K-2 /100 Power Output Drop

[Dennis Utley - AF7Y]

First the good news.  The receiver seems to be working well, even transmit power is
at a full 100 watts (for a few seconds).  Then output drops quickly
to 50 watts.  Thats the bad news.

I can set the power knob for output from 110 down to QRP power, and the first
few seconds of transmission are at the selected power.  After the
first few seconds power goes instantly to almost exactly 50% of what
was manually selected.   100 watts becomes 50, 60 watts becomes 30,
and 40 watts becomes 20 watts prox, (Peak indicated power on a
calibrated digital peak reading  meter.)  (Model P-3000 by RF
Applications, digital readout into dummy load on 40 meters.
Indicating 1/1.  Same behavior if input to 1.1/1 SWR 40 meter beam.

After this 50% drop, transmission continues normally, quality reports
are good, and all is well, except my power is 50% of what it should be.

Maximum power after a few seconds of transmission is only about 55 watts Peak
SSB.

If I turn the radio off and wait a few moments.  It will again output
full 100 watts peak SSB output for a few seconds then drop to 50%.

Has anyone seen this before?  I have done a few searches on the
reflector, but have not seen this problem discussed.  

===========================================
New Elecraft K2/100 -- Serial # 5290 .

Any help and suggestion is very much appreciated;
===========================================

Thank You,
Dennis Utley, AF7Y
7820 Caballero Dr.
Sandy, UT  84093

Emails:  [e-mail]
            [e-mail]

Phone: [phone]

[Alexey Kats]

It sounds like half of your PA shuts down somewhere close to the final
stage transistors. Could it be a bad soldered connection somewhere
which loses connection after brief warm-up? Or a partially stripped
wire in one of symmetric transformers? I'd start with checking all the
connections around PA transistors in final stage. (I had similar
experience with a powerful AF amplifier long time ago, but there was
another hint - audio was badly distorted.)

--
Alexey Kats (neko)

On Fri, Jul 16, 2010 at 1:50 PM, Dennis Utley - AF7Y <[hidden email]> wrote:
>
> First the good news.  The receiver seems to be working well, even transmit
> power is
> at a full 100 watts (for a few seconds).  Then output drops quickly
> to 50 watts.  Thats the bad news.

> If I turn the radio off and wait a few moments.  It will again output
> full 100 watts peak SSB output for a few seconds then drop to 50%.
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[Alexey Kats]

Out of curiosity, is it only happening with power set to >10 watts?
(If it is happening in QRP mode as well something must be wrong with
your main RF board and its PA, but if it's only happening in QRO mode
it's the KPA100 at fault. I think.)

On Fri, Jul 16, 2010 at 2:45 PM, Alexey Kats <[hidden email]> wrote:

> It sounds like half of your PA shuts down somewhere close to the final
> stage transistors. Could it be a bad soldered connection somewhere
> which loses connection after brief warm-up? Or a partially stripped
> wire in one of symmetric transformers? I'd start with checking all the
> connections around PA transistors in final stage. (I had similar
> experience with a powerful AF amplifier long time ago, but there was
> another hint - audio was badly distorted.)
>
> --
> Alexey Kats (neko)
>
> On Fri, Jul 16, 2010 at 1:50 PM, Dennis Utley - AF7Y <[hidden email]> wrote:
>>
>> First the good news.  The receiver seems to be working well, even transmit
>> power is
>> at a full 100 watts (for a few seconds).  Then output drops quickly
>> to 50 watts.  Thats the bad news.
>
>> If I turn the radio off and wait a few moments.  It will again output
>> full 100 watts peak SSB output for a few seconds then drop to 50%.
>

--
Alexey Kats (neko)
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[Don Wilhelm-4]

Dennis,

Such a problem is usually caused be a poor solder connection somewhere.  
The challenge is to find where.
You indicated this problem occurs on SSB.  What about CW?  Does it do
the same thing?  How about when you do a TUNE?
If the problem occurs during a TUNE, then it will be easy to find the
offending stage by the procedure below, but if it happens only on SSB,
then we should look at the KSB2 board.

Since the problem occurs when the power is set to less than 11 watts, it
is likely in the base K2, so remove the KPA100 and test only the base K2
without the KPA100 attached.  If it does the same thing, then you should
construct the RF Probe from the parts provided with your K2 kit (or tell
us that you have an oscilloscope available), so you can trace through
the transmit chain "backwards" and find the stage where the power is steady.

Let us know if my "guesses" are correct (please answer the questions
above), and we can lead you to the failing stage, and after that you can
find the failing component.

73,
Don W3FPR

Dennis Utley - AF7Y wrote:

> First the good news.  The receiver seems to be working well, even transmit
> power is
> at a full 100 watts (for a few seconds).  Then output drops quickly
> to 50 watts.  Thats the bad news.
>
> I can set the power knob for output from 110 down to QRP power, and the
> first
> few seconds of transmission are at the selected power.  After the
> first few seconds power goes instantly to almost exactly 50% of what
> was manually selected.   100 watts becomes 50, 60 watts becomes 30,
> and 40 watts becomes 20 watts prox, (Peak indicated power on a
> calibrated digital peak reading  meter.)  (Model P-3000 by RF
> Applications, digital readout into dummy load on 40 meters.
> Indicating 1/1.  Same behavior if input to 1.1/1 SWR 40 meter beam.
>
> After this 50% drop, transmission continues normally, quality reports
> are good, and all is well, except my power is 50% of what it should be.
>
> Maximum power after a few seconds of transmission is only about 55 watts
> Peak
> SSB.
>
> If I turn the radio off and wait a few moments.  It will again output
> full 100 watts peak SSB output for a few seconds then drop to 50%.
>
>  

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[Dennis Utley - AF7Y]

Thank You for your post, Alexey.  It seems you are right about the problem with my K2 being near the PA stage.  I discovered it does not happen below 10 watts.

I appreciate your input.

Dennis, AF7Y

[Dennis Utley - AF7Y]

Thank you for your response to my post, Don.  I did some more checking with a watt meter that works better at QRP levels and found I don't have the power drop below 10 watts.  Following  your suggestion I also tested the radio in CW, and Tune modes.  The results are as follows:

CW
====
Power Set at       Actual Power

110                    69
80                      49
50                      26

Tune
=======
106                    62
87                      49
50                      26

SSB
=====
100                   100 for a couple of seconds when cold
80                     40
47                     28
40                     18

The failing stage would seem to be the 100 watt amp board.

It's surprising the "set power" and output readings are so different from CW to TUNE modes.

Are there any adjustments on the PA board that might account for the power out deficit?

Thanks for your input on my problem, and judging by your many posts, you also help many others..

73
Dennis, AF7Y
===========

quote author="Don Wilhelm-4">
Dennis,

Such a problem is usually caused be a poor solder connection somewhere.  
The challenge is to find where.
You indicated this problem occurs on SSB.  What about CW?  Does it do
the same thing?  How about when you do a TUNE?
If the problem occurs during a TUNE, then it will be easy to find the
offending stage by the procedure below, but if it happens only on SSB,
then we should look at the KSB2 board.

Since the problem occurs when the power is set to less than 11 watts, it
is likely in the base K2, so remove the KPA100 and test only the base K2
without the KPA100 attached.  If it does the same thing, then you should
construct the RF Probe from the parts provided with your K2 kit (or tell
us that you have an oscilloscope available), so you can trace through
the transmit chain "backwards" and find the stage where the power is steady.

Let us know if my "guesses" are correct (please answer the questions
above), and we can lead you to the failing stage, and after that you can
find the failing component.

73,
Don W3FPR

Dennis Utley - AF7Y wrote:
> First the good news.  The receiver seems to be working well, even transmit
> power is
> at a full 100 watts (for a few seconds).  Then output drops quickly
> to 50 watts.  Thats the bad news.
>
> I can set the power knob for output from 110 down to QRP power, and the
> first
> few seconds of transmission are at the selected power.  After the
> first few seconds power goes instantly to almost exactly 50% of what
> was manually selected.   100 watts becomes 50, 60 watts becomes 30,
> and 40 watts becomes 20 watts prox, (Peak indicated power on a
> calibrated digital peak reading  meter.)  (Model P-3000 by RF
> Applications, digital readout into dummy load on 40 meters.
> Indicating 1/1.  Same behavior if input to 1.1/1 SWR 40 meter beam.
>
> After this 50% drop, transmission continues normally, quality reports
> are good, and all is well, except my power is 50% of what it should be.
>
> Maximum power after a few seconds of transmission is only about 55 watts
> Peak
> SSB.
>
> If I turn the radio off and wait a few moments.  It will again output
> full 100 watts peak SSB output for a few seconds then drop to 50%.
>
>  
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Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
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