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Old 23 January 2016, 21:27   #1
mightynerd
 
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Question A600 only 2 audio channels

I knew the audio on my A600 wasn't working correctly for a few years now and I decided to try to fix it. Basically, it was very low and noisy. After gathering some information I replaced all capacitors and changed the power supply to an ATX one. This fixed the sound and also improved the composite video output a lot but there's still one problem remaining.

Only two of the four audio channels work. It doesn't matter if I have only one or two RCA cables connected I can only hear half of the instruments, the ones from the right audio channel (I compared the sound I heard with soundtrack I have on my computer). I tried listening with headphones and changed the pan on my amplifier to only hear the left side and there was completely no sound so it's not like the left channel is just a lot lower. Pushing the RCA connectors to any direction doesn't help. One thing worth mentioning is that one of the capacitors (C304), close to most of the audio circuitry, leaked. I cleaned the acid with some alcohol before replacing the capacitors around that area.

So my question is: what can I do in order to locate where the problem is?
 
Old 23 January 2016, 23:36   #2
8bitbubsy
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Take a good picture of the audio circuitry (one part is above the keyboard connector where the two silver caps are, and another is under the floppy drive to the right where the Paula chip is). Most likely the audio circuitry for the left stereo channel has problems, maybe an open trace or damaged component. Since you did a recap, I'd take a guess on capacitor leakage being the culprit here, damaging the traces.

Also worth checking out is the A600 schematics for the left/right audio channels.
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Old 24 January 2016, 00:56   #3
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That happened to me before and changing the capacitors fixed it. Maybe something went wrong when you changed yours?
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Old 24 January 2016, 02:39   #4
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Electrolytic fluid tends to either eat away the traces between the audio caps and the audio ports or to slip under the LM347 audio amplifier destroying traces and connections.
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Old 24 January 2016, 15:50   #5
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Here are the pictures: http://imgur.com/a/fwzkC

I know I've done a terrible job at soldering the caps and I probably should have let someone else do that for me. It look very ugly but I've checked the solder points with a multimeter for continuity and they're all ok so that shouldn't be the problem.

One thing I've noticed is that according to the schematics, pin 1 and 7 on the op-amp (U15) should be directly connected to the caps behind the audio connectors. I checked if they are with my multimeter and pin 1 (bottom left corner of U15 on the first picture) is not (and pin 7 is). So I guess that could be the problem but I can't really track the trace in between.
 
Old 24 January 2016, 19:32   #6
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The top pin on the left capacitor in picture #2/#3 doesn't look to be connected to the trace at all, hard to say on the pictures, but have you tried to measure the continuity with your multimeter on those too?
EDIT: These two caps are crossover caps in the last audio stage, so if they're not connected then you will get no channel audio. I suspect that this is the issue. If you only connect one audio RCA/phono at the time, it will leave an internal connector switch open and thus the sound is bridged in mono between both connectors, so this cap could still be the problem...

Last edited by 8bitbubsy; 24 January 2016 at 20:01.
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Old 24 January 2016, 19:50   #7
Arnie
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Wow

Have you destroyed the through holes for the right audio socket?
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Old 24 January 2016, 20:01   #8
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Looks OK to me at least... Except for some mystical floating solder blob? Btw Arnie, your signature is so true!
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Old 24 January 2016, 20:32   #9
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Yes, I've checked that those two caps behind the audio jacks are connected. Continuing on what I said earlier (that pin 1 on the op-amp isn't connected to C324 as shown on the schematics), i tried to connect a wire between pin 1 on the op-amp and the plus pole on C324 and it made everything work as it should so that's definitely causing the problem. Now, I don't know how to find where the trace is damaged. I can see a trace between pin 1 on the op-amp and R327 and it's connected but I don't see any trace going towards C324.

Arnie: I accidentally broke one of the pins on the right audio connector and temporarily replaced it with some copper wire. I'll have to replace that too.
 
Old 24 January 2016, 21:51   #10
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I have some spare if you need one.
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Old 24 January 2016, 21:58   #11
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You really need to remove the op-amp before more is destroyed.
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Old 25 January 2016, 17:58   #12
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Arnie: Thanks but I think I can just buy one at a local electronics store. These are standard RCA connectors, aren't they?

idrougge: You mean from the electrolytic fluid? Will that really help?
 
Old 25 January 2016, 19:09   #13
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Remove, clean, replace. It's a part that costs something like two postage stamps.
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Old 25 January 2016, 19:16   #14
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If it is working with your jumpers, I would leave it alone. It looks like you don't have the skill to repair it properly (not to insult- neither do I) and you may just break it worse.

Either leave it or send it to someone professional.
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Old 26 January 2016, 16:07   #15
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Don't leave corrosive fluids alone in your Amiga.
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