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Thread: H7874-00 power supply

  1. #1
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    Default H7874-00 power supply

    I have a VAX 4000/300 with a bad power supply - does anyone have a working spare for sale? The system also has a R400X expansion cabinet, and I have it running temporarily by moving the supply from the R400X into the BA440 along with the 3 functioning hard drives, but I want to get both boxes running.

  2. #2

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    Have you considered attempting to fix the PS?

    The one in my Personal DecStation 5000/25 simply had one shorted tantalum cap on the 12V output, which of course makes it shut down a moment after switch on.
    IBM 5170/5053, 2 x 5150/5051
    Sun IPC, Ultra1 and SPARCclassic with tape, disk and CD
    HP Apollo 9000/735
    Silicon Graphics Indy, O2
    Radio Shack TRS-80 model 100
    Apple Mac Plus, DEC 5000/25

  3. #3
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    Quote Originally Posted by paul View Post
    Have you considered attempting to fix the PS?

    The one in my Personal DecStation 5000/25 simply had one shorted tantalum cap on the 12V output, which of course makes it shut down a moment after switch on.
    There are no obviously blown or leaking caps, nor burned areas or parts. I have no service manual.

  4. #4

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    I've found shorted caps are not always obvious.

    I'm still a learner when it comes to electronics but it is my understanding there should be some resistance across a cap. It's a big job to unsolder one end of all the caps to check resistance across each of them, but one way you can reduce the number of suspect caps is to measure the resistance across them in circuit. Although you won't get a true ohms value for the cap, nevertheless, if you get a reasonable resistance the cap won't be shorted.

    On the other hand if you get 0 ohms or very low ohms then either the resistance in the attached circuit is low OR the cap is indeed shorted.

    Either way, you can eliminate the seemingly good caps, and just focus on the subset that gives low readings. You may have to unsolder these to identifiy the exact one which is shorted, but at least the number would have been narrowed down, perhaps considerably.

    I used this technique to diagnosis a faulty disk controller, and found it reduced dozens of potenial caps to just three suspects. Sure enough, one of those was the culprit.

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  5. #5
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    >> SAFETY <<
    Obviously, disconnect power before delving into a power supply. And wait a while because it takes some time for some of the large capactors to discharge. A 10 minute wait should be enough.

    Quote Originally Posted by tezza View Post
    I'm still a learner when it comes to electronics but it is my understanding there should be some resistance across a cap. It's a big job to unsolder one end of all the caps to check resistance across each of them, but one way you can reduce the number of suspect caps is to measure the resistance across them in circuit. Although you won't get a true ohms value for the cap, nevertheless, if you get a reasonable resistance the cap won't be shorted.
    Tezza. Most in the trade would have an ESR meter in their toolkit. Have a read of http://www.flippers.com/esrkttxt.html for an explanation. You can measure in-circuit (with power off). An ESR meter has saved me many hours of work. The ESR meter I use is shown at http://www.flippers.com/esrktmtr.html and has typical ESR readings printed on the front.

  6. #6

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    Quote Originally Posted by modem7 View Post
    Tezza. Most in the trade would have an ESR meter in their toolkit. Have a read of http://www.flippers.com/esrkttxt.html for an explanation. You can measure in-circuit (with power off). An ESR meter has saved me many hours of work. The ESR meter I use is shown at http://www.flippers.com/esrktmtr.html and has typical ESR readings printed on the front.
    Interesting. Thanks for alerting me to these. I see the final sentence says that ordinary multimeters are ok for shorted caps. Of course many caps are not shorted, but are still faulty, so I can see the need for such a device.

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  7. #7

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    Quote Originally Posted by mloewen View Post
    There are no obviously blown or leaking caps, nor burned areas or parts.
    That's usually the preferred scenario.

    I have no service manual.
    None of us do, especially for power supplies for which the manufacturer would rather sell you a new one. But for someone with the right skills there is often nothing difficult about fixing one. The semiconductors, resistors, and inductors are pretty bullet proof - it's the tantalum and electrolytic caps which age and cause the PS to simply shutdown on overcurrent or excessive ripple.

    Yes, it is dangerous if you don't know what you are doing but any local electronic technician should have no problem checking the easy stuff for you at minimal cost.

    If the PS fan starts momentarily (after sitting for a while) and then stops it can almost certainly be fixed.
    IBM 5170/5053, 2 x 5150/5051
    Sun IPC, Ultra1 and SPARCclassic with tape, disk and CD
    HP Apollo 9000/735
    Silicon Graphics Indy, O2
    Radio Shack TRS-80 model 100
    Apple Mac Plus, DEC 5000/25

  8. #8

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    Quote Originally Posted by paul View Post
    That's usually the preferred scenario.
    I don't know - If it's not working, I'd rather open the thing up and find a big, smoking hole in the top of an electrolytic capacitor, or two bare metal legs where a tantalum used to be. Kind of gives you idea of where the problem might be ;-)

    Quote Originally Posted by paul View Post
    None of us do, especially for power supplies for which the manufacturer would rather sell you a new one. But for someone with the right skills there is often nothing difficult about fixing one. The semiconductors, resistors, and inductors are pretty bullet proof - it's the tantalum and electrolytic caps which age and cause the PS to simply shutdown on overcurrent or excessive ripple.
    For a lot of DEC stuff, the service manuals (and very detailed ones at that) are in fact very often available via sites like Bitsavers. But I agree that a great deal of failures aren't that hard to fix without documentation.

  9. #9

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    Did you ever get your supply fixed? I just switched on my 4000/200 and the supply is dead. Just like that. Grrr... It's been eaten by a gru. I pulled a supply out of my 4000/300 for now because I want to show off this 4000/200 tomorrow at SRCS.

  10. #10
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    Quote Originally Posted by marmotking View Post
    Did you ever get your supply fixed? I just switched on my 4000/200 and the supply is dead. Just like that. Grrr... It's been eaten by a gru. I pulled a supply out of my 4000/300 for now because I want to show off this 4000/200 tomorrow at SRCS.
    No, still on my TODO list, but way down. I found two replacement supplies on Epay to get everything up and running.

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