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HP/Keysight 66311B magnetic field leakage

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srb1954:

--- Quote from: Gulftown on November 21, 2023, 09:43:23 am ---So it must be the transformer.
I removed all secondary connections from the transformer and let it run with just the primary connected.
The power draw is around 3 watts in this condition and 16 watts with all the secondaries connected....
The effect is still the same.

--- End quote ---
Have you got a current probe of current transformer to measure the primary current?

An excessively spiky current might indicate that the transformer core is saturating. The permability of the iron would then reduce and there would be more stray magnetic field field outside of the core.

Another option is to use a variac to reduce the primary voltage a little and see if that reduces the stray field. A small drop in the primary voltage would significantly reduce any core saturation effects if present.

nctnico:
I think I have found the problem. There is a metal (steel / iron) shield missing around the transformer. Mine looks like this:

electronomicon:
I’ve got the same problem on different devices. For me, it only shows up in the combination of two similar HP/Agilent devices (HP 33120A and Agilent 34401A, either one of them) and one scope (HP 54645D). Not with my old Philips scope and neither any other than those two devices with the 54645D. It’s definitely very annoying.

I also did some investigation, like the thread starter, and it comes back to the mains transformer, too. I disconnected everything inside the 33120A, but as soon as the mains transformer has power and is close to the scope, the CRT starts wiggling around.
The transformer is open in both devices (see attached image) … should there be a shield around it that I’m missing?

I might try to wrap the transformer, to see if that helps. What thickness of copper foil would be advisable to use?

nctnico:
Copper foil won't do you much good as the problem is the magnetic field. You need to use a material which is good at transporting magnetic fields. Like iron or mu-metal.

In this thread I experimented with adding an extra shield around a transformer with good results:
https://www.eevblog.com/forum/testgear/korad-kel2010-multicomp-mp710771-review/msg5039434/#msg5039434

In the end I also moved the transformer away from sensitive circuits but the shield by itself improved the situation significantly.

Gulftown:

--- Quote from: nctnico on November 23, 2023, 12:08:11 pm ---I think I have found the problem. There is a metal (steel / iron) shield missing around the transformer. Mine looks like this:


--- End quote ---

Thanks for the suggestion, but the shield is normally there.
I just removed it to read the serial number and take a good picture of the transformer.


--- Quote from: electronomicon on November 23, 2023, 12:30:59 pm ---I’ve got the same problem on different devices. For me, it only shows up in the combination of two similar HP/Agilent devices (HP 33120A and Agilent 34401A, either one of them) and one scope (HP 54645D). Not with my old Philips scope and neither any other than those two devices with the 54645D. It’s definitely very annoying.

I also did some investigation, like the thread starter, and it comes back to the mains transformer, too. I disconnected everything inside the 33120A, but as soon as the mains transformer has power and is close to the scope, the CRT starts wiggling around.
The transformer is open in both devices (see attached image) … should there be a shield around it that I’m missing?

I might try to wrap the transformer, to see if that helps. What thickness of copper foil would be advisable to use?

--- End quote ---

When scrolling through the service manual it seems they used the same transformer in all versions HP, Agilent and Keysight, it seems weired, that no one noticed this in the more or less 30 years of manufacturing time.

Maybe mu-metal is the best option, but 30% reduction would not be sufficient i guess.
I also have an 34401A, i will try it near the scope, when i'm back at the bench.

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