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K2 10Watt RF SSB TX distortion

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K2 10Watt RF SSB TX distortion

paulb

Hi Folks

Getting a few reports of slightly ragged SSB audio TX.
check here definitely NQR ( not quite right )
Used another receiver to chase the signal path from the SSB board
to the power stages.
Distortion is in finals. They appear to be on the very edge of class B.
Deceased  resistor value R59 to increase bias current. My K2 meter may not
be accurate; previous value TX showing 0.64A , moved to 0.78A.
Received audio is now very clean.
R59 value approx 1K.
No apparent increase in temps with casual use.
serial 6534 built several years ago.

cheers

Paul b
zl1ajy

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Re: K2 10Watt RF SSB TX distortion

Don Wilhelm-4
Paul,

R59 should be 4.7k, and the increase in bias voltage is not a good thing.
It may be that the PA transistors have some current leakage from
collector to base that is upsetting the bias.

I would not even operate it in CW mode with that increased bias voltage.

I would suggest first replacing R59 and while you are in that vicinity,
arbitrarily replace Q11 and Q13 (2N2222As).  Then check the bias during
transmit.  It should be between 0.62 and 0.64 volts.  If it is not,
order the K2PAKIT and replace the PA transistors.  The Elecraft PA kit
contains matched transistors - do not substitute - 2SC1969 transistors
used to be consistent and did not have to be matched, but that is not
true for those available today.

73,
Don W3FPR

On 1/28/2015 5:56 PM, paulb wrote:

> Hi Folks
>
> Getting a few reports of slightly ragged SSB audio TX.
> check here definitely NQR ( not quite right )
> Used another receiver to chase the signal path from the SSB board
> to the power stages.
> Distortion is in finals. They appear to be on the very edge of class B.
> Deceased  resistor value R59 to increase bias current. My K2 meter may not
> be accurate; previous value TX showing 0.64A , moved to 0.78A.
> Received audio is now very clean.
> R59 value approx 1K.
> No apparent increase in temps with casual use.
> serial 6534 built several years ago.
>
>

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