LC squeaking

[olePigeon]

From my experience it was the capacitors. My LC would squeal horrendously like someone tuning an old AM radio. I recapped the motherboard and all the noises went away. I've done this one 2 LC motherboards and both times it stopped the squeal.

It's relatively easy to recap, but there's one capacitor near the audio jack that's difficult to get to without a long, thin iron. I had to remove the audio jack to get to the capacitor, then solder back on an audio jack.

I could not get a solder wick to pick up the solder successfully, so I recommend using two soldering irons to melt the solder on both sides of the capacitor simultaneously until it slides off with very gentle pressure. This trick worked for me to avoid ripping off or breaking any solder pads. Then I bought a bunch of tantalum capacitors and soldered those on as replacements.

I also recommend solder paste. You put two little dabs of solder paste on the pads, just plop the capacitor into the goop, then hold your solder iron in the goop for a few seconds. The flux evaporates and the solder melts perfectly onto the pads and capacitor. It's great stuff.

Oh, make sure you buy some isopropyl alcohol and a microfiber cloth, then clean the board and pads before soldering. Get all that leaked electrolytic crap off the board.

[uniserver]

could you give some more info as to what caps you used, how much are they, where did you buy them.. and what are the values?

i have 2 LC-II / 1 LC (2) squeal like am radio, and the other has no sound at all.

thanks alot, if you need a Paypal donation for your time let me know!

[olePigeon]

All the values are printed on the caps themselves. If you slide the board out, you can see what you need and how many. The majority are 47uf 16v, but there are 3 or 4 different kinds. The voltage doesn't matter too much as long as it is equal to or more than what's printed on the old can capacitor. I buy tantalum capacitors. They're solid, little yellow (sometimes black or other colors) blocks with a stripe on them. They look like this.

I have an electronics surplus store near my house where I buy them, so I get them from there. However, many people are happy with Digikey, Mouser, and/or Jameco. Fellow member trag sells premade kits to recap your board. I think he's your best bet.

[James1095]

Having recently got back into playing with these things, I can say that if your 68k Mac is one that has surface mount electrolytic capacitors (the little silver cans) they *are* bad and replacing them all is the *only* thing that will reliably fix the machine. There's no point in even chasing down all the random symptoms they cause, just replace them all, wash the board off and go from there. Every LC pizzabox I've been in lately has bad caps in the power supply too, haven't come across a 68K Mac yet that replacing the capacitors didn't fix 100%.

The problem is two fold, first, they leak corrosive, conductive fluid out onto surrounding components and second, once the fluid leaks, the capacitors can no longer do their job. Replace with tantalum or ceramic chip capacitors and you will never have to deal with them again.

[uniserver]

i just fixed 2 LC-II's and 1- LC-I i just replaced the 3 small 16v 10uf Caps right near the sound chip and the sound is now clear and with normal volume.

i know i should replace the other caps too… i have a limited amount of 16v 47uf caps

so far i used all 4 i had, on my powermac 6100, with scratchy sounding audio,
does anyone know what cap specifically fixes scratchy sounding audio with the powermac 6100
and no it's not the speaker

[olePigeon]

I don't know, sorry.

[James1095]

I'd bet money the problem is capacitors. Either SMT electrolytics on the motherboard are bad, causing any number of malfunctions, and unless they've been replaced, they *are* bad. Or the output filter capacitors in the PSU are bad, causing switching noise to make it out of the power supply and eventually work its way to the speaker. Every LC series of this vintage that I've been inside recently has had bad capacitors. Replace *all* of the SMT 'lytics on the MB, and all of the output filter caps in the PSU. The PSU caps are clustered together on one end, most of them are probably leaking brown liquid by now. It's less than 15 bucks in parts to replace all of them and the SMT ones on the MB and it's the only way the machine will ever be reliable.