Line In Not Responding

[Richard A. Smith]

Chris Barrett wrote:

> batteries I had lying around.  I see that there is a max voltage, I'm 
> worried I may have inadvertently fried the controller or some component 
> of the system that the mic and audio in system rely on. 

The line in is protected by a 5.1V zener and then there is a 1k resistor 
prior to the AD1888 Mic1 input.   So if you blew it up then you must 
have hit it pretty hard.

> I guess what I'm asking is what are my options?  I'd prefer to repair 
> this myself, I'm relatively comfortable around soldering irons and have 
> done some surface mount repairs and modifications before, but I'd like 
> to know what to look for?

1st in the signal path is L23 which is an ESD choke. The backside of 
that is where D13 (the 5.1V zener) connects to, then it goes to R25 
which is a 1k and then to C35 which is the AC coupling cap.  When you 
switch to DC coupling input ( ie measurement mode) C35 is shunted by a 
switch.  Then finally it goes to pin 21 of the AD1888 which is Mic1.

My suggestion is to connect up some sort of small signal AC input to the 
  Mic in and then look for that signal at each of the above components. 
  If you get to pin 21 then you must have blown up the input.  There is 
a 2nd unused Mic input on pin 22 which is routed out to TP 87.  You can 
try shorting the signal from C35 to TP 87 (or short pin 21 and 22) and 
then tweak your ALSA setup to use Mic2 rather than Mic1.

-- 
Richard Smith  <richard at laptop.org>
One Laptop Per Child