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Thread: Macintosh LC - Constant rebooting (solved)

  1. #1

    Default Macintosh LC - Constant rebooting (solved)

    Hi all,

    Just thought I would share a very unique story/fault I experienced with a Macintosh LC.

    The machine was basically sold from a deceased estate. The seller didn't know much about it other than it hadn't been used in about 20 years and was sitting in a garage when he found it. I took it home and was pleasantly surprised to find that it was quite clean inside, no real dust to speak of and no visibly leaky capacitors or PRAM battery.

    I tried to power it up however it went into a constant reboot loop. The chime would sound, delay, the monitor would turn on, then it would chime again. I figured the caps were dead either in the power supply or the logic board so I initially swapped in a PSU from a Quadra. It powered up just fine so I figured I found the culprit; however for belt and braces I also ordered a set of capacitors for the logic board.

    All the capacitors arrived with the exception of some line filter caps, so I figured I would recap everything and just fit those at a later date. The recapping of the logic board and PSU was a success, or so I thought, as the machine then booted and behaved normally for quite a while. I also checked the voltages on the PSU output and they were perfect (Pro Tip: The PSU requires a load before the voltages will become stable. Put a resistive load on each rail and then test). The line filter caps then arrived so I figured I would fit them last night. This was pretty straightforward (and done just in time as the RIFA ones had some massive cracks in them), so I refitted the PSU in short order and turned it on.

    Infinite reboot loop. Again.

    I thought I had better check the voltages again (while plugged into the motherboard) and they were fine. As dumb luck would have it I had a probe wedged into one of the ground pins (from the top of the connector) and the machine decided to boot normally for a while, right until I removed the probe.On closer inspection the pin looked a little darker than the others, although it was hard to see through the semi-transparent plastic. I then pulled the pin out of the connector and the lower portion disintegrated completely in my hands. I am guessing this isn't the original PSU for the machine and it came from something with a leaky battery which caused this pin to corrode out. Alternatively it could have been making a bad connection for some time and has 'arced away'.

    I now need to find a replacement pin to suit the LC-style motherboard molex connector. Anyone got any suggestions as to what type it is?

    Here's a relatively bad ASCII art depiction of what it looks like from the side. The pin on the motherboard slides up from the bottom of that diagram. The contact is made on the left and right sides by two spring steel 'wings'. This isn't the same as a AT type motherboard connector with the flat pins as that uses a spring steel 'loop' to make contact.

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    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

  2. #2
    Join Date
    Apr 2015
    Location
    Austin, Texas
    Posts
    688

    Default

    Is the pin spacing the same as the AT P8/P9 power connectors? Maybe you could desolder the connector on the motherboard and sub an AT style power connector, then chop the connector on the power supply and replace it with an AT style power connector.

  3. Default

    Yes, had one like this before, same problem, had a spare PSU, so didn't repaired the former one, but connectors are unusual.

  4. #4

    Default

    Good news! I dug around the molex catalogue for about an hour and manage to find some compatible pins.

    They do not have the 'pin guide' like the originals, however it is of no consequence as the width, length and thickness are all equivalent. They lock snugly into the plastic connector and also make a good contact with the PCB pins. My LC is now back to its usual self!

    Molex part number: 08-52-0072
    Element14 part number: 1756639
    RS Part number: 679-4714
    Mouser: 538-08-52-0072
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

  5. #5
    Join Date
    Dec 2014
    Location
    Sol->Earth->North America->USA->CA
    Posts
    145

    Default

    Nice. Another classic Mac saved from the recyclers furnace.

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