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Old 05 February 2016, 18:05   #1
alenppc
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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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Old 06 February 2016, 00:01   #2
Akira
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You're really focused on blaming the computer.
My first doubt here would be the power supply. Try a different one.
Any failure on the -12V line will affect audio.
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Old 06 February 2016, 00:11   #3
Khyron
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Quote:
Originally Posted by alenppc View Post
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).

Now, short of desoldering every single resistor and capacitor to test them individually, is there any other way to troubleshoot this problem?
I've got 2 Amiga's that both do this.. not sure if they were always like that, or it eventually happened.. haven't got around to working on them yet...

Start with this.. Amiga is ON and no sound playing..

on the L347/TL084
Power Supplies:
Check Pin4 and Pin 11 for +12 and -12V (Main Power)
Check Pin10 and Pin12 for between 2-3V (If bad check AVREF circuit)

OP-AMP: (2nd & 1st Stage)
Check Pin 1 and Pin 7 should be close to 2.5V (2nd stage center output)
Check Pin 8 and Pin 14 should also be close to 2.5V (1st stage center output)

depending what you find... from there I would suspect
C331/C321 and maybe C332/C322..
or the negative feedback R331/R321 on the Rev1.5 are just too high a value of resistor..

Let me know what you find..
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Old 08 February 2016, 09:53   #4
Daedalus
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Indeed, typical symptoms of a bad -12V line are distorted audio with a harsh, clipped sound, and bad serial. The rest of the computer still runs fine without -12V so you might never know anything's wrongs.
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Old 14 February 2016, 19:37   #5
alenppc
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Thanks Khyron, that was very helpful... Looks like I fixed it.

I was getting +0.34V on pin 11 of LF347. Turns out R302 was defective. Replaced it and immediately upon power up the new one burned out as well... Hmm...

C301 and C302 were giving me strange ESR values, so I replaced them. Replaced R302 again and now I have -11.10V at pin 11 and the audio sounds normal again.
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Old 02 December 2019, 00:39   #6
nikoskob
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Sorry to revive an old thread, but the LF347 on my board, had actually blown, between pins 10 and 11. I'm thinking due to a leaking C304.

Just replaced it now and have sound, but it seems a little distorted, so ill have to look at what Khyron suggested and look at those resistors too, as I have already found that R322 looks bad too.

Board has been fully recapped by the way, so that's one major drama out the way

I'll report back once I get the resistors checked and replaced.
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