Macintosh Portable audio issue

[BlaBla1985]

I'm having a weird issue with the audio in my portable, it often starts to "chirp" for lack of a better term. sometimes it's a continous chirping sound, sometimes it's random chirping and buzzing. And sometimes it's completly quiet unless it's supposed to make a sound.

The chirping will usualy mean the system is not running stable, so it will randomly crash if a sound should be played or a floppy it accessed.

I've tried to use the system without the harddrive, but the problem remains, no floppy drive, the problem remains. Disconnecting the speaker didn't help alwell (although the noise was gone offcourse). I was wondering if the mosfet that provides the 12V to the soundsystem can be taken out of circuit to test it without any audio stuff connected... but i don't know it the computer will like that. Will it then just burb out random errors or boot with no sound?

 

[MindWalker]

I just recently recapped my Portable. While testing it, I had some random audio issues, rarely loud, usually just faint noise/buzzing. A bit later I noticed that my charger was actually very sick (later on had no output at all) and needed recapping. Since that I don't think I've had those audio issues anymore (I have not used the machine that much, though). So I guess it could be a PSU related issue, do you have another PSU to test with?

 

[BlaBla1985]

I just recently recapped my Portable. While testing it, I had some random audio issues, rarely loud, usually just faint noise/buzzing. A bit later I noticed that my charger was actually very sick (later on had no output at all) and needed recapping. Since that I don't think I've had those audio issues anymore (I have not used the machine that much, though). So I guess it could be a PSU related issue, do you have another PSU to test with?

i don't use the original charger or the battery. i use a bench power supply set to 6.5V, 2.2A on the battery terminals. although that could mean that the output of that supply isn't as clean as i thought.

i have a battery that should work, and some aftermarket 7.5V PSU that came with this Portable and an original one that came with a dead model that i never got working. I could give that a shot.

how should i open that original PSU if i need to recap it? it's not screwed together i believe.

 

[BlaBla1985]

I charged the battery overnight on a 6V car battery charger and tried it out in the portable and that fixes the audio chirping and the instability. I don't think the original PSU is in good shape as the output goes up and down between 7V and 7,5V.

 

[MindWalker]

Nice! I think that the Portable is not supposed to be run without the main battery, it might cause strange issues.

If you look at my link (a write-up on Reddit) you can see that the PSU is glued and you need to crack it open. I used a vice and it worked reasonably easy and there was very little damage to the casing (which I glued back together).

 

[BlaBla1985]

I  think my bench supply lacked the power needed to run everything at once thus causing the instability. As the bench supply was connected to the battery contacts. But i'll use it like this from now on, it means i can no do some stuff with it.

i have a few large clamps and a heatgun, i'll see if i can crack it open and fix up the original PSU. Thanks for the tips.