My LC III wants to be a DJ.

[Lateralus]

When powered on, my LC III intermittently emits whining noises in varying tones through its speaker. I've swapped the speaker to no avail.

The kicker is that if I touch miscellaneous chips on the motherboard with varying degrees of pressure, I can alter the pitch and repetition rate of the whine.

Why is my LC III a beat machine?

 

[porter]

Why is my LC III a beat machine?

Beats me!

My LC II and III both make whining noises which I assume are coming from the PSU which subside after a few minutes, I assume that they are merely going supersonic rather than actually abating.

 

[Lateralus]

You might wanna check. I assumed mine was the PSU as well, but it's definitely the speaker.

Once on, it goes quit every now and then only to start the whine again a few seconds later. Definitely not something that being powered on for a while cures.

 

[porter]

Could be noise on the power rails then. Do you have a similar machine you can swap components with to eliminate them from your inquiry? Eg swap PSU etc.

 

[Lateralus]

Nope. I run a tight ship nowadays.

 

[Dennis Nedry]

Most likely it's bad capacitors on the logic board.

I have one that does this too from the speaker. It sounds like an old AM radio.

 

[Lateralus]

The board looks perfect, top and bottom. So I don't know.

Maybe my IIci spread a disease to it while they were in the closet together this past year...

Regardless, I'm more willing to screw with the LC III to keep it viable than I am the IIci. The LC III, majestically sitting next to a garbage can as I passed by on the walk home from school one day, won my heart and brought me over from the dark side.

 

[FakeWozniak]

I just got an LCIII from ebay and it has that whine/hiss/am radio noise too.

From my hardware background, I would think it was a failed capacitor too. I've noticed that while it oscillates audibly, the actual sounds going over the speaker (Apple Menu -> Control Panel -> Sounds, then clicking an alert sound like Quack) will be garbled. I would suspect the problem is isolated to the audio driver. Maybe something like a 555 Op Amp or a supporting capacitor.

Anyone isolate this further? If so, please post! I will post what I find.

 

[superpantoufle]

Don't we have twice the same problem here?

From what I understand, one could merge those two threads. Two of my pizza boxes (a LC and a LCIII) alsa wanted to become DJ's lately. It seemed to be some RF interference (from the AC board?) in the speaker. Does the noise stop when you plug in headphones?

 

[indibil]

I know it's been a long time since this post, but I have an LC3 with the same noises. I have changed capacitors of the supply and of the motherboard, and they are still there.

Has anyone figured out what causes these noises?

Thank you.

 

[ymk]

I also had oscillation after recapping.

Changing C4 to 47uF solved it for me.

 

[mikes-macs]

It’s a mystery to me. I have never heard of this problem before now. It was usually the Mac IIsi that had all of the sound problems. I’ve got a Performa 400 with no sound issues at all. Perhaps you got some device in your home that causes interference.

 

[Daniël]

I'd guess the DFAC (digitally filtered audio chip) would be at fault. They tend to die from cap leakage, and can do weird things when damaged/dying like noisy sound, intermittent sound, extremely weak signals, etc.

 

[bibilit]

Most likely it's bad capacitors on the logic board.

I agree a 100%, this screeching sound is generated by bad caps.

 

[indibil]

I also had oscillation after recapping.

Changing C4 to 47uF solved it for me.

Thank you so much! I have replaced the new 10uF C4 with another 47uF one and the noise is gone!!!!

It can't be the fault of anything in my house because I have another LCIII that doesn't have that problem, with the factory capacitors.

 

[ymk]

Thank you so much! I have replaced the new 10uF C4 with another 47uF one and the noise is gone!!!!

Glad it worked for you

Another thing LCIII owners should be aware of: C22 polarity is reversed on the silkscreen.

 

[mikes-macs]

I was hoping your Mac could DJ a retirement party next weekend? How can I re-create this on mine?

 

[indibil]

Glad it worked for you

Another thing LCIII owners should be aware of: C22 polarity is reversed on the silkscreen.

Yes, i have put C22 reversed, i know this