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Procedure for setting Bias on K3 LPA FETS

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wa8wzg

Procedure for setting Bias on K3 LPA FETS

To All...
I have had to replace the 2 FET's Q4 and Q5 on the LPA,,,'I cant not
find the info on setting the Bias..on these devices,,
Can some one point me in the right direction...
Thanks again!!
Tom
N7GP
EX WA8WZG
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Scott-2

Re: Procedure for setting Bias on K3 LPA FETS

Tom,
The procedure below is what I received from Elecraft Tech support back
in July of 2019. It worked for me after changing the LPA FETs in my K3.

73,
Scott AD5HS

=======================================================

You'll need this sequence for setting bias after the fix:

*Here is the procedure to check if your LPA module has
been adjusted properly for bias current:*

(current increases approx. 600 ma when XMIT is tapped (below 12W).

1. Remove the 100 watt amplifier (if installed) by disconnecting the fan
panel and their 2 connector wires, and the 2 wires to the circuit
breaker. Use care to not pull on the KPA3 too hard and thus let it fly
off the connectors and cause damage to it or to the chassis. Use gentle
but firm finger pressure, or pry carefully with a plastic tool (such as
the K2 green plastic alignment tool), near the two PCB connectors. This
will usually help the KPA3 start to move.

2. Locate small SMD pots R18, R19 and R24 near the center of the LPA
board. Turn the pots to minimum. (White pots fully CW, Black pots fully
CCW)

3. Turn the power knob down to ZERO.

4. Tap DISP, (display button), use VFO B to display the current drawn by
the radio. For more accurate results, insert a DMM in the positive power
supply lead that is set to a current range that can handle up to 2-3 amps.

5. Hold TEST (the right hand side of the MODE button). This puts the K3
in TEST mode with no RF drive.

6. Tap XMIT. You will see a current of approx 1 to 2A (depending on
options in radio) but the red TX LED will not come on. Adjust R18 to
increase the reading by 0.2A IE 200ma

7. Adjust R19 to increase the current another 0.2A

8. Adjust R24 to increase the current another 0.2A

9. Tap XMIT to exit bias on condition. (Current should drop approx.
600ma). NOTE: if you are just checking current and you do have the KPA3
installed, set it to NOT INST and cycle power so you do not have its
current added, thus confusing your readings.

10. Set Power to 11.9W. Hold TEST again to cancel TEST mode so RF output
is possible. The LCD shows TX NORM.

11. With PWR SET to “nor” Hold TUNE and see how much power you get on
all bands.

Elecraft Customer Support
[hidden email]
831-763-4211

=================================================================

On 11/12/2020 1:48 PM, [hidden email] wrote:

> To All...
> I have had to replace the 2 FET's Q4 and Q5 on the LPA,,,'I cant not
> find the info on setting the Bias..on these devices,,
> Can some one point me in the right direction...
> Thanks again!!
> Tom
> N7GP
> EX WA8WZG
> ______________________________________________________________
> 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
> Message delivered to [hidden email]

______________________________________________________________
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
Message delivered to [hidden email]

wa8wzg

Re: Procedure for setting Bias on K3 LPA FETS

In reply to this post by wa8wzg

Thanks for all the replies !!
I appreciate it from everyone,,
73's
Tom
N7GP
ex Wa8WZG

On 2020-11-12 19:48, [hidden email] wrote: