Jumping power on 40 and 80 meter

[Peter-64]

HI

Last weekend I found out that my K2 didn't work correctly on 40 and 80 meters.
The power on 40 is jumping between 100 and 50 watt when I do keydown
When rotating the power pot, it looks like a bad pot meter. The power is jumps up and down
when I turns it
Its worse on 40 and a bit better on 80 meters. All other bands are fine so its not the pot
meter.

To try to find the error I remove the K2/100 PA. With 15 watt its has still the same
error.

Then I removed the ssb adapter and place  C167 back and 2 jumpers W2/W3. Still the same.

When I look at the drawing there is a RF output detector near the antnne output. As I can
see the label V REFDET goes only to the SSB adaptor (pin 6)

How is the power level set in the K2?
Since 80 and 40 meter doen't share the same band pass filters I think there isn't the
problem
Also the PA is broadband so this one is working good too.
Does any one has a clue to look at?

Peter


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[Don Wilhelm-3]

Peter,

If you have the problem both with and without the KPA100 installed, the
problem does NOT lie with VRFDET because with the KPA100 in-line the
detection is done with the wattmeter in the KPA100 and with the KPA100
removed (that means disconnected), VRFDET uses the diode in the base K2 RF
board.
OTOH, if you did not disconnect the KPA100, but meerly put it into bypass
for QRP operation, the KPA100 wattmeter still is functioning.

The K2 takes the power sensed on the VRFDET signal and processes it to
compare with the requested power setting and produces the V ALC signal to
control the power output.

You might look at Q24 on the K2 RF board as well as D36, R98 and R99 on the
bottom of the RF board.  These are in the power control path for the carrier
(BFO) injection.  The ampunt of injection is controlled by V ALC coming from
the emitter of Q8 on the Control Board.  If you have recently done the
Keying waveshape upgrade to an older K2, look carefully at those changes -
an error here can cause results similar to what you report.

73,
Don W3FPR

----- Original Message -----

>
> Last weekend I found out that my K2 didn't work correctly on 40 and 80
> meters.
> The power on 40 is jumping between 100 and 50 watt when I do keydown
> When rotating the power pot, it looks like a bad pot meter. The power is
> jumps up and down
> when I turns it
> Its worse on 40 and a bit better on 80 meters. All other bands are fine so
> its not the pot
> meter.
>
> To try to find the error I remove the K2/100 PA. With 15 watt its has
> still the same
> error.
>
> Then I removed the ssb adapter and place  C167 back and 2 jumpers W2/W3.
> Still the same.
>
> When I look at the drawing there is a RF output detector near the antnne
> output. As I can
> see the label V REFDET goes only to the SSB adaptor (pin 6)
>
> How is the power level set in the K2?
> Since 80 and 40 meter doen't share the same band pass filters I think
> there isn't the
> problem
> Also the PA is broadband so this one is working good too.
> Does any one has a clue to look at?
>

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Elecraft web page: http://www.elecraft.com