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Thread: CBM 8032 - Blows Fuse - Help

  1. #1

    Default CBM 8032 - Blows Fuse - Help

    I have a CBM 8032 that used to work but now blows the fuse and does not boot when I turn the power on (blew the fuse once, replaced the fuse and tried again same result). What is the most common cause of this?

  2. #2

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    A short. Get your Ohm meter out.
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  3. #3
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    If the power circuitry checks out, you can get a current limited bench supply and inject 0.5-1v into the power rail and start checking for hot components carefully with the backside of your finger. Monitor the current draw and make sure it doesn't go too high, if you start getting multiple watts of power consumption, dial it back quick.

    Make sure you get the polarity right though else you'll cause more damage.

  4. #4
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    Quote Originally Posted by GiGaBiTe View Post
    If the power circuitry checks out, you can get a current limited bench supply and inject 0.5-1v into the power rail and start checking for hot components carefully with the backside of your finger. Monitor the current draw and make sure it doesn't go too high, if you start getting multiple watts of power consumption, dial it back quick.

    Make sure you get the polarity right though else you'll cause more damage.
    I wouldn't recommend that; triple-supply memory chips are easily blown by incorrectly applied or absent voltages.

    I assume that the fuse does not blow with the system board disconnected?

    m

  5. #5
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    Yes, the first test (before anything else) is to disconnect both the monitor and the PET main board from the power supply itself (just leaving the fuse, the switch and the mains transformer - and any RFI filter if present - in circuit) and test that configuration.

    If the fuse blows again - I would then look at whether there was any short circuit between Live and Neutral to earth or Live to Neutral.

    Actually, the first thing I would do would be to check for a short circuit from mains Live and/or Neutral to Earth anyhow - better safe than electrocuted!

    Then report back.

    Dave

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    Build yourself a"dim-bulb tester" and plug the power supply into that with the system disconnected from the power supply.

    I've got a pretty elaborate dim-bulb setup, but all you really need are 1) an incandescent lamp, 2) a lamp socket, 3) am extension cord.

    This is an invaluable thing to have when you are in the habit of obtaining old electronics.

  7. #7

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    Quote Originally Posted by MikeS View Post
    I wouldn't recommend that; triple-supply memory chips are easily blown by incorrectly applied or absent voltages.

    I assume that the fuse does not blow with the system board disconnected?

    m
    +1
    triple supply RAM will die instantly with only +5V and missing -5V VBB supply. I have a few chips that remind me that.

    Frank

  8. #8

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    Thanks all. I will test the PS disconnected from the 8032 and see if the fuse pops again. Great advice.

  9. #9
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    If you have a 'kettle' lead on your PET - it probably contains an RFI filter - and this is possibly the culprit.

    Dave

  10. #10

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    Quote Originally Posted by daver2 View Post
    If you have a 'kettle' lead on your PET - it probably contains an RFI filter - and this is possibly the culprit.

    Dave
    +1 with that. I had a CBM floppy drive that caused this problem and the RFI filter in the socket was the culprit.

    Andy

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