[TenTec] [] Running a Century 21 on external 12V power

[Carter]

On 6/7/2017 8:31 PM, Josh Gibbs wrote:
> Just when you thought this thread was done... some interesting new
> information!
>
> It seems that Ten-Tec started adding in additional shielding for the
> transformer at some point. Mine does not have the shield!
>
> For reference here is one with the shield:
>
> https://drive.google.com/open?id=0B_pjAwEfeYcdWTNrUUhPQlk4ZlU
>
> And here is mine:
>
> https://drive.google.com/open?id=0B_pjAwEfeYcdOUdHVW5VUlJmcVk
>
> I'm going to make up a shield like this with some aluminium and see if it
> helps with the hum.

Aluminum is non-magnetic, so not sure if it will do what you expect. I 
thought you had to use iron or mu metal (used to shield scope tubes) for 
magnetic shielding. From Wikipedia:
*Mu-metal* is a nickel <https://en.wikipedia.org/wiki/Nickel>–iron 
<https://en.wikipedia.org/wiki/Iron> soft magnetic alloy 
<https://en.wikipedia.org/wiki/Alloy> with very high permeability 
<https://en.wikipedia.org/wiki/Permeability_%28electromagnetism%29>, 
which is used for shielding sensitive electronic equipment against 
static or low-frequency magnetic fields 
<https://en.wikipedia.org/wiki/Magnetic_field>. It has several 
compositions. One such composition is approximately 77% nickel, 16% 
iron, 5% copper <https://en.wikipedia.org/wiki/Copper> and 2% chromium 
<https://en.wikipedia.org/wiki/Chromium> or molybdenum 
<https://en.wikipedia.org/wiki/Molybdenum>.

The high permeability of mu-metal provides a low reluctance 
<https://en.wikipedia.org/wiki/Reluctance> path for magnetic flux 
<https://en.wikipedia.org/wiki/Magnetic_flux>, leading to its use in 
magnetic shields <https://en.wikipedia.org/wiki/Magnetic_shielding> 
against static or slowly varying magnetic fields. Magnetic shielding 
made with high-permeability alloys like mu-metal works not by blocking 
magnetic fields but by providing a path for the magnetic field lines 
<https://en.wikipedia.org/wiki/Magnetic_field_line> around the shielded 
area. Thus, the best shape for shields is a closed container surrounding 
the shielded space.


    Uses and properties

Mu-metal is used to shield equipment from magnetic fields. For example:

  * Electric power <https://en.wikipedia.org/wiki/Electric_power>
    transformers <https://en.wikipedia.org/wiki/Transformer>, which are
    built with mu-metal shells to prevent them from affecting nearby
    circuitry.
  * Hard disks <https://en.wikipedia.org/wiki/Hard_disk>, which have
    mu-metal backings to the magnets found in the drive to keep the
    magnetic field away from the disk.^[/citation needed
    <https://en.wikipedia.org/wiki/Wikipedia:Citation_needed>/]
  * Cathode-ray tubes <https://en.wikipedia.org/wiki/Cathode-ray_tube>
    used in analogue oscilloscopes
    <https://en.wikipedia.org/wiki/Oscilloscopes>, which have mu-metal
    shields to prevent stray magnetic fields from deflecting the
    electron beam.

73,

Carter   K8VT