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SlateBlue

Macintosh Classic AB Recap

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Hi everyone. I thought I'd take the time to create a post about my recapping adventure, and maybe take the opportunity to ask a few questions. If the is dry reading for you, I apologize. I'm not known for being a good storyteller.

 

So, for starters - A several months ago, I acquired a Macintosh SE and a Macintosh Classic. The SE appeared to work just fine (still has original caps on board), and was in overall good condition with minimal yellowing. The Classic would boot fine, but had no sound, and the case had significant yellowing. I removed the logic board from the Classic to find that all the SMD caps were leaking. I set the SE aside and went to work on the Classic. After a botched repair attempt (a couple of lifted pads), I sent the board to Novi, Michigan for a professional recap job. The recap fixed the sound issue, but I still had yet to tackle the cosmetic issues of the machine. Pressed for time on other projects, I set the Mac Classic aside to be worked on at a later date.

 

Last week, I finally took the time to completely disassemble the Mac Classic so I could retrobright the front bezel and rear case. I also took the time to blow compressed air over the AB to remove years of dust. Upon reassembling the computer and powering it on, it seemed the hard drive would quite spin up. It would try, and with each attempt the screen would flicker. Thinking I had a bad hard drive (it worked weeks prior), I swapped a known good working one to find that the issue persisted. I then realized I likely was dealing with a voltage issue. I measured both +5v and +12v at the external floppy port and realized the voltage readings were low ~ around +4.8 and +11.65 respectively. So, I adjusted PP1 until both voltages were in an acceptable range ~ +5.05 and +11.99. I suspected that the compressed air had somehow turned the potentiometer and decreased the voltage.

 

Yesterday, after cleaning the floppy drive, I noticed the screen would appear to shake/wave slightly as the disk drive was working. Thinking again that I had a voltage issue, I measured at the external floppy port again. The voltages were ~ +5.15 and +11.77. At this point, I realized adjusting PP1 wouldn't fix the issue. Too little voltage is worse than too much, right? So, I removed the AB and desoldered the caps from the bottom left/center of the board (yellow box in attached photo) to find them all leaking. I guess it's time to recap the AB now.

 

My questions:

 

I know that I must match the capacitance and voltage ratings of the original caps. Must I also match the physical size? Can someone explain whether one should choose General Purpose or Low Impedance caps and why?

 

I searched the Interwebs and could not find a capacitor list for a Macintosh Classic Analog Board.

 

Can I get away with leaving the smaller (physical size) caps alone? None appear to be leaking.

 

Thanks!

post-6277-0-79739000-1469585216_thumb.jpg

Edited by SlateBlue

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Im gonna say that size for caps does not matter, they just have to be the same otherwise.

 

If no caps are leaking, they are bound to leak very soon. Im surprised there is no trace of anything yet. Id replace them while you have it taken apart.

 

These are guesses, so wait for a pro!

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Physical Size does not matter just as long as it's the same capacitance value and voltage rating. Try to get 105c rated caps and try to get nichicon, rubycon, nte, or panasonic. Chong X works but who knows how long they will last. Also: here's a list of brands to avoid: http://www.badcaps.net/forum/showthread.php?t=388

Thank you!

 

I ordered most of the replacement caps from Mouser, but I had to source the 470uF 50v cap from Digikey. All replacement caps were Nichicon except for one Panasonic. I completed the recap job today using caps rated for 105*C and I'm impressed with the difference it made. The Mac has a clean display and there's no more drain on CRT power when using the floppy disk drive. I measured the voltages to be +5.08 and +12.31. These voltages appear to be within an acceptable tolerance, so I did not adjust PP1. I should add that I also replaced the optoisolator. I doubt that it was bad, but I understand it has a limited life, too.

 

Here is a partial list of replacement capacitors. I left the smaller 1uF caps alone; I think there were two or three on this board. There was also a smaller 47uF cap, but I can't recall the voltage ratings of these smaller caps. In my case, it was all the larger caps clustered near the bottom of the AB that were leaking. I'll create a diagram when I get time.

 

1 - 4.7uF 250v

 

1 - 220 uF 50v

 

1- 1000uF 25v (See * below.)

 

* 1 - 1000uF 16v (You may be able to sub. 25v cap here. I did not.)

                             (There are two 1000uF 16v caps on the Classic AB. Their dimensions are different: 10mm x 20mm, and 10mm x 28mm. Does size matter? : :) If not, add 1 to this quantity.)

 

3 - 470uF 25v

 

1 - 470uF 50v (Panasonic. Acquired from Digikey as Mouser had none in stock at the time I ordered. The leads were just long enough to fit through the board after bending them to match the board spacing.)

 

1 - 2200uF 16v (See ** below.)

 

** 2 - 2200uF 10v (Again, you may be able to sub. for 16v. I did not. This cap was taller and slimmer than the 16v cap.)

 

1 - 1000uF 16v (See * above.)

 

1- 1000uF 10v (See * above.)

Edited by SlateBlue

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And the obligatory photo. Forgive the poor lighting and refresh mismatch. I'm impressed with the vivid display of the screen. This is only half brightness!

 

(Errr... The uploader automatically rotated my photo. I'm not sure how to fix it. Tilt your head or your monitor.)

post-6277-0-10722300-1469748773_thumb.jpg

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