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Thread: IMSAI 8080 Frankenstein machine ?

  1. #11
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    Quote Originally Posted by Moonferret View Post
    Congrats on the purchase and welcome to the IMSAI club!
    Ah, thanks - good to see another UK user too.
    I hope mine will arrive in the order it was shipped and am looking forward to getting it on the bench and fixing it.

  2. #12

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    Quote Originally Posted by Moonferret View Post

    That's actually a typo. From an email discussion with Todd Fisher the other year...

    "The 10 amp spec is an error. There was the original 20 amp supply that used a Signal (brand) transformer, the rectifier board mounted to the back of the front subpanel, and capacitors mounted directly to the base. That was the PS-A. The later PS-28 had all components including transformer mounted to a long circuit board. That is the 30 amp supply that lasted throughout later production. "
    Now that makes sense! I wondered about that 10A value.

  3. #13
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    It seems this is the same machine from the other thread

    http://www.vcfed.org/forum/showthrea...ght=imsai+8080

    I hope I have more luck getting it going than the previous owner.

  4. #14

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    Quote Originally Posted by zippysticks View Post
    It seems this is the same machine from the other thread

    http://www.vcfed.org/forum/showthrea...ght=imsai+8080

    I hope I have more luck getting it going than the previous owner.
    I'm not sure how I lost track of the other thread. I see he'd last posted on April 30th? I suspect he'd plugged it in with the resistor in series with the input. I suspect the resistor was a 2K but don't know as it isn't clear. That would not be enough to protect the rectifiers from a dead short. I can't see from the angle of the picture but I suspect the rectifiers are a pair of stud mounted type diodes. One or both may be shorted ( or open ), if powered on with the resistor on the primary.
    If the diodes are shorted they can damage the capacitors( if you connect the wires up properly now ). If the diodes are a pair as I believe them to be, you need to unbolt them ( assuming they are stud mounted ) and check each individually ( the windings of the transformer will make them look short if in circuit or at least in parallel ).
    The larger capacitor, of the pair, has been replaced. That may be an indication of an earlier shorted diode.
    Don't do any supply test with the front panel connect to power through the mother board or you may damage it as well.
    Let us know when you get your hands on the machine.
    Dwight

  5. #15
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    Great advice - thanks Dwight.
    I plan to be quite methodical with this machine - I just hope it hasn't already been significantly damaged.

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