Power Fluctuations on K2 just Built

[John Bahun]

I recently completed my K2 and I am noticing that my Power output is very flaky and does not seem to track the Power control knob very well.  I have tried both the TUNE button and the Key input with a Hand Key and I am getting power fluctuations with both.

I am suspecting something in the ALC circuit at this point - Anyone have any ideas on this - Since the installation of the two final transistors is kind of tricky - if they are not installed correctly - could this cause this behaviour?

Also - if I measure from the Collector of the Finals to ground - what should I measure in Ohms?  I would like to compare mine to someone else's K2 to see if I might have a partial short in my finals.

However - the way the power fluctuates - I am really suspecting something in the ALC circuit - Just wondering if anyone else has run into this issue...

Thanks in advance!

John - WB8UFA

John J. Bahun
Software Engineer - Apple Computer Inc.

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[Don Wilhelm-3]

John,

The most likely cause is an unsoldered connection or a poorly soldered one.
The problem can be anywhere in the power control loop.  The beginning of the
power control starts with the VPWR signal out of U8 pin 2 on the Control
Board, then goes through the keying control circuit and exits there as VALC
which is applied to the RF Board Q24 to control the amount of BFO injection.
The RF signal is frequency translated and amplified by the transmit chain
and the ouptut RF Voltage is sampled and rectified by diode D9 - that
voltage is then fed back to the microprocessor as the VRFDET signal where it
is compared with the power control setting by the firmware - finally VPWR is
adjusted to compensate and bring the power to the requested level.

Sooo - your problem can be anywhere in that loop.  Examine all your
soldering carefully, then check the soldering again, and when you have
completed that, check the soldering!  Use the schematic to identify the
components associated with each path and check each of them one at a time -
then when you find it, you will know what caused the problem.

Measuring the collector to ground resistance of the output transistors will
produce different results depending on your meter and the probe polarity and
the time you apply the probes - some meters will charge the big electrolytic
capacitor in the collector circuit in the process.  If you have more than
500 ohms, you are good.

73,
Don W3FPR

> -----Original Message-----
>
> I recently completed my K2 and I am noticing that my Power output
> is very flaky and does not seem to track the Power control knob
> very well.  I have tried both the TUNE button and the Key input
> with a Hand Key and I am getting power fluctuations with both.
>
> I am suspecting something in the ALC circuit at this point -
> Anyone have any ideas on this - Since the installation of the two
> final transistors is kind of tricky - if they are not installed
> correctly - could this cause this behaviour?
>
> Also - if I measure from the Collector of the Finals to ground -
> what should I measure in Ohms?  I would like to compare mine to
> someone else's K2 to see if I might have a partial short in my finals.
>
> However - the way the power fluctuates - I am really suspecting
> something in the ALC circuit - Just wondering if anyone else has
> run into this issue...
>
> Thanks in advance!
>
> John - WB8UFA
>
> John J. Bahun
> Software Engineer - Apple Computer Inc.
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
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> Elecraft web page: http://www.elecraft.com
>
>
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>

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