[Elecraft] K3 speaker amp chip bites the dust again

[Joe Subich, W4TV]

With SLP=0 and THR=20 you have essentially *no* AGC action.  That
means nothing is protecting the audio amplifier from strong inputs.

I leave it to Wayne or Lyle to calculate the overall gain from noise
floor to maximum DSP output but if one has set the audio for comfort
on a S4-S6 signal the audio amp is running along at roughly 1.5W
into the speaker.  When that S9+10 dB signal comes along and you have
no AGC even if the audio amplifier is expected to produce only 20 dB
more power, the 3.1W amplifier is going to die when asked to produce
more than 150 W of peak power.

73,

    ... Joe, W4TV


On 2014-07-01 3:28 PM, Dave Hachadorian wrote:
> I guess I've blown the speaker amp IC in my K3 again.  This is the third
> time this has happened, twice in one radio and once in another.
>
> In this case, the radio had both the AF Output Mod Kit (pair of 470 ohm
> resistors) and the E85056 Audio Protection Assembly Circuit Board
> installed.
>
> I was operating on Field Day, with headphones, as I usually do, when
> some onlookers wanted to hear what was going on.  I flipped
> SPKRS+PHONES=YES, then along came a strong signal, and poof! no speaker
> audio.
>
> AGC SLP was zero, and AGC THR was 020.  Front panel AGC was FAST.
>
> This failure is really getting old, especially at $120 a pop to get it
> fixed.
>
> Dave Hachadorian, K6LL
> Big Bear Lake, CA
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