Noise for the boys

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!

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!!

More on the withering filters case: The TS-2000 disappearing noise conundrum

A few months ago, a friend gave me his TS-2000 to check, because occasionally there was a crackling noise coming out of the speaker (and showing on the S-meter) during SSB weak signal reception (SW8KOU operates EME on 2m).
However, when I checked the rig, the noise just wasn't there. I checked for bad SMD components or soldering, but found nothing. I returned it, the noise eventually came back even louder after a few days, he gave it back to me to check it again, and guess what - the noise was absent again. I gave it back to him again, and he reported there were sporadic outbursts of noise now and then.
Eventually, the noise came back to stay, and loud it was!! It resembled lightning noise, heard during a storm on a low HF band and registered S2 ~ S5 on the S-meter. This time the noise was kind enough to be there when I powered the rig up on the test bench. It could be heard on SSB only, not on FM (with the proper test setup). I had already previously checked the rig thoroughly for bad SMD components or bad solder connections, and, having seen many "withered" ceramic filters in the meantime (in other rigs), it suddenly dawned on me that the noise could very well be emanating from a faulty ceramic filter! The schematic diagrams showed two such filters in the SSB signal's path, CF3 and CF4 in the TX-RX unit.
To make a long story short, the culprit was CF4. The filter had suffered the same degradation I describe in another post, but in a more gradual way, due to different conditions. The other cases I have examined had about 7V applied to the filter's pins, but the circuit around CF4 applied about only 1.5V to it. The degradation was there, but the symptoms and time scales were different. I measured about 1MΩ from the output pin to the nearest ground pin, which was far more resistance than the usual 20 ~ 100 Ω I had previously measured in other malfunctioning filters. That explained the crackling noise and the fact that the receiver didn't go totally deaf, like in the other cases. The lower voltage was slower to act on the filter, and the degradation was milder. I opened up the filter, and, there you are, I could immediately see the effects of oxidation on the phosphor bronze spring plate. I removed the thin element at the output side, and sure enough, it showed the telltale signs clearly (see the photo, click on it to enlarge). After carefully cleaning the edges in the way I have described, the resistance reading was more than 40ΜΩ. I soldered the filter back in place, and the noise was gone. My friend tells me that the receiver is very quiet now (but let's see what happens when eventually CF3 (with 7V applied to it) breaks down, too!).
 I will shortly publish the solution to this vexing problem, consisting of just adding two DC-blocking capacitors at each filter (the example mod will be performed in an ailing TM-D710). The same simple concept, however, holds for the modification of all similarly affected rigs.