KPA-100 Problems

[David Walker-8]

Hi Guys,

I am having problems with the KPA-100.  80m reports of distorted audio, like a vibrating effect associated with it.  And even when I have the PO adjusted to a max of 15 watts it seems to transmit up to 60w out.  I know it's not RF feedback caused by antenna because I have the same problem with a dummy load.

Any suggestions would be appreciated.

Dave
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[Don Wilhelm-3]

Dave,

Is this a new KPA100? or have you had it for a couple years?  Some (but very
few) KPA100s had a similar situation a while back.

If it is recent, I have no immediate answers, but if it is older, put the
change on it for the increased TR switch voltage (change R4 to 39k) and add
a 1k resistor in parallel with RFC3.
With these changes, check the RF envelope in CW mode for ripple
(modulation).  The CW RF envelope should be nice and smooth at the top and
bottom.  Then re-check the audio in SSB mode.

If you have a recent KPA100 with these resistor changes already on it, I
would be suspicious of RFC15 on the bottom of the RF Board - check with an
ohmmeter to be certain it is not open.  Why RFC15 would show a problem only
on 80 meters is beyond me, but it is worth a check.

I don't know if that will cure the power out situation (I doubt it), but
that may a different problem involving the power control circuits in the
base K2 and the wattmeter circuit in the KKPA100.

73,
Don W3FPR

> -----Original Message-----
>
> I am having problems with the KPA-100.  80m reports of distorted
> audio, like a vibrating effect associated with it.  And even when
> I have the PO adjusted to a max of 15 watts it seems to transmit
> up to 60w out.  I know it's not RF feedback caused by antenna
> because I have the same problem with a dummy load.
>
> Any suggestions would be appreciated.
>
> Dave
>

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[Steve Kavanagh]

A few recent units have shown problems that at least
sound similar to this one. See, for example:

http://www.ac6rm.net/mailarchive/html/elecraft-list/2005-03/msg00676.html

http://www.ac6rm.net/mailarchive/html/elecraft-list/2005-12/msg00408.html

These two, at least, were due to the KSB2 ALC circuit
having insufficient attenuation range to control the
output of K2/100's with above-average overall transmit
gain on the lower bands.  The fix involves adding a
resistor and changing another in the KSB2.  Gary
Surrency at Elecraft was able to provide me the
necessary details.

73,
Steve VE3SMA



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