Bob & Plato bring up some interesting points. On top of what they stated about the weight of a steel chassis, one also has to think about rigidity and overall strength. When one looks at the amount of posts on the web about goods damaged in shipment, the thought of a transformer breaking free from a soft metal chassis (like aluminum) or the possibility of that same type of chassis twisting / denting far more easily from rough treatment starts to sink in. I do agree that this would act as a "reasonable" shield AND minimize reactance to the internal magnetic field though.
As to Greg's comments, sorry if i was unclear. I have to remember that this is an audio forum and not an RF forum. As such, my explanation and experience has been with the low level circuitry in RF gear being overloaded by high level RF that was emanating from within the same chassis. By removing the cover(s) on some high power amplifiers and transmitters, the RF is allowed to "leak" into the atmosphere. The high level signal is therefore less concentrated near the low level circuitry that was being driven into oscillation and the problem was fixed ( kinda sorta ). Sorry for the confusion, as i was coming from an "i'm still at work" point of view.
This does bring up some valid points about how equipment is designed and tested though. Since engineers and technicians basically build and test everything with the lids removed for ease of access and being able to take test measurements, it is QUITE possible that equipment could measure differently with the lids on. Being able to get around this and take "accurate" measurements would take some real ingenuity on the part of manufacturers.
As i have stated before in other threads pertaining to similar matters, one manufacturer had a product that ran like gangbusters until the lid (which was metal) was slid into place. Once this was done, the amplifier would do some REALLY squirrelly things. Rather than go through, redesign the amp, change all of the parts in production, etc... they simply went from a metal plate to a hard plastic plate. Besides being cheaper for them to produce, it took care of the instability problems.
As such, Greg is correct in stating that the lid of your audio gear would normally act as a shield and keep rf out. Removing it would expose the internal circuitry to whatever interference happened to be floating in the air. This might or might not be a big deal, depending on where you live and how many "good buddies" you had driving around the area. Sean
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