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| Fig. 1 Can you spot the missing part? (click to enlarge) |
A few days ago, the digital TV broadcasts commenced (at long last) in my area, so I promptly installed a digital receiver and enjoyed perfect reception of those long awaited, crystal-clear TV signals. My initial joy vanished in a moment, though, when to my great dismay I found out that the digital receiver completely trashed my reception on HF with S9++ raucοus buzzing garbage all over the bands. I confirmed that the culprit was indeed the shiny new DVB receiver, by disconnecting it from the mains supply - then I proceeded to examine the entrails of that stinking rat. Not many surprises there - the usual, dirt-cheap switching power supply, where several corners have been brutally cut to keep the dreaded cost down. Take a look at the first picture (Fig. 1) - do you see anything missing? As the magnanimous host I am widely reputed to be, I have added a red arrow to help you figure out the answer - no frigging line filter at all, the one and only position for such a component had been jumpered in the most barbaric way. So the harmonics of the power oscillator that makes up the heart of the power supply are very efficiently radiated from the mains power lines - and I can assure you that those signals, although extra strong, don't carry any intelligence at all!
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| Fig. 2 The upgrade in place! |
This is a very sad story indeed, causing serious grief to many unfortunate radio amateurs who suffer dreadfully degraded reception by the scores of big and small switching power supplies that infest the modern home (perhaps you have read my relevant QST 7/2004 article about "dirty", dirt-cheap computer switchers).
Those power supplies are very cheap, but in this case cheap comes at a high price for radio enthusiasts who seek joy in the form of distant, low level signals. What's a young man (or lady) to do, then?
Other than replacing the offending switcher with a better quality one, which oftentimes isn't feasible at all, you could add the missing line filter elements yourself. Old computer power supplies will yield the necessary components for the tinkerer to lash-up a common mode line filter (Google it up and learn, don't expect everything on a platter from me!) - unless they're also rats themselves.
Take a look at Fig. 2 (click to enlarge), where the filter has been added to the rat, transforming it to something infinitely more tolerable. The toroid came from the line filter of a good, old computer switcher and worked fairly well, reducing in a single step the trash to about S2 - S3 worst case across the HF bands. (Someday I might get down to improving this even more.) The toroid has a bifilar winding, with each wire connected in series with each of the the mains conductors, just before they exit the case.
Disclaimer: This mod requires working with dangerous mains voltages. Be cartain that you're up to it before tampering with a switching power supply, they definitely don't excuse ANY kind of mistake, and may immediately violently explode to show that fact!!
