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Thread: Oh, my aching Mac

  1. #1
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    Default Oh, my aching Mac

    Working on an SE that would only boot from floppy; diagnosed the problem as a stuck HD (Miniscribe 20). Miniscribe will not spin up and is now officially toast.

    In the process of disassembly, however, when removing the power pigtail from the HD at the socket on the analog board, I was distressed to find the socket came off the board with the pigtail. I can repair this, but my question for the General Assembly is: While I'm in there, should I also replace the 16V 4700uF electrolytic cap that is next to the flyback? If so, with what? I have read they are prone to failure and a beefier cap is recommended.

    Second picture below is scraped offa Tezza's site, A and B point to items referenced above.

    Thanks for your suggestions,

    -CH-Miniscribe 20.jpg Analog Board.jpg

  2. #2

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    If the socket came off due to cold solder joints, you should be able to repair it by soldering it back in. Remove the old solder from the pads first, then use new solder. Does the capacitor appear to have bulged and/or leaked? If so, replacing it while you have the board out is a good idea. Find a cap with the same form factor or one that can be easily adapted to fit, the same or higher voltage rating with the same (or next higher) capacitance value.

  3. #3

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    If you want to replace the cap choose a low ESR, higher voltage rated item that will physically fit.

    For example 50V 4700uF

    You will find that newer caps are much smaller than the older ones. Size also goes up with voltage; so you will find that a modern 16V 4700uF capacitor is actually much smaller than the old one in the SE, whereas the new 50V 4700uF *might* be the right size.

    If you end up replacing the other caps on the analogue board beware of the bipolar one. It looks like an electrolytic however its got BP stamped on it. Use a high voltage polyester to replace that particular one.

    When I recap Mac analogue boards I measure up the physical dimensions of each cap and tabulate them. I then look at catalogues and match up the capacitance & sizes, ensuring the voltage rating is equivalent or higher than specified.
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

  4. #4
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    Quote Originally Posted by 3pcedev View Post
    If you want to replace the cap choose a low ESR, higher voltage rated item that will physically fit.

    For example 50V 4700uF

    You will find that newer caps are much smaller than the older ones. Size also goes up with voltage; so you will find that a modern 16V 4700uF capacitor is actually much smaller than the old one in the SE, whereas the new 50V 4700uF *might* be the right size.

    If you end up replacing the other caps on the analogue board beware of the bipolar one. It looks like an electrolytic however its got BP stamped on it. Use a high voltage polyester to replace that particular one.

    When I recap Mac analogue boards I measure up the physical dimensions of each cap and tabulate them. I then look at catalogues and match up the capacitance & sizes, ensuring the voltage rating is equivalent or higher than specified.
    I have read about people replacing this cap with ceramic and with higher capacitances, etc. Pina seems to think that a larger body is a plus, for the sake of heat dissipation. 50V no question, but higher capacity as well?

    -CH-

  5. #5

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    Quote Originally Posted by clh333 View Post
    I have read about people replacing this cap with ceramic and with higher capacitances, etc. Pina seems to think that a larger body is a plus, for the sake of heat dissipation. 50V no question, but higher capacity as well?

    -CH-
    I wouldn't change the capacitance. I can't remember what that capacitor does exactly; but it's capacitance was chosen for a reason. Things like smoothing circuits don't care if you increase the capacitance; but other circuits do.

    The other thing to remember is that these machines are now only used as a hobby. It's not like your leaving it on for 10+hrs a day, every day of every week. If you replace the caps even with the 'as designed' types it will probably last you a lifetime. Pina's guide is great and technically correct; however it's targeted at a time when these machines were used day in day out.
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

  6. #6
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    Quote Originally Posted by 3pcedev View Post
    ... Things like smoothing circuits don't care if you increase the capacitance; but other circuits do.
    Like RC timing circuits, e.g.?

    Thank you for your reply. I will match capacitance but use a higher voltage.

    My lifetime ain't what it once was but I'd like to do right by the next owner of these machines.

    -CH-

  7. #7

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    Quote Originally Posted by clh333 View Post
    Like RC timing circuits, e.g.?

    Thank you for your reply. I will match capacitance but use a higher voltage.

    My lifetime ain't what it once was but I'd like to do right by the next owner of these machines.

    -CH-
    If you want to go for maximum lifetime choose capacitors which have a high temperature rating (and high MTBF). I usually go for 5000 to 10000hr capacitors at a rating of around 80-100 deg C.
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

  8. #8

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    That Miniscribe drive might be suffering from stiction (the heads getting stuck to the platter). Try rotating the interruptor (the thing it warns you not to rotate!) to unstick them, and then apply power. That should get it going again, although likely only temporarily.

  9. #9
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    Quote Originally Posted by vwestlife View Post
    That Miniscribe drive might be suffering from stiction (the heads getting stuck to the platter). Try rotating the interruptor (the thing it warns you not to rotate!) to unstick them, and then apply power. That should get it going again, although likely only temporarily.
    Thank you for the suggestion. I read that tip during my research; maybe at Chris Adams' site (http://www.ccadams.org/se/) or Low End Mac (http://lowendmac.com/), but to no avail. I also read that Miniscribes, once they begin to fail, repeat the pattern ad infinitum. This and the fact that it is of small capacity has me pursuing another drive as a replacement.

    -CH-

  10. #10
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    Quote Originally Posted by vwestlife View Post
    That Miniscribe drive might be suffering from stiction (the heads getting stuck to the platter). Try rotating the interruptor (the thing it warns you not to rotate!) to unstick them, and then apply power. That should get it going again, although likely only temporarily.
    I gave it another shot this morning; instead of gently turning the interruptor a few detents I ratcheted it back and forth until it felt freer. Attached a 5/12 VDC supply and it started its whirring and clacking. So I attached it to the Adaptec 2940 that is on this machine and the Adaptec controller recognized it, unlike before. From the controller's disk utility I ran a verify. It found and relocated two faulty sectors. Booting into Linux I was able to see the drive, mount it and examine its contents. I used ddrescue - a variant of dd - to make a copy of the Miniscribe.

    Next I rebooted, ran a low-level format from the Adaptec controller, mounted the drive once again and, using hformat from the hfsutils suite, created one HFS partition named "untitled". Shoulda called it "Lazarus", maybe?

    Thanks for the suggestion.

    -CH-

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