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No sound from speaker on Mac 512K

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Hi all,

I've got a Mac 512K which is having sound issues with the speaker. Tried hooking up a speaker to the external audio jack, and while I do get audio, it's somewhat attenuated, and I have to crank up the volume on the speakers to hear it. Unplugging from the jack, I'm able to hear sound from the speaker, but it's very faint. (Unfortunately, I don't have a disk to generate a continuous test sound, so I'm just booting Dark Castle, and letting it play the intermittent audio from the splash screen.)


One other odd thing: Seems like I'm getting some sort of feedback from the disk on the audio signal, as changes in the drive motor speed, or head movement are generating short beeps out the audio.


Tried a MacPlus logic board with the existing power board, and that works fine, at least telling me the female side of J7 and the cable to the power board (and thus the speaker) are fine.


Also, I was able to probe PA0:2 (pins 2 thru 4) of the VIA and see them all asserted. Thus, the paths in the 4016 for the three volume-adjust resistors are all active, effectively setting the volume to its highest. (Verified this by tweaking the volume settings in Dark Castle, and saw the changes reflected in the VIA pins.)


Thought it might be the external audio jack, as I believe it has some mechanism to cut out the internal speaker when an external is hooked up. I do, however, get continuity when checking between R7 and R1, with no external speaker. So, seems like that's not an issue.


Any thoughts? Without another Mac 512K logic board to probe how things should be for a working board, I'm somewhat stuck.

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OK, it gets interesting. I had previously noticed a bit of corrosion (silver migration, perhaps?) between R18 and CR3. post-8385-0-62989600-1486534616_thumb.jpg


I had cleaned it up pretty well, and replaced R18, just in case.


Started thinking about that part again, and resoldered the diode this time. Low and behold, I get a nice, loud "beep!" from the main speaker during boot-up. It eventually reverts back to the much attenuated audio, but I'll replace the diode and hopefully that will permanently resolve the problem. Fingers crossed...

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