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Thread: List of Capacitors for Macintosh Classic

  1. #11
    Join Date
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    Quote Originally Posted by Timo W. View Post
    On my power board, there were no signs of leakage either and the Classic still worked, too. But knowing the caps were 30+ years old, I wanted to renew them anyway. As soon as I had desoldered the first one, that fishy smell came up and the cap was wet underneath. That large cluster of caps at the speaker was particularly bad with 6 of 9 leaked.
    I thought I'd take your advice and at least have a look before powering up and well its not good news. I suspect most of the caps on adjacent to the motherboard power lead have leaked, 1 rifa has cracked but more so there is signs of a short on the primary side of the transformer. The board is well and truly cooked, black and crumbling.

    Few pics







    I can certainly replace and clean up around the caps but the damage at the transformer is another thing. If that's gone there's probably not a lot I can do. Any advice for testing a transformer for a fault, can I measure it on the multi-meter?
    Last edited by Glen M; July 3rd, 2020 at 12:52 PM. Reason: spelling

  2. #12

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    Wow, that looks bad (but fixable). Give it a good clean with IPA to see how bad the damage actually is.

    No idea about the transformer, but since it's supposed to work with AC, a multi-meter won't help you much. It will most likely measure near 0 ohms.

    What makes you think there's a short on the primary side?

    btw, your power board is slightly different than the one in my Classic (see here). It only uses one 220 F, 400V cap instead of two and I can spot at least one cap that has a different capacitance value. Check my list of caps against your board before ordering the caps.
    Last edited by Timo W.; July 3rd, 2020 at 01:19 PM.

  3. #13
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    Quote Originally Posted by Timo W. View Post
    Wow, that looks bad (but fixable). Give it a good clean with IPA to see how bad the damage actually is.

    No idea about the transformer, but since it's supposed to work with AC, a multi-meter won't help you much. It will most likely measure near 0 ohms.

    What makes you think there's a short on the primary side?

    btw, your power board is slightly different than the one in my Classic (see here). It only uses one 220 F, 400V cap instead of two and I can spot at least one cap that has a different capacitance value. Check my list of caps against your board before ordering the caps.
    Where that is turned to charcoal is that not the primary side of the transformer? I removed it for a better look and it actually looks ok. Hoping it was just a short caused by the electrolyte.

    I've also noted down all the caps and there position across the board. There is only the 1 big cap on the 220v PSU with a jumper link in place of the second one.

  4. #14

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    Quote Originally Posted by Glen M View Post
    There is only the 1 big cap on the 220v PSU with a jumper link in place of the second one.
    And a 470 F, 50V one where mine had a 1000 F one. Wonder why that is. Don't you have 230/240V mains in Ireland as well?

  5. #15
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    Quote Originally Posted by Timo W. View Post
    And a 470 F, 50V one where mine had a 1000 F one. Wonder why that is. Don't you have 230/240V mains in Ireland as well?
    Yeah it's 230v here. The board didn't look to have any work done to it previously so can only assume its always had the 470 cap.

    Maybe a different revision of the board?

  6. #16

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    Most likely Rev. B then, because the service manual mentions that you can not mix revision A and B power boards and the according tubes, and your power board has a different connector for the tube's collar.

    Still suspicious that the second large cap is missing and has a jumper wire installed instead. That would normally indicate it's for 120V mains.

  7. #17
    Join Date
    Apr 2015
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    Austin, Texas
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    Quote Originally Posted by Glen M View Post
    Where that is turned to charcoal is that not the primary side of the transformer? I removed it for a better look and it actually looks ok.
    When the PCB burns like that, it becomes conductive. You'll need to remove the transformer and test for resistance between the burned area. If there is resistance, you'll need to start digging into the board to get all of the burned contamination out. If it goes too far into the board, you may have to resort to drilling a small hole through the entire board and then using a tiny file or saw blade to cut a slot between the two traces to get rid of the path between them.

    To prevent it from happening again, you can insert plastic wafers through the slot and secure them to form a wall between the traces. You may also want to apply more conformal coating to the PCB because it looks like it's flaking off in several areas. When the copper is exposed to the air, it starts to tarnish/corrode and its electrical properties change. It also allows high voltage to more easily "escape" and potentially arc over to something nearby. Clear fingernail varnish works decent enough and is cheap if you can't find the purpose made stuff.
    Last edited by GiGaBiTe; July 4th, 2020 at 11:41 AM.

  8. #18
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    I've cleaned the effected areas and as you can see a fair bit of the solder mask has come away. I will be coating this with some green nail varnish after I'm finished the recap.



    Problem though. Were the PCB was burnt at the transformer 1 pad just fell off as I desoldered. From looking at other pictures this pad has no traces from it so is it just there to help secure the transformer in place? I do note this leg of the transformer has continuity with the leg above it (looking at the picture), that continuity within the transformer itself when measured removed from the board. What should I do here, just leave it floating or try to plug the hole with solder?


  9. #19
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    Apr 2015
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    Austin, Texas
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    If there is no trace going to the pad, it was just there for mechanical support. It should be fine just floating.

    As for the copper, it looks like it has tarnished significantly, I would do something to neutralize the oxide before you put any sort of conformal coating on it. Krud Kutter works if you soak the end of a cotton swab with it and rub it on the bad areas. It usually only takes a minute or so to work of light scrubbing, then rub it dry with the other end of the cotton swab.

    https://www.walmart.com/ip/Krud-Kutt...dSellerId=1194

  10. #20
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    Jun 2020
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    Tennessee, USA
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    Wonder what made that analog board so unhappy as to burn like that, is that a common problem when caps go bad? If so my preventative maintenance spiel may be happening sooner rather than later on my Plus.

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