the amp heating up the traps. If the SWR is low on 20 SSB with the amp on that rules traps out.
I've had the SWR climb high due to a balun about to go and then the SWR is sky-high once the balun goes. Once again check the SWR across the whole band with the amp on and amp off. Rule out any traps and the balun before you conclude it's the amp. An SWR of 2.4 to 1 in the 20 CW section sounds very high. I had a trapped tribander (Cushcraft A3S) and the SWR never got that high on wider bands never mind 20 meters. Lastly, does the amp have slugs to tune the SWR for each band? That's my last idea but I've had those slugs in tube amplifiers. I've never had a SS amplifier.
From: Jeff Clarke <ku8e@...>
To: Elecraft-K3 <Elecraft-K3@groups.io>
Sent: Thu, Jul 2, 2020 3:23 pm
Subject: [Elecraft-K3] KPA 1500 ATU Issue
A friend of mine has a KPA1500 that's having strange issues with the ATU. The SWR on his Hygain TH6 tri-bander is kind of high in the CW band because it's tuned for SSB. When he goes down to CW on 20 meters it's maybe a 2.4 :1 SWR. When you switch in the ATU with the amp in the standby position it gets the SWR down to an acceptable level. When you put the amp into the OPER position the SWR jumps up and causes the amp to fault. It also does this also for his 80 meter and 40 meter dipoles when you get away from the resonate frequency on those bands. Same issue with the TH6 on both 15 and 10 meters where it's not near the resonant frequency. This makes the amp pretty much unusable on frequencies where the SWR is around the 2:0 range or a little above that without putting an external antenna tuner is line after the output. He doesn't want to use it this way and risk blowing the final transistors again.
According to the KPA1500 manual "At lower amplifier outputs up to about 800 W, or when using your driving transceiver “barefoot” at 100 W, the ATU will match loads up to an SWR of 10:1 on 80 through 10 meters and 5:1 on 160 and 6 meters. At 1500 W the tuner will match an SWR of about 3:1" The SWR on his antennas are well below these SWR readings but the ATU doesn't seem to be working as designed?. My friend is pretty frustrated right now because he just got his amp back from Elecraft (after having no amp for 4 months) to repair two final transistors that blew. He's only owned it for about 10 months. He was really looking forward to being HP again since he's not a LP guy. Lucky for him it was still under warranty and was fixed for free. I wasn't so lucky with my KPA500 and it cost me $600 to ship and have it repaired by Elecraft for blown final transistors. Now he's not sure he wants to own a solid state amplifier that's so picky about SWR (and having to use an external antenna tuner) that he's just thinking about just selling it and going back to a tube amplifier.
Has anyone else had a similar issue with the ATU? I remember when he first bought the amp a little less than a year ago and we hooked it up it didn't act this way.