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    HP54520A problem, first hints

    My HP54520A scope is breaking down... After a few minutes of activity it burns the mains fuse (4A 250V). Before I open it for a visual inspection, do you have any hind what might be the problem?

    Maybe something with the PSU, when it is heated?
    A leaky PSU electrolytic? (how to check that?)
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    Re: HP54520A problem, first hints

    Quote Originally Posted by neazoi View Post
    My HP54520A scope is breaking down... After a few minutes of activity it burns the mains fuse (4A 250V). Before I open it for a visual inspection, do you have any hind what might be the problem?

    Maybe something with the PSU, when it is heated?
    A leaky PSU electrolytic? (how to check that?)
    I did a self test on the menu. All self tests are passed ok.
    The problem blows the fuse after a few minutes/tens of minutes so I cannot test anything.
    Any speculations what might cause such a problem?
    I bet it is something with the PSU
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    Re: HP54520A problem, first hints

    Some HP instruments have a mains input filter that with age can become internally conductive and blow the mains fuse.
    However, when this happens there is normally bad smelling gas coming out of the filter.
    The solution to that problem is to replace the mains filter. I found one at Mouser for my HP signal generator.


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    Re: HP54520A problem, first hints

    Quote Originally Posted by std_match View Post
    Some HP instruments have a mains input filter that with age can become internally conductive and blow the mains fuse.
    However, when this happens there is normally bad smelling gas coming out of the filter.
    The solution to that problem is to replace the mains filter. I found one at Mouser for my HP signal generator.
    I did not notice any smell though. How can I check this filter for being conductive? Disconnect it from the rest of the instrument and check for conductivity between the mains terminals?
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    Re: HP54520A problem, first hints

    I've seen that in my HP function generator too. The easy check is to temporarily disconnect it and bypass it. They tend to fail under stress and measure fine at low voltage. I would agree with the PSU diagnosis, does that model have a cooling fan and if so, is it free to spin?

    Brian.
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    HP 54520A Blown Fuse Problem

    Hi I continue the problem from this older thread
    https://www.edaboard.com/showthread....em-first-hints
    Please read this thread first.

    Now I have removed the mains RFI filter which is the one shown in the picture.

    Shall I plug the filter in 220v and switch in it's power switch? (included in the filter)
    Or the blown fuse problem may happen only under load conditions?

    Is there any way to check this filter otherwise, (the components inside)
    I am willing to open it, is there any toxic oil inside I should be aware of?

    One thing that worries me is that the switch is connected after the filter, so if there is a problem in the filter itself should the fuse be blown? see the schematic onto the filter.
    Last edited by FvM; 30th July 2019 at 16:58. Reason: Merged with previous thread
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    Re: HP54520A problem, first hints

    Quote Originally Posted by betwixt View Post
    I've seen that in my HP function generator too. The easy check is to temporarily disconnect it and bypass it. They tend to fail under stress and measure fine at low voltage. I would agree with the PSU diagnosis, does that model have a cooling fan and if so, is it free to spin?

    Brian.
    Brian, I bypassed the RFI filter module shown in this thread. Again after a minute or so, the fuse (I have added in the bypass circuit) blown away. So it is not the RFI filter.

    What else could cause such a behaviour? Can it be a big electrolytic in the PSU module? Could this short circuit after a monute or so? What other components can cause such a behaviour?
    Professional engineering is the top, but amateur engineering is more fun.
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    Re: HP 54520A Blown Fuse Problem

    is the fuse the right size?
    is the input set to the correct line voltage?
    (i blew up an HP Arb by forgetting that detail and running it on 220 when it was set to 110)
    how is the 'scope grounded compared to the grounds on the probes?

    this might help
    https://www.servicemanuals.net/HP/54...48/detail.aspx



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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by wwfeldman View Post
    is the fuse the right size?
    is the input set to the correct line voltage?
    (i blew up an HP Arb by forgetting that detail and running it on 220 when it was set to 110)
    how is the 'scope grounded compared to the grounds on the probes?

    this might help
    https://www.servicemanuals.net/HP/54...48/detail.aspx
    Yes the fuse if of the right size (4A)
    The line voltage is set correctly.
    I have the service manual but this does not even have schematics!

    As said, the scope works ok for a minute or so then the fuse is blown.
    Just to mention that the scope was working without any problems a few months ago.
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    Re: HP 54520A Blown Fuse Problem

    The service manual is attached but it doesn't show a full schematic.
    Am I right in thinking the scope works properly until the fuse blows?

    Brian.
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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by betwixt View Post
    Am I right in thinking the scope works properly until the fuse blows?
    Brian.
    Yes, correct. The scope works and measures ok until the fuse blows after a minute or so (variable). I noticed that in the SMPSU and in the CRT PSU there are two separate fuses, which are not blown. Maybe the mains fuse is blown before they do I do not know. Although not recommended I have replaced the mains fuse from 4A to 5A, just to see what happens. And the fuse is blown away too.
    The next step I am thinking is to remove the PSU case and measure the big electrolytics. I think this was a common problem with SMPSUs and that is why I hate them, they always fail at some time.
    But before I do so, and to avoid having to buy an in-circuit tester (pulse) I would like your advice of what you would suspect that causes this blown fuse, so as to try to locate the problem. Any advice will be really helpful. Ii is too bad to throw this away just for a PSU issue (I think).
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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by betwixt View Post
    The service manual is attached but it doesn't show a full schematic.
    Am I right in thinking the scope works properly until the fuse blows?

    Brian.
    I have verified that the main PSU is the problem. I connected the PSU to the mains but disconnected all the output cables from the PSU that power the oscilloscope. The mains fuse still burned out after a minute or so. I have also bypassed the mains RFI filter and the result is the same.
    So definitely the problem comes from the mains PSU module.

    Now, what components you suspect should I check, having in mind that the scope is fully working for a minute or so?
    Visually I see no damage with a quick inspection.
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    Re: HP 54520A Blown Fuse Problem

    Just a WAG - Capacitors across the supply lines that have gone leaky?
    Susan


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    Re: HP 54520A Blown Fuse Problem

    That would be my guess too Susan.

    The fuse looks to be in the AC input to the PSU and as a fault in the output side would probably result in shutdown rather than a blown fuse, my suspicion would be the input filter up to the two big black reservoir capacitors.
    It doesn't appear to have a PFC circuit so there is probably a direct connection from the big transistor's collector, through the transformer to the positive side of one of those capacitors. See if you can break the link or failing that, isolate the transistor collector connection (NOT the base or emitter unless you can do both!) or remove it completely and see if the fuse still blows. If it does, move step backwards and see if you can isolate the bridge rectifier by removing it or unsoldering both AC legs.

    Brian.

    update: I found this on the internet, please note the caution on it. It looks promising it is the same PSU:
    Last edited by betwixt; 7th August 2019 at 07:32. Reason: added attachment
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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by betwixt View Post
    That would be my guess too Susan.

    The fuse looks to be in the AC input to the PSU and as a fault in the output side would probably result in shutdown rather than a blown fuse, my suspicion would be the input filter up to the two big black reservoir capacitors.
    It doesn't appear to have a PFC circuit so there is probably a direct connection from the big transistor's collector, through the transformer to the positive side of one of those capacitors. See if you can break the link or failing that, isolate the transistor collector connection (NOT the base or emitter unless you can do both!) or remove it completely and see if the fuse still blows. If it does, move step backwards and see if you can isolate the bridge rectifier by removing it or unsoldering both AC legs.

    Brian.

    update: I found this on the internet, please note the caution on it. It looks promising it is the same PSU:
    Always amazingly helpful! :)
    I also suspected the primary side of the PSU. Something overheated, a faulty component that fails after heated, a leaky capacitor, things like this. Problems related to non working at all, wrong output voltages, or lower voltages under load would lead me to the secondary side of the PSU, or to the feedback (opto) circuit.
    I also replaced the mains (inside the RFI filter) fuse with a 5A instead of a 4A (nimimal), although not recommended. Then the internal fuse of the PSU was blown and the in-RFI-filter fuse started to deteriorate but not blown.
    Thanks so much for the help, I will keep you updated

    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
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    Re: HP 54520A Blown Fuse Problem

    There's apparently no overcurrent limit or shutdown on the primary side. A short on the secondary may probably cause input overcurrent. I wonder if triggering of the crowbar SCR is the final cause.

    PSU failure statistic suggests dryed out electrolytic cap as most likely reason, highly stressed semiconductors (rectifier, linear regulator, power transistor) are next candidates.


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    Re: HP 54520A Blown Fuse Problem

    With this type of fault, you will have to reduce the circuit to find the fault. first try removing R13 and R14 and see if the fuse still blows. If not reconect R13 R14 and disconect CR24, and see if the fuse still blows then disconect CR25, CR26, CR27 and then CR31 to CR34 in turn checking if the fuse still blows. This can get expensive in fuses so what I do is get some fine stranded multi core cable and solder a single strand in place of the fuse.



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    Re: HP 54520A Blown Fuse Problem

    Quote Originally Posted by FvM View Post
    There's apparently no overcurrent limit or shutdown on the primary side. A short on the secondary may probably cause input overcurrent. I wonder if triggering of the crowbar SCR is the final cause.

    PSU failure statistic suggests dryed out electrolytic cap as most likely reason, highly stressed semiconductors (rectifier, linear regulator, power transistor) are next candidates.
    I think I am going to check the mkp caps and the big electrolytics first. First in circuit then desolder them and check their capacitance and resistance with a multimeter. But is would be wiser to trace the problem by removing R12, R13 and R14 (assuming these are there on my PSU) which in fact isolates the HV side from the rest of the PSU. If the fuse is still blown this would be an easy fix. If not I will proceed with the rest of suggestions in this thread.

    UPDATE
    Ok here is the circuit and here is the input thing. The silkscreen under it says "RT1". This is also in mat finish resign, so I guess it is a thermistor.

    The labels on it are not very clear but it says
    UEI
    10SP 010

    I am not sure if these are zeros or Os.

    The c rack is shown in the red circle. I do not know if this would cause such a behaviour (blow the fuse after a minute) but it would be worth it to replace as it seems damaged.
    Any ideas of a suitable replacement?

    - - - Updated - - -

    Is it this part?

    https://www.ebay.com/itm/10pc-NTC-Th...gAAMXQya1Q6iy7
    Last edited by neazoi; 7th August 2019 at 16:12.
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  19. #19
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    Re: HP 54520A Blown Fuse Problem

    RT1 is NTC for inrush current supression. If it's broken and connects and disconnects fastly, it may possibly cause problems, but the possibility seems rather unlikely to me.



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    Re: HP 54520A Blown Fuse Problem

    It is unlikely that that part would cause the problem of the fuse blowing after a minute. It is a inrush suppressor so that after a minute its job is done and no possible fault with it will cause the fuse to blow.



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