Fallen off metal part inside of HP 8921A

[EIKA]

Hi guys!

I bought HP 8921A and was fallen down to concrete floor just by one of corners. Cabinet got some small bent at the bottom corner (God save the HP's designer of rubber "legs"!), but device passes all tests and in specs, LOL. HP rulez forever!!! 

But there is a one small part become free and rattled and knocked inside of device.
I dissembled the device and took it out. Here is how it looks like:





It's metal disc D=35 mm, probably stainless. One of the sides is polished and shiny, but another one contains self-adhesive glue (what become a bit dry for years).

Questions:

1. What is this?
2. Is it genuine HP part?
3. Is it critical one / what function did it perform in the device?

[ttx450]

I know nothing of the HP unit, but looks like water jacket freeze plug.  which seems odd in a electrical device, as that type could come lose easily.  thats probably why you see glue on it, guessing it is not original.  is there a calibration area that is accessible in size you stated?  those kind of plugs have perforated sides that snap in place, not sure what they are called.  I am wondering if this is whats going on?

[EIKA]

It cannot be water jacket freeze plug, because it's flat, less than 0.5 mm of thickness. And device was never disassembled before. There are no holes in cabinet that can accept so huge part other than Memory card slot at the front. But disc was found on the back panel, where are vent perforation is located.

[ttx450]

there are many plugs that are flat.  I installed to many to count that are flat, usually with slight concave.  maybe someone that has worked on one will catch the thread.. good luck

[Gyro]

It looks like it might be a panel knock-out for something optional. The glue might be from a label.

Try looking / feeling around to see if there is a label with a soft area or non obvious hole behind it. It's too thin for an outer panel, but maybe an internal screen / subassembly?

[TheSteve]

I don't remember seeing such a plug on any of the internal assemblies. As others have stated it looks to be a knockout of some sort, you'd think from the back panel. If you can find an obvious spot where it came from you can reinstall it, if not I wouldn't worry too much.

[MikeK]

Yeah, looks like a panel knockout.  A photo of where it came from would help.

[EIKA]

100% that it's not panel knockout. It's stainless, with self-adhesive layer and have no small remaining parts of metal that were used to hold on panel.

Also, I see traces on the glue from fastening to a pipe or something similar.

Looks like it was a end cap for something.

It it doesn't look like HP Test Set internal part, I can suppose, that some idiot or just kid put it through Memory Slot.

[ddrl46]

This metal disc may have been a shield for the fan, see attached PDF, page 105:
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/HP/8920A,%20B,%208921A.pdf

The cooling fan on all 8920As with Option 050, and 8920Bs, and 8921As has a small
metal disk mounted for magnetic spurious shielding. This metal disk has been found to
come loose or fall off in some early production instruments. A service note has been
issued to cover this particular failure (Service Notes: 8920A-12, 8920B-03, 8921A-05).
The Test Set should be checked for proper installation of the fan disk. RF Assemblies
A16, A17, and A18 may require removal to inspect the fan. Refer to the Repair section of
the manual (see "AF, Digital, and Most RF Assemblies," in chapter 3, on page 75) for
information on assembly removal and installation. If the fan disk is missing the instrument
should be very carefully checked to see if the disk has become lodged elsewhere. Electrical damage can occur as a result of a loose disk. The fan disk can be replaced by ordering
part number 08921-00003.

[Gyro]

That looks like a pretty conclusive 'watch out for the metal disc that we accidentally let float about inside' statement.

Presumably it must be made of stamped Mu-metal (don't flex it) to have any effect on the magnetic field from the fan without enclosing it. I'm surprised that anything that small can have a measurable effect.

[EIKA]

Thank you very much, really appreciate your input.

This is exactly what I looked for. It explains everything!

Funny story.