Macintosh Plus Bang and Smoke

[mikemike690]

I just brought a Macintosh Plus, took it apart to see the corrosion, with no damage luckily, and I decided to take the risk to start it up, anyway plugged it in and booted a disk and everything was fine, then after a couple seconds I hear quite a loud bang and smoke coming out of it. Does anyone know what causes this and how to fix it? Thanks.

 

[MacTech68]

Could be the Class X & Y mains filter capacitors. They are very specialized components and MUST be replaced by the correctly rated components.

Here's a thread outlining where they are:

https://forums.macrumors.com/thread...oads-the-power-breaker.1947360/#post-22411786

 

[mikemike690]

Could be the Class X & Y mains filter capacitors. They are very specialized components and MUST be replaced by the correctly rated components.

Here's a thread outlining where they are:

https://forums.macrumors.com/thread...oads-the-power-breaker.1947360/#post-22411786

Thanks, I'll post a photo in the morning.

 

[nightcap965]

It sounds as though the magic smoke has escaped.

You'll probably find a number of capacitors have leaked. How are your soldering skills?

 

[mikemike690]

Just opened it up now, and from a quick look it's only one small gold box thing looks like it's bent the foil over it back. Here are some pictures it's C33.
http://imgur.com/mvsAy87
http://imgur.com/lWRLDVp

 

[MacTech68]

Just opened it up now, and from a quick look it's only one small gold box thing looks like it's bent the foil over it back. Here are some pictures it's C33.
http://imgur.com/mvsAy87
http://imgur.com/lWRLDVp

Wow. Not seen a Class Y fail - but now I have.

I's suggest replacing both Class Y 4700 pico farad capacitors. Remember to check the lead spacing.

Looks like that machine has had a battery leak. What does the other side of the board look like?

 

[mikemike690]

Wow. Not seen a Class Y fail - but now I have.

I's suggest replacing both Class Y 4700 pico farad capacitors. Remember to check the lead spacing.

Looks like that machine has had a battery leak. What does the other side of the board look like?

Yeah it has but nothing really major right? It's just got the corroded contacts. The other side of the board is fine, I'll upload a picture after school. Also what do you mean by the lead spacing? And do you know anywhere I could buy these? Or would I just be able to go to a local electronics shop. Thanks

 

[MacTech68]

Yeah it has but nothing really major right? It's just got the corroded contacts. The other side of the board is fine, I'll upload a picture after school. Also what do you mean by the lead spacing? And do you know anywhere I could buy these? Or would I just be able to go to a local electronics shop. Thanks

I would advise against buying these on ebay or other auction sites, since these capacitors are very strictly safety rated.

The lead spacing on this type of component it fixed to a certain width. If you get the wrong lead spacing, you can't fit it into the holes.

I'll get back to you with a lead spacing.

 

[mikemike690]

I would advise against buying these on ebay or other auction sites, since these capacitors are very strictly safety rated.

The lead spacing on this type of component it fixed to a certain width. If you get the wrong lead spacing, you can't fit it into the holes.

I'll get back to you with a lead spacing.

Okay thanks for that

 

[MacTech68]

For C33 & C36 on the 240Volt International Analog Board, your Y2 capacitors are -

4700 pico farad <------- THIS IS CRITICAL
Class Y2 <------- THIS IS CRITICAL
300Volts AC (or at least 275VAC)
lead spacing 10.2mm <------- THIS IS CRITICAL Give or take 1mm
Width 5.1mm
Length 13.5mm
Height 10.5mm

For the Kemet Evox Rifa (the original parts)
Old Kemet PN P272HL472M300
New Kemet PN PME271YA4470M

For C37 on the 240Volt International Analog Board, your X2 capacitor is -

0.47 micro farad (470 nano farad)
Class X2 <------- THIS IS CRITICAL
275Volts AC
Width 15.3 <------- THIS IS FAIRLY CRITICAL
Height 22
Length 30.5 <------- THIS IS FAIRLY CRITICAL
Lead Spacing 25.4mm <------- THIS IS CRITICAL Give or take 1mm

New Kemet PN PME271M647K
Old Kemet PN P276EL474K275


However, provided you meet the above criteria, you should be fine.

From your pictures, it looks like the other capacitor C37 may have already been replaced.


As for where to buy, looking at a few in your location, many don't sell Class X2 or Y2 caps or they only sell Class X2.

 

[feroX88]

Do you think is possible to use 250V for C33 and C36 instead of 300V?
I have found below model that seems to be the same as the original.

PME271Y447MR30 - Safety Capacitor, 4700 pF, Y2, PME271Y Series, 250 V, Paper (MP)

 

[MacTech68]

That should be ok, however, I'd be cautious, even though Kemet states:

"Excellent self-healing properties ensure long life, even
when subjected to frequent overvoltages",

it's the lifetime limit of the 'self-healing properties' that make these things eventually go bang AFAIK.

So a higher rated voltage will withstand voltage spikes for a longer lifetime overall.

Depends how stable your line voltage really is. If it's often over-voltage, then I'd go with the 300VAC - it might just last a little longer.

Interestingly, they are factory tested to 3 times their rated DC (NOT AC) voltage - but doing that more than once can damage the capacitor:

"The 100% screening factory test is carried out at
3,000 VDC. The voltage level is selected to meet the
requirements in applicable equipment standards. All
electrical characteristics are checked after the test.
This test may not be repeated due to potential capacitor
damage. KEMET is not liable for any failures that result from
repeating the test."

 

[feroX88]

So that's fine. Thank you very much for the great answer and explanation. Guess at this point is better to use a high rated voltage, other values seem to be identical.

Do you have a complete list of the other capacitors with the compatible replacement?

At this point, do you think is better to recap the whole board? don't wanna risk another explosion
I'm searching now for a complete list of old and new compatible capacitors, hope I'll find something and I'll share it.

 

[MacTech68]

No list for the 240Volt (International) Analog Board. Compnent silk screen numbers differ wildly from 110V to International versions.

As far as exploding capacitors go, the 3 mentioned above are pretty much the ones that fail violently.

However, depending on how late your board was repaired by Apple, you may have recent/upgraded replacements of other capacitors.

A decent picture(s) of the board should reveal if anything is truly amiss.

 

[feroX88]

So, I have opened the Macintosh and TRIED to take some pictures of both Analog and Logic board. I didn't unmount the analog board because I couldn't discharge the cathode, I don't have the alligator clips, I'll definitely do it next days.

The situation is worse than I thought, I found oxidation on the copper (especially on the logic board), rust and liking battery residue. The capacitors except the above mentioned seem to be ok, but I'm not an expert so any suggestion on their status is welcome.

Pictures
https://drive.google.com/open?id=10JJ5UaCifEKmAs4dCq4fJSIEReiNE0Jl

For remove the green copper and liking battery residue should I use the contact cleaner spray or isopropyl alcohol?

 

[MacTech68]

This particular machine's insides becomes less of a repair and more of a restoration.

That means a lot of tedious, skilled work is required to keep it going far into the future.

However, If you repair the worst looking bits, and can get it working for now, that may be all you care to do. But, this stuff will most likely keep eating away at itself.

If this was something you wished to give the best chance of working in 10 years time, my recommendation is to clean as much of the green copper oxide as you can. You may even need to use a tiny wire brush on components on the Analog board. Removing each component one by one, cleaning and inspecting is likely necessary for longevity.

The solder side of the Analog board may even have blackened joints and tracks. This is where corrosion has already begun. Such joints really need to be cleaned back to copper and resoldered and re-coated. Some tracks may have been "eaten" through, causing an open circuit that needs to be bridged. Again, this is very tedious work.

For the Logic board, similar methods should be used. It will no doubt look worse than it does now after cleaning but that's where this machine's life has got to. Some tracks may need to be checked for open-circuits just like the analog board, the tracks can be effectively etched right through.


For the rusty metal chassis, I'd remove it and ask somebody with a sand-blaster to bring it back to near new condition.

Please note that I have NOT undertaken repairs such as this on a Mac 128/512/Plus, but this is how I would proceed. You need to find a balance between getting it to work and how much time and skill you can throw at it. Sadly, due to leaking clock batteries that's just how it is.