Mac LC Whistling/Screeching

[CompuNurd]

I just finished recapping a Mac LC, so I fired it up only to hear a screeching/whistling sound from the speaker. It's not a solid sound, it randomly fades pitch up and down. Other than this, the LC works fine and there aren't any artifacts in the video as far as I can tell. I think it did this before the recap.

 

[Unknown_K]

MY LC3 whistled before it died, recapping fixed it for me. I would check you got the polarity correct on the capacitors. Also did you use aluminum analytics for the recap or some other kind of capacitor (they all have different characteristics in audio circuits that might lead to noise)?

 

[Elfen]

I did a recap on am LC III not too long ago and the SCSI died. Turned out to be too much solder on the pad on the cap near the SCSI Port and it was shorting out with another pad that was under it. Desolderig and resoldering the cap with much less solder fixed it.

I'm sure that you have the same problem in or around the audio circuit, though there are several caps there. Unsolder each one, inspect the pads and put them back in with a tiny amount of solder. It should fix it up.

 

[CompuNurd]

Which caps are the audio caps?

 

[Elfen]

The area around the audio jacks:

C2, C4, C5, C7, C13 and C8

 

[Elfen]

Sorry, I looked at the wrong board (an LC III) when I posted those numbers. But it is the caps in the back by the left hand side by the audio jacks.

 

[olePigeon]

Squealing is from bad caps.  Just replace them all, should be right as rain.

 

[uniserver]

i would wash the LC in hot soapy water,  then scrub all around that big  group of 10uf caps there…  and try to scrub under the I/C;s around there too, with the bristles of the tooth brush,  i bet you have some cap goo some where.

then rinse again with hot water… and compressor dry off.

i'v had some LC boards do that after a re-cap, that is half of the reason i pretty much wash/scrub all boards after i remove the caps and clean the pads.

 

[CompuNurd]

Tap or distilled? I have washed the board three times now, but there seems to still be buildup of some type of goop around the ICs. Not sure if it is cap goop or the left over minerals from the tap water. I am just using regular dish soap, I am yet to buy the ammonia I need for cleaning them.

 

[uniserver]

it doesn't matter what you wash the board with,   as long as it gets gets the electrolyte off.

Also the next process, The one that matters the most, is Drying the Board with a compressor.

- Blasting all the residual water off the board and from underneath the I/C's right after the wash.

Then the occurrence of mineral build up is a non issue.

The other kinds of goo that can still be on there like Rosen flux, etc..  that needs to be removed with acetone.

nothing else will cut it.

If you do all that and still get some squealing coming from the speaker, maybe start checking the traces and VIA's for resistance

maybe you have a half eaten up trace or Via causing a varying resistance changing with temperature.

If you take a hair dryer and heat up that sound area of the PCB, with the computer on, you will notice a change in pitch of the squeal.