Pwr supply noise problem

[conve36]

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I modified a black & decker 12.5v 6amp power supply to power my radio equipment at home. All I had to do was put a new end on the power cable. Now my problem is when I have the power supply plugged in, my scanner (which is a dedicated highway patrol scanner live feed on VHF-low) will stop on a frequency and just puts out static. As if the squelch was open, but I have it turned all the way closed. As soon as I unplug the power supply it goes away. If I run the scanner on battery power the noise is still there. But whn I unplug the antenna it goes away. I've tried several antennas. Moved the power supply accross the room, with no luck. The power supply is made to power those portable 12v coolers, but works great for powering my 40w radio! Theoretically speaking, if I was to wrap the power supply in aluminum foil, would that shield it? Or a metal box maybe? Thanks!

 

[davidmc36]

Does it have a two prong plug or a three prong one. Maybe it just needs a ground. If it was made for running a cool-a-tron I expect it may not have much for shielding/filtering. May even have a simple rectifier in it and if you put the 12v DC on an oscilloscope it may have a lot of noise along with it.

 

[kandrey89]

Uhmmm,
I won't pretend I've ever dealt with noise problems. but here are some things you should think about.

The battery is a DC power supply, it is in fact a constant power supply source much better than a AC-DC power supply block because of the way it pumps energy. As far as I understand radios, the source of the noise could be coming from 2 places based on the power supply.
The power supply is emitting RF interfeerence:
1. causing the RF translation through the scanner radio's power supply port
2. causing the RF translation through the media(air) onto the receiving antenna

You did not mention what happens to the scanner when the RF interference exists, and you use a different power supply source. Does the scanner still pick up the interference? Are you absolutely sure that you have eliminated all the components and that the battery supply IS in fact the only component that when connected is causing the interference?

I'm not sure if this will work or not, but in theory it should, if the power source is fluctuating and thus causing RF interference, then the Volt Meter in AC mode should read something, not be = 0 or ~0.0x V. Although it's possible that cheap volt meters might not read this or read it correctly since the frequencies at which you are reporting interference is much higher than the 60 Hz AC which is what the volt meter is tuned for. Oscilloscope is a better tool for measuring line interference.

I don't think I've ever heard of a battery producing such results, since a battery is a chemical source of power, and is by nature very linear and VERY DC, as oppose to AC.

 

[Kennrth]

Based on what you have stated it appears that the noise source is not the Power Supply. It is most likely nearby because it can be both conducted and emitted. It is getting into your radio via the antenna and the power line.
While on Battery try to sniff out the offending device. If you are lucky it maybe in your home. Then you need to stop it at the source.
It is most likely an electronic device plug in to Ac power outlet or a light dimmer control. It may be within a few houses from your home.

The third prong if there is one is earth ground and will not function as an rf shield. In fact isolating the earth ground would most likely lower the noise level since the earth ground will conduct rf.

You can purchase an isolation transformer which isolates earth ground and is intended to block high frequency noise. And keep the power cable to the radio short as possible.

You can swamp the noise pretty effectively by terminating the DC Power Supply End (radio side) with 2 types of capacitors. A 10uF Electrolytic (observing polarity) With voltage rating 3x expected dc. This will smooth out low frequency ripple and switching noise. And add a 1000 pF 50V ceramic capacitor in parallel with the 10uF which will bypass part of the high frequency component. Would take some soldering and wire splicing.

You can contact these people and explain where in the spectrum your interference is from and they can recommend an inline dc rfi filter
Corcom - Filters: EMI/RFI Filter Technology and Filtering Solutions from Tyco Electronics

There is one worst case scenario. Your radio itself maybe generating the offending frequency internally called a Birdie. By adding an antenna or external power supply it becomes regenerative. There is nothing that can be done for this. It is a product of the radios mixers.

 

[zz0468]

If I'm reading your post correctly, you have this noise problem whenever the power supply is running, regardless as to whether it's actually powering the radio or not. And removing the antenna from the radio removes the noise.

So... the noise is entering the radio via the antenna. It's affecting low band ONLY? A Black and Decker power supply??? Is it a switching supply?

Poorly designed switching supplies (read that as supplies not designed for communications equipment) frequently have had no effort made to reduce or eliminate the plentiful noise that switchers tend to radiate.

You might try putting ferrites on both the AC cable, and the DC cable coming from the supply. The interference could be radiating from the cables themselves.

Another solution would be to chuck the crappy supply, and buy a higher quality supply intended for powering communications equipment.

 

[rfradioconsult]

If the power supply is a "universal type" in other words will operate on 110 to 240 Volts; this type of supply are VERY bad about generating RFI. The cheapest solution is to buy a proper power supply and quit trying to fix something that most likely cannot be easily fixed.

 

[conve36]

The power supply works PERFECT with no problems at all powering my Motorola Convertacom and 35w power amp. Everyone tells me my audio is crystal clear with no feedback at all. Again, the only problem is it seems to radiate into my low-band scanner (CHP scanner).

What I have tried was to run an extension cord all the way accross the house and into the garage but it made no difference. What I am about to try is keeping the power supply accross the house and run the power cable from the supply accross the house and into the radio equipment and see if that helps.

I had a $70 power supply from radio shack that I took back when I found this power supply at walmart on clearance for $9 and works just as good if not better. (Besides the interference, which hopefully will fix)

 

[davidmc36]

I had a $70 power supply from radio shack that I took back when I found this power supply at walmart on clearance for $9 and works just as good if not better. (Besides the interference, which hopefully will fix)

Soooo, are you saying that everything was fine with the $70 unit from Rad Shak, but the interference started when you got the $9 unit from Wally World?

 

[conve36]

Soooo, are you saying that everything was fine with the $70 unit from Rad Shak, but the interference started when you got the $9 unit from Wally World?

Yes. But again, it works great with no noise at all to anything BUT the CHP low-band scanner. Also, this scanner is not capable of PL tones, or that would fix my problem.