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  1. #21
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    Re: HP 54520A Blown Fuse Problem

    I will try to search on the web, to find information about checking electrolytics but also the polypropylene HV types. If you have any hints let me know
    My best hint is don't follow this line of investigation. A component failing after a minute with 300+ Volts across it is unlikely to show a problem with a low voltage DVM probe. You would either have to replicate the operating conditions or just replace it anyway. Given their relatively low cost I think replacing is by far the best option. Even if not 'faulty' there is a good chance they are now out of spec after all those years.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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  2. #22
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    Re: HP 54520A Blown Fuse Problem

    suggestion:
    find an identical 'scope on ebay or a used equipment place
    get it for spare parts
    if it turns on, but the 'scope doesn't work, you have a new power stage

    useless suggestion:
    when all else fails, go to destructive testing
    replace the fuse with a bigger fuse or a short
    this forces a component to be the fuse
    then hope that the component(s) that blow up are the bad ones,
    and they don't take other things with them
    this is not a good method



  3. #23
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    Re: HP 54520A Blown Fuse Problem

    The output voltages and the crowbar circuit should be checked first.
    There are 8 different voltages involved in the crowbar triggering. 7 of them can be adjusted.
    The +15.5V output can't be adjusted, and it is the voltage that is shorted by the crowbar.
    The purpose of the crowbar is to blow the fuse when there is an overvoltage.

    Edit: maybe R7 on the primary side should be used to adjust the +15.5V output.



  4. #24
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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by std_match View Post
    The output voltages and the crowbar circuit should be checked first.
    There are 8 different voltages involved in the crowbar triggering. 7 of them can be adjusted.
    The +15.5V output can't be adjusted, and it is the voltage that is shorted by the crowbar.
    The purpose of the crowbar is to blow the fuse when there is an overvoltage.

    Edit: maybe R7 on the primary side should be used to adjust the +15.5V output.
    What is the crowbar?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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  5. #25
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    Re: HP 54520A Blown Fuse Problem

    What is the crowbar?
    SCR A2, on page 2 and page 7.



  6. #26
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    Re: HP 54520A Blown Fuse Problem

    There is no R13 and R14 in my circuit, instead the line is connected directly to c3. So I think I am going to desolder the q3 as Brian suggested. Maybe disconnect R12 too.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



  7. #27
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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by betwixt View Post
    My best hint is don't follow this line of investigation. A component failing after a minute with 300+ Volts across it is unlikely to show a problem with a low voltage DVM probe. You would either have to replicate the operating conditions or just replace it anyway. Given their relatively low cost I think replacing is by far the best option. Even if not 'faulty' there is a good chance they are now out of spec after all those years.

    Brian.
    Well, I desoldered Q3 completely. I also desoldered R12. I also changed C22 and C23 with other used ones I had.
    The PSU still blows the fuse. So thankfully I hope it is something in the HV side and not related to the secondary. I say thankfully, because this is an easier fix.
    So first I will replace this cracked thermistor, before trying any other components pulls in the HV side.

    What's your thoughts, am I in the right path?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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  8. #28
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    Re: HP 54520A Blown Fuse Problem

    Measure the voltages over C22 and C23 before the problem happens. The center voltage should be stable and half of +V.
    If the leakage is too high, the center voltage can slowly drift away and cause problems after a while.



  9. #29
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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by std_match View Post
    Measure the voltages over C22 and C23 before the problem happens. The center voltage should be stable and half of +V.
    If the leakage is too high, the center voltage can slowly drift away and cause problems after a while.
    Ok, but I have replaced these caps with other used ones I had and it still blows the problem. So I do not think this is the fault. But thanks for the hint.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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