Occasional hum

[CageyH]

I agree, hence my stubborn attitude to fix it.

[Macca]

I agree, hence my stubborn attitude to fix it.

I just don't have the energy to seriously tackle it right now. There are a few things I have not tried yet that might work.

I have music with no hum with the Philips, that's good enough for the moment.

Ground loops are the worst thing ever, so hard to solve. And even when you do fix them you don't really know exactly why the fix worked.

[CageyH]

I have been thinking about this during a quiet day at work.
There may have been a basic point I have overlooked.

The phono stage is built into an anodised aluminium case. The anodising has been removed to ensure a good contact is made for earth bonding.
However, the mating face of the case are still anodised. To get good EMI/RFI rejection, then ideally these should be metal to metal contact, so I may have to remove a spot locally on each joint to improve the shielding, or fitting a small bonding strap between parts of the case.
I shall investigate further.

Aluminium oxide is non-conductive, so removing locally it seems sensible.

[struth]

if it screws into the case then the screw should theoretically be a good conductor.

[CageyH]

Unless they are inserted into an anodised countersink, using anodised screws?

[Barry]

If they are self-tapping screws, then they will make an electrical contact. Even if the screws have an anodised thread, the act of screwing them in will tend to remove the anodised surface.

Is it possible for you to show a photo of the unit and the fixings?

[CageyH]

They are not self tapping. I will add a photo, but it is a bog standard Chinese aluminium case.
Countersunk M3 (or something close) screws.

[struth]

you could always re tap the hole. even same size should clean it if its got any on it... scratching inside around the hole may work a bit at least. Caustic Soda will remove it also.. also known as Sodium Hydroxide.

[CageyH]

This is very similar, if not the actual case - https://www.audiophonics.fr/en/diy-e...l-p-14211.html

[CageyH]

you could always re tap the hole. even same size should clean it if its got any on it... scratching inside around the hole may work a bit at least. Caustic Soda will remove it also.. also known as Sodium Hydroxide.

I have a set of taps in the garage.

At the moment, I am not touching it, as it is quiet, but I was thinking about why it was so difficult to resolve, and what could I do to shield it better, including ideas of copper lining etc. which should not be needed if I have good electrical bonding between the component parts of the case.