K2 diminishing power on 15 to 10 meters

[Chuck Teague]

I just finished installing the KAT2 and in testing discovered that power output begins dropping on 15 mtrs. I removed the KAT2 and tested the plain K2 into a dummy load, with the same results.  I redid the transmitter alignment but it didn't change the results, which are, with a 10 watt setting:
80 mtrs: 10 watts
40 mtrs: 10 watts
30 mtrs: 10 watts
20 mtrs: 10 watts
17 mtrs:  8 watts
15 mtrs:  3 watts
12 mtrs: 1.5 watts
10 mtrs: 1.5 watts
160 mtrs; 1 watt

There is no high current with any of these readings--it is about what you would expect for the power output listed.  An external meter between the radio and dummy load confirms the K2 internal meter. It appears that the drive is tapering off, but I'm at a loss for what would cause that.

Power supply is 13.8 volts, indicating 13.3 on K2 display and never dropping below 13v.

This is a pretty much loaded radio with SSB, DSP, NB, 160 meter option--pretty much everything but the KIO2, 60 meters and transverters, and has been in use as a K2/100 for several years delivering expected output on all bands.  I just recently pulled the KPA100 out of it to convert it back to QRP.

Where do i start and what do I look for?

Thanks,

Chuck Teague
NN7U

[Don Wilhelm-4]

Chuck,

Did you ever increase the value of R98 on the bottom of the RF board to
keep the power from oscillating?  If so, you have now removed the
additional gain of the KPA100 and may need to restore the original value
of R98.  Remove the bottom cover and check the value of R98 - if it is
greater than 270 ohms, change it.

If that is not the problem, read on ---

Often a drop off in power on the higher frequency bands is traced to a
problem with the Transmit Buffer U9.  First be certain Q20 is doing its
job - measure the DC voltage a U9 pin 3 - it should be near your power
supply voltage during receive and then half that value during transmit.  
If it stays at the higher voltage, the Q20 has failed.

The next thing to try is re-flowing the solder with a hot (750 degF)
soldering iron.  Look at the schematic for the RF Board sheet 2 in the
left center of the sheet to identify the components involved - re-solder
those.

If those steps do not solve the situation on the 17 through 10 meter
bands, you will have to do some signal tracing to find out where the
fault is.  Turn to Appendix E page 14.  You will *not* be following the
instructions for signal tracing that appear in the manual, but the list
of points to check are shown starting halfway down the left column.

Set the power knob to 10 watts and leave it there throughout the
following tests.

You will be comparing the RF voltage at each of those points for all
bands 20 meters and above.
Since 20 meters gives you normal output, that is your "standard" band.  
Do not expect to see the RF voltages that are listed in the manual as
"expected voltages" - you are doing this differently.

While doing this exercise, keep in mind the way the K2 controls power.  
It reads the power at the output, compares that to the requested power
and controls the drive (by increasing/decreasing the BFO injection into
the Transmit Mixer).  What that means is that on a band where the power
output is lower than the requested value, you should see a higher RF
voltage that that seen of a good working band - a higher RF voltage
means the stage is working.  When you get to a stage where the RF
voltage is *lower* than the voltage on the "good" band, you have found
the failure point.

I doubt anything I have said above will lead you to a solution for the
160 meter band - I would believe that is a different problem, so figure
out what is going on for 17 thru 10 meters first, and then you can
attack the 160 meter problem.

73,
Don W3FPR

On 7/21/2012 6:50 PM, Chuck Teague wrote:

> I just finished installing the KAT2 and in testing discovered that power
> output begins dropping on 15 mtrs. I removed the KAT2 and tested the plain
> K2 into a dummy load, with the same results.  I redid the transmitter
> alignment but it didn't change the results, which are, with a 10 watt
> setting:
> 80 mtrs: 10 watts
> 40 mtrs: 10 watts
> 30 mtrs: 10 watts
> 20 mtrs: 10 watts
> 17 mtrs:  8 watts
> 15 mtrs:  3 watts
> 12 mtrs: 1.5 watts
> 10 mtrs: 1.5 watts
> 160 mtrs; 1 watt
>
> There is no high current with any of these readings--it is about what you
> would expect for the power output listed.  An external meter between the
> radio and dummy load confirms the K2 internal meter. It appears that the
> drive is tapering off, but I'm at a loss for what would cause that.
>
> Power supply is 13.8 volts, indicating 13.3 on K2 display and never dropping
> below 13v.
>
> This is a pretty much loaded radio with SSB, DSP, NB, 160 meter
> option--pretty much everything but the KIO2, 60 meters and transverters, and
> has been in use as a K2/100 for several years delivering expected output on
> all bands.  I just recently pulled the KPA100 out of it to convert it back
> to QRP.
>
> Where do i start and what do I look for?
>
>

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[aj4tf]

Interesting, I am working on a KPA100 problem right now.  The original problem was reduced output on 15m, but when I disassembled the KPA100, I found a burned trace on the circuit board in the bias current circuit feeding Q4.. which seems unlikely to have caused a problem only on 15m.  I plan to put it back together tomorrow and test it.

[Alan Jump]

Not having the schematics in front of me and not being familiar with the unit...PCB traces don't usually burn on their own. Did Q4 check as good? Might be worth some desoldering to put an ohmmeter on it.

73 de N5ILN

Sent from my iPhone

On Jul 21, 2012, at 7:40 PM, aj4tf <[hidden email]> wrote:

> Interesting, I am working on a KPA100 problem right now.  The original
> problem was reduced output on 15m, but when I disassembled the KPA100, I
> found a burned trace on the circuit board in the bias current circuit
> feeding Q4.. which seems unlikely to have caused a problem only on 15m.  I
> plan to put it back together tomorrow and test it.
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/K2-diminishing-power-on-15-to-10-meters-tp7559611p7559616.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

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[aj4tf]

Yes, it's good.  I just got it back together and it appears to be working.

[Chuck Teague]

Problem solved by resoldering everything around TX Buffer U9. Many thanks to Don Wilhelm W3FPR for his sage advice.  The radio would be going back to Elecraft now without his insisting I try it one more time.

Chuck, NN7U