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View Full Version : Mac 512Ke analog board suppression caps failure



lizardb0y
February 15th, 2011, 12:13 AM
I recently bought a boxed "working" Macintosh 512Ke, 240V model. It was working when I first plugged it in, with a slight "wave" to the picture. After a minute or so the magic smoke leaked out and the screen went blank.

I finally managed to get the case apart. There are two screws recessed under the carry handle which are tough to get at, and I couldn't reach properly with the tools I had. After spending a good part of Sunday looking for a Torx T15 screwdriver with a long shank I gave up, cracked torque on the screws with a Trox "L" tool that had only about 20 degrees movement inside the handle cavity, and removed them using a long 3mm blade screwdriver. I fear this trick won't really work to do them up again.

All of the standard caps on the analog board look OK to the eye, but there is a large rectangular capacitor (C37?) right next to the fuse and toroidal transformer which has burst, and the package is completely unreadable. According to this picture (http://www.flickr.com/photos/damianward/3933693662/) of a 240V analog board it is a (probably dual package) 0.47uF class X2 capacitor.

A very useful forum posting (http://myoldmac.net/cgi-data/forum/phpBB2/viewtopic.php?p=730&sid=6eb3c6a394cbf743482a48ebd07558cd) suggests that the board has two 470pF 250V Class Y caps, C33 and (C36 or C37), which seem to fit this part, and this rather handy PDF (http://home.earthlink.net/~gamba2/images/plus_analog.PDF) describes 4.7nF caps.

So, I'm confused. If I normalise these all to pF:

470pF = 470pF
0.47uF = 470000pF
4.7nF = 4700pF

My questions are:

Which caps do I need?
I can't find Class Y caps anywhere - will Class X2 caps be OK? (I assume so seeing as that's what's fitted)
Where can I get them, preferably in New Zealand or Australasia?

Thanks,
--
Andrew

lizardb0y
February 16th, 2011, 09:06 AM
I have had it confirmed by a local collector that C37 on the International analog board is a 0.47uF X2 capacitor, and have duly ordered 5 (being the minimum order).

lizardb0y
March 14th, 2011, 01:02 AM
It's turned into a bit of a drama. After waiting a couple of weeks I called the vendor who finally informed me that they were out of stock and new stock wasn't expected until July. I ordered the caps from another shop, and they arrived quickly. Yesterday and installed the replacement cap, and the fun began. I'm writing it up here: http://www.vintage8bit.com/content/macintosh-512ke-adventures

RetroHacker_
March 14th, 2011, 05:38 AM
Ah, the fun of those metalized capacitors. While you're in there, change all of 'em.

I've had several of those fail. Sometimes they don't even cause the machine to fail. Most recently, the X2 cap in a Bell + Howell Apple II+ went out. It's always fun to see smoke billowing out of the side of the computer, and the machine still running...

-Ian

cyrano
March 14th, 2011, 08:10 AM
Also, check for hotspots. There's a 4 pin power supply? connector on the logic board that is to close to a cap that gets very hot. The connector itself burns in and the isolation even melts after years. Even the solder point cracks up because of the heat.

I must have done hundreds of these back in the days. Even the Mac+ still had that problem. Since it only showed up after years, it didn't seem to bother Apple. And standard repair was a logic board swap. ouch...

Chuck(G)
March 14th, 2011, 08:45 AM
Polyester caps are a good reliable replacement for those polystyrenes. Good replacements if you don't care about identical looks.

lizardb0y
March 14th, 2011, 09:43 AM
Also, check for hotspots. There's a 4 pin power supply? connector on the logic board that is to close to a cap that gets very hot. The connector itself burns in and the isolation even melts after years. Even the solder point cracks up because of the heat.

Thanks. I'll look for that. Any advice if I do find it? Other than replace the analog board that is :)

RetroHacker_
March 14th, 2011, 09:51 AM
The four pin connector of which he speaks is the yoke connector. It frequently cracks it's solder joints and usually takes out the horizontal deflection (so called "vertical line disease"). There is also one electrolytic, I forget which one, that always went on those. But, for now, just resolder the yoke connector (and the other connector on the board) while you're in there.

-Ian

cyrano
March 14th, 2011, 09:53 AM
Thanks for clearing that up. My memory isn't what it used to be ;-)

lizardb0y
March 22nd, 2011, 03:51 AM
I received the replacement parts and soldered them in. At the same time I checked the yoke connectors and looked for any signs of overheating on the electrolytic caps. Everything looked good, so I powered up the Mac once more, half expecting another pop, fizz and Magic Smoke Experience.

Instead all I got was a nice "Dong" sound and a good, stable "?" floppy icon. Unfortunately I won't be able to spend any more time on it for a few days, but I have just gone about aquiring a Powerbook 100 with the external SuperDrive, manuals and system disks. It sounds like it works (described as "screen lit up and there was a mouse pointer"), though it may not boot - possibly a HDD failure. For approx $3 USD this will hopefully be enough for me to make my own 800K System floppys :)