[Elecraft] [K2] K2: KPA100 transmitted audio distortion 80 meters

Wed Nov 11 11:19:56 EST 2009

Even though I have little helpful to report, I felt I should post the
outcome.  In short, my KPA100 had a spontaneous and unexplained recovery
from its 80 meter distortion.  The audio as monitored by another receiver,
while transmitting into a dummy load, is now absolutely without audible
distortion, whether the amp is on or not.    I did confirm, through swapping
my KPA100 while it was distorting, with one owned by a friend, that the
KPA100 was the problem, not the base K2.  However, once it healed itself, 
nothing I did would make it distort again.  My attempts included tapping
components, cooling components, placing the amp in and out of the case,
rearranging the ribbon and speaker cables, changing the supply voltage, and
even changing the type of power supply.  Previously, none of these same
actions would stop the distortion.  

Thanks Don, and others who sent offline suggestions.  

Dick, K0KK

Don Wilhelm-4 wrote:
> 
> Dick,
> 
> Before doing anything drastic like swapping out K2 or parts of them, 
> might I suggest that you may have a problem with RF-in-the-shack on 80 
> meters.  Since you do not have the problem on other bands, that is high 
> on the suspect list.
> Just for a test, connect a 66 foot wire to the back of the K2 and run 
> the other end out somewhere (preferably outside).  The far end should be 
> insulated for it will likely have high voltage RF on it (treat it like 
> the end of an antenna).
> If that wire fixes the problem, you for certain have an RF Feedback 
> problem there - check your antennas - the proper place to fix RF in the 
> shack is out at the antenna - use choke baluns at the antenna feedpoint, 
> and possibly another in-line choke at the point where the feedline 
> enters the shack.
> 
> If it has not already been done, ground the shell of the mic jack - you 
> can do that by removing the left side panel, scrape the green solder 
> mask away from a spot right next to the base of the mic jack and pool 
> some solder between the copper ground and the body of the jack - it 
> takes a lot of heat (use a large iron) to transfer enough heat to the 
> mic jack so solder will flow on it, but with enough heat or enough time, 
> the task can be accomplished.
> 
> 73,
> Don W3FPR
> 
> radiotelegrapher wrote:
>> Don,
>>
>> It's good to see you're still carrying out your much appreciated good
>> work
>> here.  I'd been away from this forum for a a couple years.
>>
>> I changed  R4 back to 100K.  Dressing the ribbon and speaker cables
>> doesn't
>> help.  Right now the KPA100 is outside the K2, on it's back, and cables
>> are
>> stretched tight and clear of the KSB2 board.
>>
>> I haven't done the KI6WX mod.
>>
>> For sure it's only 80 meters that has the problem.  On other bands, it's
>> almost indistinguishable whether the KPA100 is on or off, and audio is
>> excellent.  But on 80, both the cracking and general distortion start
>> when I
>> cross 10 to 11 Watts.  CW is fine, although there is about a second of
>> warble in the tone the first time I key after changing the band to 80). 
>> USB
>> behaves the same as LSB.
>>
>> For a next diagnostic step, I'm thinking I could borrow my buddy's K2,
>> try
>> his KPA100 in my K2, and mine in his, and likewise for the KSB2 boards. 
>> Seem reasonable?   
>>
>> Dick, K0KK
>>
>>
>> Don Wilhelm-4 wrote:
>>   
>>> Dick,
>>>
>>> It puzzles me that it only appears on 80 meters.  Do change R4 back to 
>>> 100k - the higher voltage is not required with the updated KPA100 (blue 
>>> toroids), and is undesirable.
>>>
>>> Do you have the KPA100 ribbon cable and speaker cable tucked up into the 
>>> corner of the KPA100 and dressed as far away from the KSB2 board as 
>>> possible? (remove the right side panel to check and tuck-in if
>>> necessary).
>>>
>>> Do you have the KI6WX increased gain mod on your KSB2.  If so, have you 
>>> made the corresponding changes to the ALC section of the KSB2?
>>>
>>> 73,
>>> Don W3FPR
>>>
>>>
>>> radiotelegrapher wrote:
>>>     
>>>> I have moderate distortion of transmitted SSB audio from my KPA100, but
>>>> ONLY
>>>> on 80 meters.  The distortion seems worsen and improve from time to
>>>> time,
>>>> with no apparent reason, and tapping components doesn't give a clue. 
>>>> Listening on a nearby receiver, and transmitting into a dummy load on
>>>> 80
>>>> meters, the audio consistently sounds fine with the KPA100 off (at 10 W
>>>> and
>>>> less), but is distorted with KPA100 on (at 11W and higher).  The
>>>> distortion
>>>> is particularly noticeable at the start of a new syllable, which sound
>>>> like
>>>> a loud crack.  A constant amplitude, like a tuning fork by the
>>>> microphone,
>>>> sounds fine, so I suspect the TR switch or ALC.  My build is Rev 3,
>>>> Jan.
>>>> 18,
>>>> 2007.  Searching the archives, I've found the usual causes, but the
>>>> fixes
>>>> are already included on my build:  C31 is .22 mF, and R33 (1K)
>>>> parallels
>>>> RC3. I tried changing R4 from 100K to about 39K, but it didn't help. 
>>>> Any
>>>> ideas?  Dick, K0KK
>>>>   
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>>>>
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>>>>       
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