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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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    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



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  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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    Re: HP 54520A Blown Fuse Problem

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



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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!



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  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!



  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!



  10. #30
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    Re: HP 54520A Blown Fuse Problem

    Hello neazoi,
    You need to replace the thermistor. It cracked for a reason.
    Thermistors are notorious for going short circuit.
    Your case is a rare one. They normally short immediately.
    After many years, or even within a shorter time, they do change their
    molecular structure and no longer work as they should. Varistors
    also fail, and sometimes can cause a fire.
    Make sure the 4A fuse is a time-delay type (Slow-blow).
    Regards,
    Relayer



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  11. #31
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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.
    Hi,
    I have disconnected R12 and Q3 (isolating the input of the PSU completely) and the fuse is still blown.
    So the problem is somewhere on the input side of the PSU.

    Now, I have replaced the thermistor with a new one of the same type, although the new ones I bought seem that have a slightly lower diameter disc. Fuse is still blown.
    I have replaced the C22 and C23 with new ones just in case. Fuse is still blown
    I have replaced C1 and C51 (the caps near the input common mode choke) with 1.5uF instead of 1uF. Fuse is still blown with a bigger explosion (broken glass). At this point I think I have blown the thermistor (I am not sure) because it opened at half. Since this was not a capacitor problem , I reinstalled the original 1uF caps there.

    At this point I need to ark are the materials in the thermistor really toxic especially to breathe, since I accidentally blown it? I was in another room during the big bang, but I came back after the explosion.

    Then I replaced the C3. fuse is still blown.


    The only components that I have not replaced is the bridge and the varistor. But I measure the bridge in circuit and it seems right. Could one of these this cause the problem of blown fuse after 1 minute or so?
    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!



  12. #32
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    Re: HP 54520A Blown Fuse Problem

    Try to connect an old-fashioned light bulb instead of the fuse. If you are lucky the light bulb will stay off until the problem happens, and it will turn on when the fuse normally blows.
    If it stays on, you can try to find the current path by measurements.



  13. #33
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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by std_match View Post
    Try to connect an old-fashioned light bulb instead of the fuse. If you are lucky the light bulb will stay off until the problem happens, and it will turn on when the fuse normally blows.
    If it stays on, you can try to find the current path by measurements.
    I think is is the bridge rectifier. After I deslodered it, no fuse was blown. I have replaced all other parts except the varistor and the fuse was always blown. So it must be the bridge.

    This is the first time I have come across to a bridge that delays to fail, even under no load!
    That's new to me.
    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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