New Zealand Vintage Computer Forums

Yesterday I received my new Macintosh 512Ke described by the seller as "in good working order". This afternoon I finally had a chance to power it up. It did turn on, the power on beep was duly issued, as was the "?" floppy screen. All seemed pretty good for a minute or so until, while I was digging around for a boot floppy, the screen went away and the magic smoke came out. A lot of magic smoke, and it kept coming well after turning off and unplugging the power. I guess I know what I'm doing tomorrow.

I wondered where that 512 Mac went on Trade Me. Now I know! I was flirting with the idea of going for it myself.

I've had a number of magic smoke episodes. The have never been as bad as they look. In all cases there were old rice-paper filter caps in the power supply. Easy to spot and fix. Hopefully this is what's happened to this fat Mac.

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 Sundy 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 of a 240V analog board it is a (probably dual) 0.47uF class X2 capacitor.

A very useful forum posting 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 describes 4.7nF caps.

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

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

Which caps do I need? Can anybody help?

Hmm..confusing indeed.

Your conversions seem right. Is the Mac Plus board the same as a 512k though? The diagram and the PDF are for a Mac plus. The PDF does speak of "Apart from minor variations...". Maybe this cap is one of those minor variations? However, that doesn't explain the discrepancy between the doc and the image though? Also the docs in the forum specify another value as you say.

Another thought. Would there be differences in cap values between US boards and those designed to work at 240V? This might explain the difference.

Heres a good repair guide http://68kmla.org/files/classicmac2.pdf - Sadly from my quick skim I didnt notice it mention what C37 is on an international analog board.

On the other hand, that might not matter as Larry Pina tells me (well his book "Macintosh Repair and Upgrade Secrets" does anyway) that C37 on an international analog board is a .47mfd 250V Cap - and looking at this international Mac analog board on my lap (C 1983-84) I can confirm that is what that cap is.

Radio Spares will have them - you might just have to buy 5

The analog boards on the 128/512/plus are all interchangeable - they beefed up some bits along the way, but essentially the are all the same thing.

gavo wrote:analog board is a .47mfd 250V Cap - and looking at this international Mac analog board on my lap (C 1983-84) I can confirm that is what that cap is.

Sorry, that is micro farads BTW - old school way of representing it I guess (uF, mfd, micro farad).

gavo wrote:Heres a good repair guide http://68kmla.org/files/classicmac2.pdf - Sadly from my quick skim I didnt notice it mention what C37 is on an international analog board.

On the other hand, that might not matter as Larry Pina tells me (well his book "Macintosh Repair and Upgrade Secrets" does anyway) that C37 on an international analog board is a .47mfd 250V Cap - and looking at this international Mac analog board on my lap (C 1983-84) I can confirm that is what that cap is.

Radio Spares will have them - you might just have to buy 5

The analog boards on the 128/512/plus are all interchangeable - they beefed up some bits along the way, but essentially the are all the same thing.

Brilliant - thanks for the tip. RS has the part in packs of 5 as you said.

lizardb0y wrote:

gavo wrote:Radio Spares will have them - you might just have to buy 5

Brilliant - thanks for the tip. RS has the part in packs of 5 as you said.

I placed the order for the replacement capacitors just after posting that last message, and they still haven't arrived! Maybe I need to carefully stash the 512Ke away and start on another project in the interim

Yes, I get frustrated waiting for stuff sometimes. I've got limited workspace. When you have a sick computer in parts all over it it's hard to do other things. The options are to either wait or pack it up carefully so you can get onto other projects in the meantime.

At the moment I'm waiting for ICs for 2 broken 400k Lisa floppy drives and they are taking a while.

I've decided I'll have to get a 128k or 512k Mac at some stage. It's a classic computer of historical significance and hence there is a gap there in my collection.

Bah. I sent RS an email asking what had happened to my parts, and they replied that they would not be available from the manufacturer until late June (they still show as "In Stock" on their site). I asked if they had equivalent parts they could supply quickly, and they sent me a link to the same part!

The RS part is a Rifa metallised paper cap, same as the original. There are metallised polymer caps available from JayCar, and Farnell (Element-14) seem to have some possible parts so there is hope yet.

Andrew,

I've used modern equivalents of those old caps from Jaycar to replace ones of mine that have blown with no trouble at all. If you find a metallised polypropelene cap that has the same spec, I'd put that in.

lizardb0y wrote:Bah. I sent RS an email asking what had happened to my parts, and they replied that they would not be available from the manufacturer until late June (they still show as "In Stock" on their site). I asked if they had equivalent parts they could supply quickly, and they sent me a link to the same part!

Thats pretty poor! I ordered some kit (caps) off them not so long back and had a really good experience - I guess because the stuff I ordered was in stock. I've just had a quick squiz in my stash but dont have that cap. Bugger.

gavo wrote:

lizardb0y wrote:Bah. I sent RS an email asking what had happened to my parts, and they replied that they would not be available from the manufacturer until late June (they still show as "In Stock" on their site). I asked if they had equivalent parts they could supply quickly, and they sent me a link to the same part!

Thats pretty poor! I ordered some kit (caps) off them not so long back and had a really good experience - I guess because the stuff I ordered was in stock. I've just had a quick squiz in my stash but dont have that cap. Bugger.

Yeah, my experience with RS has always been good in the past. They've oferred me my money back, so I'm ordering them from Farnell as soon as I can make up a $10 order! Does anyone else have any components they need?

Capacitors arrived this morning, so I'll probably try the fix tonight. Yahoo!

Let us know how it goes!