unidentified smoky smell from Mac 512K

[bigmessowires]

OK, I bought *another* compact Mac today, a Mac 512Ke. It booted up fine from a 6.0.8 floppy, and everything worked fine. I left it on while I went to another room for about 10 minutes, and when I returned there was a strong smoky/melted badness smell in the room. The 512Ke was still running normally, but I switched it off immediately.

Later, I opened up the Mac and took it apart, expecting to find scorch marks or fried electronic goo somewhere. To my surprise, everything looked fine. My nose says the residual smell is strongest around the analog board, but I can't be sure. Nothing on the analog board looks obviously leaky or burnt or otherwise broken. The logic board looks perfect.

What else should I look for that I might have neglected? Am I risking high-voltage death if I run the Mac with the case off, and try to localize the smell further? Is it possible this smell is somehow normal for a Mac that's sat in storage for 15 years, and will dissipate if I let it run for a while? Seems unlikely...

 

[Bunsen]

It's possible it was just dust on hot components. I'd suggest giving it a thorough vacuum and/or compressed air blowout and seeing if the smell returns.

 

[mcdermd]

Or it is the big, blue capacitor up on the top front of the analog board (C1). Nearly all of my 128k/512k/Plus analog boards have had this cap go bad. When it does go bad, it has a burnt plastic smell. JDW here put together a great analog board component replacement spreadsheet:

http://spreadsheets.google.com/ccc?key=0AnmQLg8d1xThcE96S1Z0S3oxOUFiMkJEYy1nNzNuZEE&hl=en

 

[bigmessowires]

Thanks, I'll double check C1. If that capacitor does go bad (or is in the process of going bad), but the Mac is still running normally, are there symptoms other than smell?

I'm starting to think that my only option is to turn it back on, and wait until either:

1. some component fails catastrophically (smoke, burning, venting) so I can identify it, or

2. the smell goes away (not so likely, but I can hope!)

 

[superpantoufle]

Am I risking high-voltage death if I run the Mac with the case off, and try to localize the smell further?

Absolutely not. That is, not until you stick your fingers in some wrong place!

 

[bigmessowires]

OK, believe it or not, the smoky smell just went away by itself!

I took "Smelly" outside, took off the case, turned it on, and let it sit for 20 minutes. There was no bad smell, other than a faint unpleasant smell if I put my nose right up to the Mac. I also tried swapping in a different logic board, to see if the smell was coming from Smelly's analog board or logic board. But since everything worked fine and I never smelled more smoke, the test was inconclusive.

The only suspicious capacitor I noticed on the analog board was C38, right behind the power plug. It's got some ancient-looking package that I've never seen before, and it looks cracked.

I then took Smelly back inside the house, put it all back together, and it's been running for about 90 minutes so far. As long as it continues to run and doesn't smell like it's about to burn down the house, I'm not going to attempt any pre-emptive capacitor replacements.

By the way, what do people recommend for cleaning corrosion and battery acid off terminals in the battery compartment? A wire brush, or some kind of solvent maybe?

 

[Mac128]

Have you adjusted the voltages? If not, you may have a problem waiting to happen. It's entirely possible some component was stressed after being turned on for the first time and was pushed to the breaking point. This or other parts may fail in short order, but if your voltages are out of whack you have yourself a meltdown ready to happen.

Also, I can't think of a single instance in which the logic board emits a bad smell. Not enough voltage runs through it to make melty/smokey things happen.

 

[bigmessowires]

Smelly's looking pretty good: the voltages are 4.95V, 12.18V, and -12.39V.

 

[MidnightCommando]

Hey there!

I've been following PlusToo with some interest, by the way.

C38 is a metallised paper capacitor, I believe - you used to see that sort of cap in older telephones (rotary dialing types) all the time. You should be able to replace it with a metallised polyester cap of suitable voltage and value. That said, it's not necessary to replace it right now. I would anyway.

For cleaning corrosion off the battery terminals, I personally recommend a cotton swab coated in pure sulfuric acid, though I doubt many of my fellow soldiers will agree with me there. If you do use that idea, make sure to rinse them off with distilled water thoroughly after, and I suggest a very thin coat of solder over the previously-corroded area to make sure there's enough metal to contact the batteries.

If you're a little tamer about cleaning, some fine (160 grit) emery cloth should do the job.

(p.s. desolder the battery holder from the analogue board /before/ application of further corrosive chemicals, for the love of god)

 

[bigmessowires]

I've been following PlusToo with some interest, by the way.

Thanks! I should probably get back to working on it, instead of driving all over Silicon Valley collecting every vintage compact Mac that's for sale.

C38 is a metallised paper capacitor, I believe - you used to see that sort of cap in older telephones (rotary dialing types) all the time. You should be able to replace it with a metallised polyester cap of suitable voltage and value.

Metallized paper cap sounds right. And coincidentally, I just installed a rotary dial phone in my work room yesterday! Bell 500, still works great.