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Old 05 February 2016, 18:05   #1
alenppc
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Join Date: Apr 2012
Location: Canada
Age: 41
Posts: 843
Another A600 audio issue...

I have an A600, rev 1.5 motherboard that has recently developed an audio issue. The audio sounds "broken" (scratchy, not all of the frequencies sound quite right, some sound "overamplified" and some underamplified).

The first thing I did was to replace the 22uF audio caps behind the keyboard connector, the only ones that had not yet been replaced, but the problem has persisted.

Next, I checked the connectivity of the entire audio circuit section ("Paula" and "Audio power" section in the PDF schematics), everything checks out fine.

I tried replacing the Op-amp but no difference.

I measured the output at Paula's pin 34 with a multimeter while playing a mod and it varies between 2.4 to 2.9V, the same reading occurs at the + of the left audio cap. I don't have an oscilloscope.

Next, I compared the readings to a rev 1.3 motherboard that has no audio issues (I don't have another 1.5), and noticed that Paula on that one produces much lower voltage when playing the same mod - between 1.8 to 2.4V, and it seems much more constant at pin 34 than at + of the left audio cap, so clearly something is amiss somewhere.

Also, I noticed that some resistors on the rev 1.3 motherboard have different values than those on the 1.5.

Now, short of desoldering every single resistor and capacitor to test them individually, is there any other way to troubleshoot this problem?

Last edited by alenppc; 05 February 2016 at 20:40.
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