[Elecraft] Elecraft Digest, Vol 252, Issue 13

Thu Apr 17 17:29:24 EDT 2025

Is that not alot of power for ft8.  Maybe the software is throttling you back.

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Sent: Thursday, April 17, 2025 12:00:18 PM
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Subject: Elecraft Digest, Vol 252, Issue 13

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Today's Topics:

   1. Intermittent low output KPA500 (phil-z at comcast.net)
   2. Re: Intermittent low output KPA500 (Jack Brindle)
   3. Re: ? Intermittent low output KPA500 (G)
   4. K2/100 Fault finding (Steve Bunting)
   5. Re: K2/100 Fault finding (Michael Carter)

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Message: 1
Date: Wed, 16 Apr 2025 14:01:11 -0400
From: phil-z at comcast.net
To: Elecraft at mailman.qth.net
Subject: [Elecraft] Intermittent low output KPA500
Message-ID: <8765654B-73FD-4298-AB39-500CF3445934 at comcast.net>
Content-Type: text/plain;       charset=utf-8

In the last few days I have noticed that when operating FT8 using my KAT500/KPA500 combo with an IC-7610, my output seems low on the first or second transmit cycles after switching bands. If I am chasing DX I usually run about 120 to 160 Watts, which is about 6 to 8 Watts on the transmitter. I am seeing only 50 or 60 Watts on the amp when I first transmit. After a couple cycles it goes to the higher output and is stable after that.

When changing bands, I switch the radio, with the amp in Standby, then switch bands on the amp before transmitting. Sometimes I use Tune in WSJT-X running just a few watts with the amp in Standby for the amp to automatically switch bands. My antennas have low SWR on most bands, so the tuner is often in Bypass.

This seems to have just started and most often occurs on 20 Meters using a GAP 20 Meter Mono-band.

I also run CW and some SSB but don?t notice the power meter as much as on FT8. I don?t thing it?s happening there.

Has anyone seen this or a similar problem?

Phil N3ZP

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Message: 2
Date: Wed, 16 Apr 2025 13:41:34 -0500
From: Jack Brindle <jackbrindle at me.com>
To: phil-z at comcast.net
Cc: Elecraft at mailman.qth.net
Subject: Re: [Elecraft] Intermittent low output KPA500
Message-ID: <B32A38AF-8B99-4322-88B4-C5E4C5CE0C40 at me.com>
Content-Type: text/plain;       charset=utf-8

Place a watt meter between the transceiver and the KPA500 and measure the TX output into the KPA. I think you will find the transceiver output is low at those times.

This is fairly common in the K3/K3S world when the tx needs to be recalibrated. I would suggest that your 7610 may need recalibration as well.

The KPA500 is a linear amplifier - it has fixed gain. The only thing in the KPA500 that could change the gain is a 3db pad in the front end that is invoked when the controller sees something amiss, but not so bad as to declare a fault (yet). You would see a flashing red LED on the KPA when this is invoked. Since you didn?t mention a flashing LED on the KPA, I can only assume the pad is not being invoked. This then points at the driving tx for low output.

73,
Jack, W6FB

> On Apr 16, 2025, at 1:01?PM, Phil Zminda via Elecraft <elecraft at mailman.qth.net> wrote:
>
> In the last few days I have noticed that when operating FT8 using my KAT500/KPA500 combo with an IC-7610, my output seems low on the first or second transmit cycles after switching bands. If I am chasing DX I usually run about 120 to 160 Watts, which is about 6 to 8 Watts on the transmitter. I am seeing only 50 or 60 Watts on the amp when I first transmit. After a couple cycles it goes to the higher output and is stable after that.
>
> When changing bands, I switch the radio, with the amp in Standby, then switch bands on the amp before transmitting. Sometimes I use Tune in WSJT-X running just a few watts with the amp in Standby for the amp to automatically switch bands. My antennas have low SWR on most bands, so the tuner is often in Bypass.
>
> This seems to have just started and most often occurs on 20 Meters using a GAP 20 Meter Mono-band.
>
> I also run CW and some SSB but don?t notice the power meter as much as on FT8. I don?t thing it?s happening there.
>
> Has anyone seen this or a similar problem?
>
> Phil N3ZP
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> Message delivered to jackbrindle at me.com

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Message: 3
Date: Wed, 16 Apr 2025 19:05:44 +0000
From: G <gt-i at gmx.net>
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] ? Intermittent low output KPA500
Message-ID:
        <trinity-2945b112-3061-499f-83ab-1614daa961cd-1744830344655 at trinity-msg-rest-gmx-gmx-live-6c784bf6f5-mlk6j>

Content-Type: text/plain; charset="UTF-8"

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Message: 4
Date: Thu, 17 Apr 2025 12:04:37 +0100 (BST)
From: Steve Bunting <stephen.bunting at virgin.net>
To: "elecraft at mailman.qth.net" <elecraft at mailman.qth.net>
Subject: [Elecraft] K2/100 Fault finding
Message-ID: <448014196.3592505.1744887877175 at mail.virginmedia.com>
Content-Type: text/plain; charset=UTF-8

Hi everyone,

I have a K2/100 from an SK sale that works just fine in qrp mode but not above 10W on any band (into a good load). The final transistors get warm with the radio set at 20W and test good afaik.

Referring to the test voltages in the manual, I find that D14 does not get 115v on the cathode when transmitting above 10w, which suggests I have a T/R switch problem. Q6 and Q7 have the correct voltages on them, although my (old) semiconductor tester fails to identify them at all. I am not sure I trust that result.

R13 tests good, but is very hot when transmitting at for 3 or 3 seconds at the 20W level.

I am a bit stuck - Can anyone suggest what I do next?
Thanks
Steve, M0BPQ

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Message: 5
Date: Thu, 17 Apr 2025 12:36:39 +0000
From: Michael Carter <Mike.Carter at unh.edu>
To: Elecraft Reflector Reflector <Elecraft at mailman.qth.net>
Subject: Re: [Elecraft] K2/100 Fault finding
Message-ID:
        <BL3P223MB03856357CDB14F0A550FE1B8E4BC2 at BL3P223MB0385.NAMP223.PROD.OUTLOOK.COM>

Content-Type: text/plain; charset="iso-8859-1"

Hi Steve,

The absence of the high voltage reverse bias supply on the cathodes of D12 and D14 when in TX key-down state suggests two possibilities

  1.

  1.
Q6 has failed with a permanent drain-to-source short. That would provide a path for current through D14 as well as through D13 when in transmit mode for power settings above 10 watts.
  2.
The high voltage bias supply has failed.  This is easily checked by measuring the DC voltage at the junction of R11 (510k) and C77 (0.015 uF).

Please let us know what you find.  If your old transistor tester is designed to test NMOS FETs, I would be surprised.  It's best to just measure the drain voltage of Q6, first in RX mode, then in TX mode.  Be sure the transmit power control is set above 10 watts to engage the PA module.  In RX mode that Q6 drain voltage should be low, less than 1 V, while in TX mode it should be quite high, at least 90V DC.

73,
Mike, K8CN

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End of Elecraft Digest, Vol 252, Issue 13
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