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 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?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'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.
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/54520A/661548/detail.aspx
Am I right in thinking the scope works properly until the fuse blows?
Brian.
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.
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:
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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