Hey there everyone! New member here.
I'm an electrical engineering student with a lot of experience working on PCBs. SMD soldering isn't an issue, to put it plainly. I was recently given my first Mac! It's a Mac LC (and I only just realized how many different Mac LCs there are) and the person who dropped it off said it was working last year but the last time he tried to fire it up it was completely dead. He made it sound like he was pretty sure it was hardware-related. Well, the first rule of troubleshooting is "Thou shalt check power supply voltages!"
So I pulled the power supply and brought it to my bench. Fired it up and the three rails (+12, +5 and -5) were showing pretty bad voltages. If I remember, the 12 was around 2V, the +5 about 0.5 volts, and -5 was about -4, the only one even close to spec. I shut it off and pulled the board out and there was a whole bunch of grunge in one area, which I cleaned up with isopropyl. I then realized two caps were leaking, so I pulled those, cleaned up the top of the board where they were, and put one of the new ones in. Then I fired it up and the fuse blew! Stupidly, i left one of the caps off because I was trying to test for polarity, and it's quite possible this is what caused the fuse to blow. (It doesn't make sense to me but I suppose it's possible)
I double-checked to make sure I put the caps in the right way (even experience doesn't stop stupid mistakes). I'm %99 sure I did, as I'm sure it was the two positive rails that I replaced the final filter caps for. So the positive side of the cap is facing towards the output pins. Yeah I just double checked it, and the other final one is the other way round, with negative facing the output pins, so it's clearly the -5 rail. It's interesting, you can also tell which rail is which based on their current output vs the size of the final cap. The -5 is rated at only 0.075A (75mA) and it's got the smallest cap, whereas the 5V is rated at 3.25A and it's got the big 1000uF cap. I replaced both the 5 and 12V ones, the -5 one looks fine. I actually replaced them with very high-quality caps so they should last a long time.
Anyway, just wondering if people have experience with these power supplies, I can't seem to find a whole lot of info online, and a schematic or even block diagram might help. I'm going to pick up a few more fuses, but first I'm going to do some probing and see if something is shorted to ground. The corrosion did a bit of damage to the board, but I've tested continuity and all the traces are fine. It looks a little messed up but there isn't any functionality impairment. I think it's possible that the fuse blew because I had only one of the two caps in when I flicked it on (I was trying to double-check polarity).
I can only see one 7805 regulator, I'm not sure how they're producing the +12/-5 yet. If anyone knows if the regulator tends to go after a while I'd appreciate the info! I might just replace it anyway.
I will let everyone know how this goes and if it's successful I'd be more than happy to fix other LC power supplies for free. (I'm almost %100 sure I'm going to be successful) Just shoot me a PM or an email (if you're in the Ottawa area) and I'd be happy to help.
Thanks for any info!