Lamp fixture causing RFI?

[Rob's Radio-Active]

I don't think he was meaning it needed to be grounded to shield your guitar from the lamp, I think he meant that if the housing is not grounded, it can radiate like an antenna.
That's what I think, too. Anyway, if you can get behind the socket, maybe you could install an "across the line" type safety capacitor to shunt out the harmonics pulses.
cheers
Rob

[g1]

Agree with Tom, the fixtures (entire fan assemblies) may be the same model, but it doesn't mean one of them isn't defective.

[AdmiralB]

Given all that has been said and tried how about the following as a next step?
Set up the conditions with the fixtures and bulbs that make one room bad and the other room good.
Then swap the complete fixture and associated bulb assemblies between rooms and tell us if the problem moves with the fixture or stays with the room.

Good idea, and now I'm getting somewhere.

Swapping fixtures left the noise in the room, i.e. the 'new' fixture exhibited the same issue.

The 'scope showed nothing unusual on the waveform.

But I tried something - I disconnected the neutral at the ceiling box, and tied the fan/lamp neutral to earth. And the noise vanished.

So somewhere between the box and the panel, I've got what I presume to be poor joint on the neutral.

[Tom Phillips]

...So somewhere between the box and the panel, I've got what I presume to be poor joint on the neutral.

Well that was a fast response. In my experience I have found many wiring jobs that included many wire nut joints between a given fixture and the breaker panel. Also some outright mistakes along the way. Best of luck to you in finding the ultimate source of the problem.
Cheers,
Tom