K3S TX current

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K3S TX current

ab2tc
Hi,

I have recently made a discovery during a routine checkup that my K3S draws
*much* more TX current on 20 m than any of the other bands. Using Data A
FSKD mode at the 100W level it draws about 22A versus 16-18 A on all other
bands. DC voltage from the Powerwerx SS-30DV is 14.2V as measured at the
binding posts. The K3S voltmeter indicates 14.2V in receive and 13.7V in
transmit mode. Current is measured with the K3S built-in ammeter (~1A in
receive). Power output is 100-102W as measured with an LP-100A on all bands
40m - 10m. TX IMD is not noticeably affected as measured using the built-in
2-tone generator. I measure maybe 33dB (5th order worst case) below PEP on
40m slowly worsening to 30dB on 10m, all within acceptable limits for a
solid state 12VDC powered transmitter.

What's up with this? I don't know how long this has been been a "problem" as
this is not something I routinely monitor. My much older K3 (with the old
bipolar PA) does *not* exhibit this behavior. Is this indicative of a
problem with the 20m low-pass filter perhaps? All measurements are done with
a Ridge Electronics dummy load. All RF connections are tight and the LP-100A
indicates 1.01 SWR on this dummy load in all cases. BTW my serial # is
10480, which is an early serial #. It was back to Elecraft twice in its
first year for fixes to the transmit signal path to improve TX IMD.

Knut - AB2TC

PS. I am not worried about the electricity bill, but whether this is
indicative of a potential future serious failure.



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Re: K3S TX current

Randy Farmer-2
Interesting. My K3S (10175) and K3 (3260) exhibit the exact same
behavior. My K3S had a problem with low power stability on 80 meters
that resulted in field replacement of the 100W PA module, but the 20
meter current draw didn't change. My working hypothesis is tolerance
stack-up in the 20 meter LPF, but that's just a supposition. Both radios
have been used for fairly serious contest duty, the K3 since 2009 and
the K3S since 2016, with no reliability problems.

73...
Randy, W8FN

On 9/5/2020 10:20 PM, ab2tc wrote:

> Hi,
>
> I have recently made a discovery during a routine checkup that my K3S draws
> *much* more TX current on 20 m than any of the other bands. Using Data A
> FSKD mode at the 100W level it draws about 22A versus 16-18 A on all other
> bands. DC voltage from the Powerwerx SS-30DV is 14.2V as measured at the
> binding posts. The K3S voltmeter indicates 14.2V in receive and 13.7V in
> transmit mode. Current is measured with the K3S built-in ammeter (~1A in
> receive). Power output is 100-102W as measured with an LP-100A on all bands
> 40m - 10m. TX IMD is not noticeably affected as measured using the built-in
> 2-tone generator. I measure maybe 33dB (5th order worst case) below PEP on
> 40m slowly worsening to 30dB on 10m, all within acceptable limits for a
> solid state 12VDC powered transmitter.
>
> What's up with this? I don't know how long this has been been a "problem" as
> this is not something I routinely monitor. My much older K3 (with the old
> bipolar PA) does *not* exhibit this behavior. Is this indicative of a
> problem with the 20m low-pass filter perhaps? All measurements are done with
> a Ridge Electronics dummy load. All RF connections are tight and the LP-100A
> indicates 1.01 SWR on this dummy load in all cases. BTW my serial # is
> 10480, which is an early serial #. It was back to Elecraft twice in its
> first year for fixes to the transmit signal path to improve TX IMD.
>
> Knut - AB2TC
>
> PS. I am not worried about the electricity bill, but whether this is
> indicative of a potential future serious failure.
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Re: K3S TX current

ab2tc
Hi Randy,

Does your K3 #3260 have the newer MOSFET PA? I believe the K3 switched the
PA from bipolar to the newer MOSFET design at some point (mine is #82!). I
agree with you that the 20/30m LPF ought to be a prime suspect. Everything
else is common and 14MHz should be very close to the center of the total
frequency range of the transmitter. My K3S has not had any reliability
problems either, but it sees fairly light duty with SSB only and no heavy
duty "run mode" contest use.

BTW, I checked the current draw on 30m (on the K3S) which *shares the same
LP filter*. It is an average 17.5A. But 30m is lower in frequency and thus
farther away from the filter cutoff. 40m, 15m and 10m LP filters are also
shared with a lower frequency band, but do not cause an elevated current
draw.

Knut - AB2TC


Randy Farmer-2 wrote

> Interesting. My K3S (10175) and K3 (3260) exhibit the exact same
> behavior. My K3S had a problem with low power stability on 80 meters
> that resulted in field replacement of the 100W PA module, but the 20
> meter current draw didn't change. My working hypothesis is tolerance
> stack-up in the 20 meter LPF, but that's just a supposition. Both radios
> have been used for fairly serious contest duty, the K3 since 2009 and
> the K3S since 2016, with no reliability problems.
>
> 73...
> Randy, W8FN
>
> On 9/5/2020 10:20 PM, ab2tc wrote:
> <snip>





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Re: K3S TX current

Randy Farmer-2
The K3 PA is the original module, but I don't know what devices it uses.
Perhaps someone with inside knowledge could provide a S/N range or
approximate date for the changeover. I built my K3 from a kit in July
2009. As I recall, It draws about 17.5A with key-down CW on 20 meters.
The K3S PA module was replaced in July 2016. I don't know if the
original K3S PA showed the elevated current draw before it was replaced.

73...
Randy, W8FN

On 9/6/2020 2:07 PM, ab2tc wrote:

> Hi Randy,
>
> Does your K3 #3260 have the newer MOSFET PA? I believe the K3 switched the
> PA from bipolar to the newer MOSFET design at some point (mine is #82!). I
> agree with you that the 20/30m LPF ought to be a prime suspect. Everything
> else is common and 14MHz should be very close to the center of the total
> frequency range of the transmitter. My K3S has not had any reliability
> problems either, but it sees fairly light duty with SSB only and no heavy
> duty "run mode" contest use.
>
> BTW, I checked the current draw on 30m (on the K3S) which *shares the same
> LP filter*. It is an average 17.5A. But 30m is lower in frequency and thus
> farther away from the filter cutoff. 40m, 15m and 10m LP filters are also
> shared with a lower frequency band, but do not cause an elevated current
> draw.
>
> Knut - AB2TC
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