Line In problem ...

[Joe Subich, W4TV-3]

After installing the KRX3 in my original factory assembled
K3 (s/n 622), I find the "Line In" is unresponsive.  If I
run the Line In and Monitor levels at maximum, I can just
barely hear the input in the background.  

Line In was working normally prior to installing the KRX3.
Any idea where to start looking?  

73,

   ... Joe, W4TV
 

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[Joe Subich, W4TV-3]

The problem ends up being an open L4 on the KIO3 Audio I/O
board.  Obviously it was simply coincidence that the Line
input failed shortly after installing the KRX3.  

I've simply tacked a jumper around L4 at the present time
and will decide on a permanent fix later.  The more important
question is why L4 would fail at all?

73,

   ... Joe, W4TV
 

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of Joe
> Subich, W4TV
> Sent: Sunday, September 28, 2008 2:52 PM
> To: 'Elecraft List'
> Subject: [Elecraft] Line In problem ...
>
>
>
>
> After installing the KRX3 in my original factory assembled
> K3 (s/n 622), I find the "Line In" is unresponsive.  If I
> run the Line In and Monitor levels at maximum, I can just
> barely hear the input in the background.  
>
> Line In was working normally prior to installing the KRX3.
> Any idea where to start looking?  
>
> 73,
>
>    ... Joe, W4TV
>  

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[P.B. Christensen]

> I've simply tacked a jumper around L4 at the present time
> and will decide on a permanent fix later.  The more important
> question is why L4 would fail at all?

Joe,

Possibly a high DC offset from the external sound device or an accidentally
high AC audio level at the KIO3's LINE IN jack.  If for example a sound card
produced a high DC offset from it's LINE OUT jack that were present on the
K3's LINE IN connector, then I could see where L4 could possibly fail.  At
least as far as I can see in the K3 schematics, there's presently no
coupling cap as part of the KIO3's LINE IN circuit

Since R13 (T1's series input R) limits the LINE IN current, it would take a
substantial DC offset (or very high AC audio level) to cause L4 to fail.  If
DC was the root-cause, then one possibility to avert an occurrence in the
future is the placement of a 4.7uF coupling cap ahead of the KIO3's LINE IN
jack -- or in the alternative, between the KIO3 LINE IN jack and T1.  With
this cap value, the R13 series build-out R, together with the T1 primary
would present a -3dB turnover point of ~ 30 Hz when driven from an external
sound device with a low-Z output.

OTOH, perhaps L4 was on the ragged-edge of failing as a defective component
anyway.  Anyone know a way to autopsy an SMT inductor? <g>

Paul,  W9AC

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