"I wouldn't be surprised if the noisy circuit has mounting holes on the
pc board for RF suppression components that they added to pass a
compliance test, then
left off in production."
No surprise there, if so. 20 years ago, I worked for a company
designing much of what we use to communicate online now. Prototype
power supplies were designed, and tested, exceeding specs. They were
reviewed state-side, found good, and put into production. Somewhere
around 10,000 - 15,000 units were ordered to be distributed with the
product as there was a considerable price drop at those numbers, far
less than the FCC/CE/UL certified units from Digikey, at under $3 a pop.
Later, it was found out why. ALL suppression, and filtering
components were removed. It was easier to send out the bad units, they
worked, than find a legitimate supplier in Asia. Eventually, the
company went under, and all assets auctioned off. God only knows where
the garbage SMPSUs were later used.
Kurt
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