[Elecraft] K3 speaker audio gone!

[hawley, charles j jr]

Was there a serial # that included the current limiting?

Sent from my iPhone

On Aug 28, 2012, at 1:18 PM, "Wayne Burdick" <n6kr at elecraft.com> wrote:

> Dave,
> 
> Do you have the AF amp current-limiting mod in place? The parts for  
> this mod are installed on the RF board. They've been in production for  
> quite some time, and we can send them to you on request.
> 
> With the limiter installed, we've seen near-zero problems with the AF  
> amp IC.
> 
> 73,
> Wayne
> N6KR
> 
> 
> 
> On Aug 28, 2012, at 10:25 AM, Dave Hachadorian wrote:
> 
>> I have blown the speaker amp twice, in two different radios, by
>> using a speaker with AGC OFF.  POOF! Gone before you can react by
>> turning down the audio.
>> 
>> The factory guys report that they have been unable to duplicate
>> the problem.  Unless you are an EXPERT solderer, replacing the
>> speaker amp chip is best done by the factory.  The job will run
>> about $125 with shipping, maybe a little less if you ship only
>> the DSP Board.  I don't use the speaker amp any more.  I still
>> think there is a transient susceptibility problem in that
>> circuitry that they have been unable to find.
>> 
>> 
>> Dave Hachadorian, K6LL
>> Big Bear Lake, California
>> 
>> 
>> 
>> 
>> 
>> 
>> -----Original Message-----
>> From: Mike Goldstein
>> Sent: Tuesday, August 28, 2012 9:07 AM
>> To: elecraft at mailman.qth.net
>> Subject: [Elecraft] K3 speaker audio gone!
>> 
>>        I've lost audio from both the internal speaker in the
>> K3,
>> and from the SPKRS output jack on the rear panel.
>> 
>>        At the end of the the first week of using a new NESCAF
>> audio
>> filter (which drives my speaker) with the K3,  I changed bands,
>> listened briefly on CW, with the NESCAF filter in the circuit.  I
>> then changed to SSB, and because the NESCAF filter was adjusted
>> for a
>> very narrow audio bandwidth, I decided to switch it out of the
>> circuit, while the K3 was delivering audio, at a comfortable
>> listening level..
>> 
>>        The audio filter was driven from the SPKRS jack on the
>> back
>> of the K3, using a stereo plug, with one channel floating.  The
>> SPKRS
>> jack was CONFIGured as SPKRS = 1.
>> 
>> A front panel toggle switch on the filter panel turns off the
>> power
>> to the NESCAF filter, and re-routes the speaker audio connection
>> from
>> the filter output, to the K3 output directly. I had done this
>> often,
>> for the first week of operations.
>> 
>> This time, however, I briefly saw a "HI CURR" indication on the
>> front
>> panel of the K3, and then my speaker audio vanished.
>> 
>> Unplugging all connections from the rear panel SPKRS jack, and
>> the
>> front panel PHONES jack, no longer allows the K3's internal
>> speaker
>> to produce audio.  Nothing I tried would result in any audio from
>> the
>> external speaker, after connecting that speaker to either of the
>> phono jacks that were tied to the rear SPKRS jack, through the
>> stereo plug.
>> 
>> The INPUT jack on the SCAF filter (to which the K3's SPKRS jack
>> was
>> connected) passes through an isolating capacitor, then a 27K
>> resistor, before being connected to the chip in the audio filter.
>> 
>> Resetting SPRKS = 2, in the  CONFIG menu, had no effect on
>> producing
>> audio in the external, or internal speaker.
>> 
>> I still have audio  at the PHONES jack on the front panel, so I
>> can
>> still use the K3.  It subsequently accounted for 380 QSOs in the
>> NAQP
>> CW contest, providing it usual flawless performance.
>> 
>> I still have audio at the PHONES jack on the rear panel, as well.
>> 
>> My theory (and Gary at Elecraft Service agrees it might be the
>> cause)
>> is that the audio chip in the K3 didn't like the abrupt change of
>> load impedance, from 27K (approx.) to infinity, to 8 ohms, while
>> it
>> was delivering audio..
>> 
>> I've since soldered a 15K resistor across the input jack of the
>> audio
>> filter, and will do likewise for any other audio filters I
>> encounter, and use.
>> 
>> Puzzling, huh?  73 de  VE3GFN
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