The AM Forum

Most of us home brewers just grab a transformer or a choke and mount it to the chassis. Every transformer or choke I have seen, the bottom of the feet was painted. Does any one take the time to scrap of the paint before mounting. I know, the bolts and nuts will ground the case but ARE YOU SURE. Nuts will loosen up in time. The reason I ask is I had a pretty heavy piece of mod iron and had a complaint that there was a hum in the audio. Last resort was to clean the feet and remount, HUM GONE. Just thought I would throw this out there and see if any of it would stick.
Regards,
Gary...WZ1M

That is way too simple and only a transformer guy would think of grounding the shield can.

The real question is:

Do you TREAT that connection against rust/corrosion?

Especially the dissimilar metals, like screwing copper to aluminum...

Of course steel to steel (or Tin) as well...

Thanks for the info.  I will be replacing an old transformer with a new old stock one that has never been used in my NC-183.

I don't see how an ungrounded modulation transformer could possibly induce hum in the audio, unless some other component is grounded to the transformer, trusting its case to serve as a grounding point.  Better to let other components have their own dedicated ground points.

Mine is permanently ungrounded.  I have it mounted on insulators so that the transformer core electrically floats free.  I usually float the cases of modulation transformers, modulation reactors and filter chokes, particularly if they are operating at more than about 40-50% of their rated DC working voltage.  Since the winding is at full HV DC potential, grounding the core puts unnecessary strain on the internal insulation, which may be fragile old paper that has deteriorated with age over the decades. Insulation breakdown between windings or from winding to case is a common problem with ancient iron. Just make sure never to touch a floating transformer case while the HV is on.

Possibly a poor connection, through the paint or rust, was somehow causing the problem. Maybe some kind of weird rf coupling was the culprit. I have seen inexplicable hums and buzzes go away after flaky but seemingly unrelated connections were repaired.

I do always ground low level audio input, interstage and output transformers.  Sometimes transformers have a separate grounding terminal so you don't have to rely on the mechanical mounting hardware on the transformer.

Just a thought, but perhaps that xfmr had an internal electrostatic shield between the primary and secondary windings, although I have never seen the use of such a shield in hi-level (i.e. > a few watts) audio magnetics.

Electrostatic shields are common in low-level audio xfmrs, such as microphone and line-level input and interstage units, and power xfmrs as well. These shields effectively isolate the two windings from capacitively coupling to each other, and when an electrostatic shield is provided, it must be connected to ground to realize the full capability of the xfmr. The shield is of further benefit in power xfmrs, in that a lot of the noise present on the AC line is isolated from capacitively coupling to the secondary.

Just my 2 cents.

73,

Bruce

On my big rig ALL the Iron is mounted on a 1 inch thick wood board.  No iron goes to Ground. This way, You cant arc over to the case of the iron.

C