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Thread: Mac Classic low voltage on 12 V rail

  1. #11
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    Old electrolytics on switching power supplies of that era often went bad (without exploding). "Low ESR" was pretty new and many electrolytics simply cooked dry with the high-frequency ripple currents.

  2. #12

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    Quote Originally Posted by NeXT View Post
    If it's the original drive I would be weary of stictation. It's possible the moving of the drive from the classic to your PC for testing might of jarred it loose for the time. Have you tried spinning up another disk on the classic?
    like ur post i was facing the same problem 1 month ago i did the same thing and after that i put the previous one it was workin............

  3. #13
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    Default Mac Classic Analog Board Capacitor Repair

    The brown capacitors were the only caps to test bad. They could not be visually identified as being bad unless removed and inspected underneath and electrically tested. Location and Capacitor Type: CP2 1000uF 35V, CP8 2200uF 16V, CP12 1000uF 16V, CP36 2200uF 10V, CP6 2200uF 10V, CP7 1000uF 10V, and CP11 220uF 25V. The CP11 capacitor is located near the step-up transformer and these are all the brown, high temperature type caps that went bad. Refer to pictures for details. Note that I had replaced CP12 in the picture.

    Anthony
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  4. #14
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    Important: After repairing, if you find the voltages at the rails to not be within spec, adjust the variable resistor PP1 on the power supply analog board. It is small and can be found at about the center of the board. Clockwise will increase output voltages. Be careful, it is sensitive. Only use this if bad elements have been repaired/replaced on the board, otherwise you risk damaging the machine.

    Anthony

  5. #15
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    Quote Originally Posted by afaiello View Post
    The brown capacitors were the only caps to test bad. They could not be visually identified as being bad unless removed and inspected underneath and electrically tested. Location and Capacitor Type: CP2 1000uF 35V, CP8 2200uF 16V, CP12 1000uF 16V, CP36 2200uF 10V, CP6 2200uF 10V, CP7 1000uF 10V, and CP11 220uF 25V. The CP11 capacitor is located near the step-up transformer and these are all the brown, high temperature type caps that went bad.
    Wonder if the failure has anything to do with Mr. Jobs' insistence that there be no fan in the Mac?

  6. #16
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    Ha ha, this one has a fan fortunately. Wasn't it the SE model that didn't have a fan?

  7. #17
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    Macs before the SE didn't have fans. The SE was the first Mac (IIRC) that used a fan. Things were getting pretty hot in that box. Obviously, the fan in the Classic is under-specified.

    The Apple Museum has this interesting tidbit on the SE fan:

    The relatively cheap cooling fan was very annoying to several SE users. Some did anything possible to get rid of it. There were several better cooling fans for the SE on the market but my favorite product was the Mac Chimney. Literally, it was a chimmeney that allowed cool air to enter the SE into the CPU. A soon-to-be-scanned picture I saw of the product makes the SE look like a little Mac house!

  8. #18
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    Many thanks again for fulfilling my request of detailed repair. I should be picking up my Mac, and I get the choice of nabbing a Classic or Classic II, this weekend. Plans fell thru this past Sunday due to fathers day and rain. Deal even includes an CMS external storage device! =)

    I'll be following your footsteps in the next week or 2. Once it arrives I'll be sure to add to this thread or start another, and start work on the webpage I promised.
    Last edited by twolazy; June 20th, 2011 at 01:34 PM.
    '. \ / .'
    '. .'``'. .'
    ......:::::::`.....`::
    Currently seeking a Compaq Deskpro 386

  9. #19

    Default Mac classic low voltage on 12V and 5V rail

    Hi to everyone, I know this is an old thread but I have the same problem and I do not know what to do.
    I changed all the browns capacitors e the big CP1, but the voltage rails remains low. Then I changed the diodes: DP5 and DP7 with MBR360 (but I'm not sure of these model) and the DP6 diode with MBR1045.
    Now as soon as connect the power supply, the fuse blown. I double checked the welds and seem to be ok.What can it be?
    Can you tell me the exact code of DP5 and DP7 diodes?
    Thank you very much! Bye

  10. Default

    Now as soon as connect the power supply, the fuse blown. I double checked the welds and seem to be ok.What can it be?
    Check the Mosfet, can be shorted, is ref IRFBC40 and located at position QP2, if Ok can be the Bridge rectifier.

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