[sdiy] Shielding plastic cases

[Mike]

> > One layer of aluminium foil with some adhesive copper tape along
> > the edges and across the middle was enough to prevent the ignition
> > from interfering with the car radio. Not totally effective, but
> > good enough to let us enjoy the programme. I'd say the attenuation
> > was on the order of 30 dB.
>
> I agree, I think this is probably worth the small effort to do it. My
> old Dr Rhythm drum machine had its PCBs wrapped up in this way, so
> someone at Roland thought it was worth it too!
>
A lot of this is so that the unit will pass the EMC regulations, by not
emitting a lot of digital noise, and having heard the effect that some early
digital cellphones had on a friends hearing-aid, I quite agree too.

There are some very good conductive paints made for the job. HP used this on
the inside of the venerable HP85 desktop, as an example.

I get quite a laugh by the antics of some guitar effects friends. They
carefully shield the box, and then hang a long aerial, (cheap guitar lead),
and a great magnetic pickup on the end! One of the lads was quite amazed by
how much extra top end he got, when I made and fitted  him one of the Till
FET preamps into his guitar. He thought it was a treble booster - it was
actually the low output impedence of the Till cancelling out the effect of
the Hi Z pickup and the 300pF/Metre of his guitar lead. I added some tiny
caps and a ferrite core - at one of his gigs he regularly got a local taxi
radio coming out of his amp. More baffling was a  mystery regular clicking
noise. He's had an injury to his left arm, and temporarily moved his watch
to his right wrist. The mechanism is a quartz driven tiny stepper motor -
the pickups were truthfully receiving the magnetic pulses

Which neatly brings me to another point: just shielding the box is often not
enough.

Mike