[Elecraft] K3 - Speaker/headphone problem

[Don Wilhelm]

And I might add that the first paragraph of that same application note 
explains the 'why' - I quote:
"This modification protects the K3's audio amplifier if either speaker 
output is connected to a short circuit or very
low impedance load. Two 470 ohm, 1/4 watt resistors are added to the 
output circuit to suppress potentially
damaging voltage spikes that might otherwise occur."

A mono jack *will* short out the right channel.

73,
Don W3FPR

Tom Hammond wrote:
> Older K3's were susceptible to damage if a MONO plug was inserted into
> the STEREO jack on the back panel.  Subsequent to that, there was both
> a hardware and a software mod to new(er) K3s to eliminate the threat of
> damage from that happening.
>
> UNfortunately, I cannot quote book, verse, and phrase of exactly when
> this 'factory' change occurred.  I believe it's spoken to in the K3
> Mods section of the Elecraft web site... QRX... lemme check... OK...
> here's the info from http://www.elecraft.com/K3/k3_app_notes.htm and down
> at "K3 AF Output Mod Kit. Rev C":
>
>    This modification is needed for all K3s shipped approximately
>    before 01/12/09 containing either a Revision "XB" or Revision
>    "A" KIO3 board. You must inspect the KIO3 board to see whether
>    the modification has been made. Modified boards will include
>    the leaded resistors. You can see one of the resistors by
>    removing the top cover and inspecting the side of the KIO3
>    board facing away from the rear panel. Photos of what the added
>    resistors look like are in the document.
>
> Hope this helps.
>
> 73,
>
> Tom Hammond   N0SS
>
>
> At 15:12 01/03/2010, Keith Hamilton wrote:
>   
>> Doesn't the manual state that a mono plug is ok? I better unplug my speaker.
>> I must have read wrong. Thanks a lot for the post!
>>
>> Keith W8GX
>>
>>
>> On Jan 3, 2010, at 3:57 PM, Dave Perry N4QS wrote:
>>
>>     
>>> Tony,
>>>
>>> Same thing happened to me when I plugged my Kenwood mono speaker into the
>>> stereo speaker jack on the back of the K3.  I did this without 
>>>       
>> thinking from
>>     
>>> the front of the radio when I was rearranging equipment in the shack.  The
>>> speaker worked for a couple of days until the audio output transistor
>>> failed.  I could then only hear audio from the headphones.  I had 
>>>       
>> to send my
>>     
>>> K3 back to Elecraft in order to replace the audio output 
>>>       
>> transistor.  It was
>>     
>>> under warranty so they repaired it for free.  
>