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Thread: Dead AT&T M24/PC6300 PSUs - help!

  1. #1

    Default Dead AT&T M24/PC6300 PSUs - help!

    Need help fixing a couple of dead Olivetti/AT&T PC6300 power supplies. One is completely dead, with no 5v or 12v output. The other has +5v and +12v, but no -12V.

    Focusing on the completely dead one for a moment:
    • I have the (poor resolution) schematic for the PSU. Unfortunately, the portions directly near the 5v/12v outputs are illegible.
    • Testing the +5v rail and +12v rail at the output connector shows both shorted to ground both with and without a load attached.
    • The PSU is comprised of 2 boards (stage 1 & 2?). I've confirmed on the primary board (the one with AC input) that the big caps after the rectifier bridge are generating 160vDC, which seems appropriate given 120vAC input. Beyond that I'm not seeing anything on the second board.
    • I've been probing with a multimeter on the second board for a handful of days now, even removing and testing many components out of circuit (diodes, caps, transistors), but everything tests good so far.
    • Just about every component that I know how to test has tested fine.
    • Testing the diode leads on the 3-pin power regulators (the ones that look like diodes) shows the diodes are not blown, but I'm not sure how to test the third power pin. Is this only done with the PSU powered up?
    • There are 3 transformers: 2 Gammatron and one ordinary transformer. I'm not sure how to test these, but what's the likelihood of a transformers shorting out?


    I haven't yet started on the second PSU that's just missing -12v, thinking the completely dead one would reveal a much more obvious issue. But that's not been the case so far. Diagnosing using a schematic is not by strong suit. I'm a trace and replace bad components guy. Any assistance or guidance is greatly appreciated.
    “The Bex religious impulse does have its collective side. As in, collect everything and hold on to it whether you remember where it came from or not.”
    —Alberto Fossa, Privateer 2

  2. #2
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    If you have multiple rails on the secondary side shorted to ground, you'll need to start pulling parts off the power rail until the short goes away. I'd suspect the mosfets/transistors were the culprit because once you get to the secondary side, there's very little that could short out.

    Quote Originally Posted by normanator
    Testing the diode leads on the 3-pin power regulators (the ones that look like diodes) shows the diodes are not blown, but I'm not sure how to test the third power pin. Is this only done with the PSU powered up?
    Diodes don't have three legs. You're either testing a dual diode package where the anodes or cathodes are commoned together, transistor or mosfet. DO NOT test these while powered on, you'll destroy them.

    Here are two guides for testing transistors or mosfets with a multimeter:

    https://vetco.net/blog/test-a-transi...04-12-25-37-07
    http://electronicsbeliever.com/how-t...-is-defective/

  3. #3

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    Well, I guess just one is a dual diode BY511150. The others are power amps, rectifiers and voltage regulators, some of which include an internal diode across 2 pins so I tested their diodes successfully.

    Thanks for the links. I watched a few tutorials on Youtube, but I'll check these out too.
    I retested the rails after removing some of the parts, but not for diodes and transistors that tested ok with the multimeter. Clearly those aren't shorted right? What's the likelihood that a transformer would be shorted?

    On the PSU with everything except -12v, I'm guessing it's the LM7912 negative 3-pin regulator. That one's cheap enough, so might just replace it and see if that PSU will resurrect. But the fully dead one is quite frustrating.
    “The Bex religious impulse does have its collective side. As in, collect everything and hold on to it whether you remember where it came from or not.”
    —Alberto Fossa, Privateer 2

  4. #4
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    Please don't forget, that the M24 PSU can not work without an external load. It even can get damaged from this.

    Can you share the circuit diagram?

  5. #5

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    Quote Originally Posted by 1ST1 View Post
    Please don't forget, that the M24 PSU can not work without an external load. It even can get damaged from this.

    Can you share the circuit diagram?
    Way ahead of you. I'm generally ohm-ing it out unpowered looking for the short. The one time I did power it on to measure output behind the rectifiers, I plugged in a floppy drive just to be safe. Sounds like I made the right call. I'm paranoid poking around with it powered anyway, because like you said in another post, there's barely enough room to have both boards plugged in. That, and I'm worried about torching something (see below).

    Circuit diagram is here. Unfortunately, the PSU page is not exceptional resolution. http://olivettim24.hadesnet.org/docs/m24schem.pdf

    I've got one PSU that works except for the -12v, so I'm going to start with replacing the LM7912. (I think I zapped the -12v when trying to probe the +/-12v output terminal on the PSU. )
    The second one is shorted on both rails, and I'm still looking for the culprit. This one came to me doa from a recycler, so this one wasn't me!!
    “The Bex religious impulse does have its collective side. As in, collect everything and hold on to it whether you remember where it came from or not.”
    —Alberto Fossa, Privateer 2

  6. #6

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    If I remember the Olivetti M24 power supply has 15 volts output. And retains charge even when powered down for maybe 20 minutes, but you can discharge it yourself.

  7. #7

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    Quote Originally Posted by tonata View Post
    If I remember the Olivetti M24 power supply has 15 volts output. And retains charge even when powered down for maybe 20 minutes, but you can discharge it yourself.
    Yes, thanks! Excellent advice for anyone working on a PSU! I noticed on my one powered-on test that the big caps on the primary side (160 volts!) discharge very slowly. Watched them count down on the multimeter. EEVBlog posted a Compaq Portable PSU repair video, and showed that one has extra components that bleed the caps down really fast. That's what clued me in to this phenomenon.
    “The Bex religious impulse does have its collective side. As in, collect everything and hold on to it whether you remember where it came from or not.”
    —Alberto Fossa, Privateer 2

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