Strange problem with K3s/10: could use some suggestions

[Bill Leonard N0CU]

I am having a strange problem with my K3s/10: the max output power on 20M drops from 11.5W to 7.5W as the freq is moved from 14.0 MHz to 14.35 MHz. All of the other bands stay above 11W across their respective bands. The other interesting characteristic is that if I lower the power to 5W, there is no drop- it stays at 5W across the entire 20M band.

Other relevant data:

-All data is taken post Cal

-The drop on the 12V line when I key up the transmitter (set for 12W) is the same at both ends of the 20M band: 13.9 to 13.4V as measured by the K3s.

-Output power is measured with an LP-100A and is the same as shown on the K3s.

-The 20M BPF & LPF are flat to a few tenths of a dB and the SWR<2:1 across the band.

-The voltages and waveforms on the drains of the two FETs are identical, and look OK to me.

-The bias on both FET gates is ~2.6VDC.

-The RF input signal to the KLPA3A (as measured at the XVTR output) is the correct level and remains constant on all bands as the freq is moved across the band. All spurious are down >50 dB. I also verified that the signal level remains constant over the 20M band when the output power is dropping. This suggests that the problem is not with the AGC or with any components ahead of the KLPA3A.

Since the problem does not occur at reduced (5W) power, it appears to be due to some non-linearity. The FETs would be a logical candidate if it were not for the fact that the problem only occurs on one band. I checked the temps on all of the 20M LPF toroids and didn't observe any rise.

I have no idea where else to look for a band specific non-linearity and have run out of ideas on what else to check. Any suggestions would be greatly appreciated.

Bill  N0CU

[Jerry Moore]

I don't see what you have connected to the output past the meter dummyload
or antenna.

-----Original Message-----
From: Elecraft [mailto:[hidden email]] On Behalf Of Bill
Leonard N0CU
Sent: Thursday, January 26, 2017 12:01 PM
To: [hidden email]
Subject: [Elecraft] Strange problem with K3s/10: could use some suggestions

I am having a strange problem with my K3s/10: the max output power on 20M
drops from 11.5W to 7.5W as the freq is moved from 14.0 MHz to 14.35 MHz.
All of the other bands stay above 11W across their respective bands. The
other interesting characteristic is that if I lower the power to 5W, there
is no drop- it stays at 5W across the entire 20M band.

Other relevant data:

-All data is taken post Cal

-The drop on the 12V line when I key up the transmitter (set for 12W) is the
same at both ends of the 20M band: 13.9 to 13.4V as measured by the K3s.

-Output power is measured with an LP-100A and is the same as shown on the
K3s.

-The 20M BPF & LPF are flat to a few tenths of a dB and the SWR<2:1 across
the band.

-The voltages and waveforms on the drains of the two FETs are identical, and
look OK to me.

-The bias on both FET gates is ~2.6VDC.

-The RF input signal to the KLPA3A (as measured at the XVTR output) is the
correct level and remains constant on all bands as the freq is moved across
the band. All spurious are down >50 dB. I also verified that the signal
level remains constant over the 20M band when the output power is dropping.
This suggests that the problem is not with the AGC or with any components
ahead of the KLPA3A.

Since the problem does not occur at reduced (5W) power, it appears to be due
to some non-linearity. The FETs would be a logical candidate if it were not
for the fact that the problem only occurs on one band. I checked the temps
on all of the 20M LPF toroids and didn't observe any rise.

I have no idea where else to look for a band specific non-linearity and have
run out of ideas on what else to check. Any suggestions would be greatly
appreciated.

Bill  N0CU



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[Nr4c]

Is this with antenna or dummy load?

Sent from my iPhone
...nr4c. bill

> On Jan 26, 2017, at 12:01 PM, Bill Leonard N0CU <[hidden email]> wrote:
>
> I am having a strange problem with my K3s/10: the max output power on 20M
> drops from 11.5W to 7.5W as the freq is moved from 14.0 MHz to 14.35 MHz.
> All of the other bands stay above 11W across their respective bands. The
> other interesting characteristic is that if I lower the power to 5W, there
> is no drop- it stays at 5W across the entire 20M band.
>
> Other relevant data:
>
> -All data is taken post Cal
>
> -The drop on the 12V line when I key up the transmitter (set for 12W) is the
> same at both ends of the 20M band: 13.9 to 13.4V as measured by the K3s.
>
> -Output power is measured with an LP-100A and is the same as shown on the
> K3s.
>
> -The 20M BPF & LPF are flat to a few tenths of a dB and the SWR<2:1 across
> the band.
>
> -The voltages and waveforms on the drains of the two FETs are identical, and
> look OK to me.
>
> -The bias on both FET gates is ~2.6VDC.
>
> -The RF input signal to the KLPA3A (as measured at the XVTR output) is the
> correct level and remains constant on all bands as the freq is moved across
> the band. All spurious are down >50 dB. I also verified that the signal
> level remains constant over the 20M band when the output power is dropping.
> This suggests that the problem is not with the AGC or with any components
> ahead of the KLPA3A.
>
> Since the problem does not occur at reduced (5W) power, it appears to be due
> to some non-linearity. The FETs would be a logical candidate if it were not
> for the fact that the problem only occurs on one band. I checked the temps
> on all of the 20M LPF toroids and didn't observe any rise.
>
> I have no idea where else to look for a band specific non-linearity and have
> run out of ideas on what else to check. Any suggestions would be greatly
> appreciated.
>
> Bill  N0CU
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/Strange-problem-with-K3s-10-could-use-some-suggestions-tp7626121.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

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[Bill Leonard N0CU]

Sorry I forgot to mention the load.

The output of the K3s goes to the RF sensor for the LP-100A then to an MFJ 260C dummy load. However, the problem was first noticed on my 20M antenna, then again when trying to drive my HB SS Pwr Amp. The reason I would like to get this resolved is that my PA needs ~9W to reach full power output.

Bill  N0CU

[Don Wilhelm]

Bill,

Are all your antenna connections tight? - finger-tight is frequently
not enough for SO-239/PL259 connections, use pliers to tighten them a
bit more than finger-tight (after making sure the PL-259s are seated in
the "notches").

I would suggest doing the TX CAL procedure once again.
If that does not correct it, contact K3support for further assistance.

73,
Don W3FPR

On 1/26/2017 12:01 PM, Bill Leonard N0CU wrote:
> I am having a strange problem with my K3s/10: the max output power on 20M
> drops from 11.5W to 7.5W as the freq is moved from 14.0 MHz to 14.35 MHz.
> All of the other bands stay above 11W across their respective bands. The
> other interesting characteristic is that if I lower the power to 5W, there
> is no drop- it stays at 5W across the entire 20M band.
>
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