Railfan Technology > power supply interference

I have the Radio Shack PRO-107 scanner and it performs reasonably well. When I have it hooked up to a power adapter, whether AC or DC, I notice that there seems to be a fair amount of noise/interference that causes the unit to stop scanning on a channel that is just noise. This does not happen when running on batteries. The problem does go away when I adjust the squelch, but then I would be reducing the sensitivity of the unit.

Any recommendations? Some sort of filtered power supply? I generally run it on NIMH batteries, but like to listen when I am recharging them.

Thanks for any suggestions.

Interesting problem but as often is the case, there's not enough information to answer your question properly. My first question would be if it was a hum or a static type noise. I'll take a few stabs at it anyway.

If it's a hum, lets try this route. When you say power supply, are you using a "wall wart," one of the transformer you plug into the power outlet? If so, you might try finding another that has the same voltage and current capabilities. Also make sure the plug has the same polarity as the one you are using. Normally it's positive center conductor and negative shell, however I have seen some that were opposite. Also, make sure that any wall wart has the current capacity required for your scanner. Exceeding capabilities of the supply will cause poor regulation and filtering. Also, make sure that it does have a DC output, not AC. There are a few of those around. Another possibility would be to buy a low power (2 to 5 Amp) 12 volt power supply. That would be a more expensive option.

If it's a static type noise, you might try this route. Again, the power supply could have a problem. Not knowing what type power supply you have, it might have a switching type regulator in it and insufficient or defective filtering. Try a different supply. Depending on how bad the problem is, you might be able to put a filter on the power leads. RadioShack (and others) have clamp on ferrite filters (Catalog #: 2730105), normally $5.99 but online it shows $3.00. I worked at the phone company and occasionally we had customers that had equipment that was susceptible to strong radio signals. We would usually put a filter on the line at the side of the house or business and when we tested at the demarcation point, the problem was gone. Often the customer still had a problem. The problem was the house wiring serving as an antenna and we couldn't do anything about that. On several occasions I suggested they obtain these filters and usually it would clear up or reduce the problem. If you apply one of these, make several turn of the wire from the power supply through the filter. The more, the better. Put the filter as close to the radio as you can. RadioShack calls it a "Snap Together Ferrite Choke Core"

That's a few things I can think of. Just make sure all connections are good an tight if you have access to any of them. Hope one of those fixes you problems.

John

To elaborate on wa4umr's advice.

Some things to try:

I'd put your scanner on AM mode as it is easier to detect interference
than FM. Leave the frequency band the same but select a channel where
there is no radio traffic. Leave the volume at a level where you can
clearly listen for a reduction in static.

Make sure the radio does not move during the test.

Cheap and dirty:

Try wrapping the power cord in aluminum foil, being careful not to
get the aluminum foil near the prongs of the power supply.


Less cheap, less dirty and less likely to horrify the techincal types:

Common mode RF chokes:

Plan "A" Air Core RF choke.

Use a prescription pill bottle about .75" - 1.0" in diameter. To be used
as a round mandrel for the coil.
Use a prescription pill bottle as they are usually made from polystyrene
which works well as a coil form in the VHF band.

Wind the cord from the power supply ("wall wart") around it so:

If the cord is flat like a very small lamp cord, make sure it lays flat
against the surface of the pill bottle. No twists, no kinks.

If the power supply cord is round, make sure there are no kinks.

Wind the coil such that each turn is spaced about the width of the cord from
the next turn. For example, if the cord is 1/8" wide, make the winding so
that the distance from the right edge of turn #3 is 1/8" from the left edge
of turn #4.

Start the winding as close to the body of the power supply as is feasible.
You can initially hold the wire in place with electrical tape. If the
technique is successful, the wire can be held in place with adhesive.

Start with about 8 turns and perform the "squelch test" as below.

Add or subtract turns for minimum noise.

If this "air core" technique is ineffective, go to "Plan"B."


The "squelch test."

Use the squelch control as a rough indicator so:

With the scanner on AM and in a fixed position, plug in the power supply
(often referred to as a "wall wart") and adjust the squelch so the noise
from the power supply just stops. Note the position of the squelch control.

Adjust the squelch and determine if it takes less squelch to silence the
radio. Less squelch would indicate an improvement.


Plan "B" Ferrite-core RF choke.


Mount the snap-on ferrite choke as close to the housing of the
power supply as you can.

You may find it necessary to use multiple turns through the core
such that you open the core, put another turn around the core half,
close it and check for reduction in interference.

Perform "squelch test."

If this seems like it is making progress, open the core and put another turn
through it, snap closed and repeat "squelch test."

It is important that the turns do not have any twists or kinks and that the
turns lie right next to each other.

If you have no reduction in noise even with any more than 6 turns through
the core, it may be time for "Plan C" which will require you to send me a PM.

Be Well,

Tim Colbert K3HX

A primer on dealing with RFI (Radio Frequency Interference) can be found
here: http://www.audiosystemsgroup.com/RFI-Ham.pdf

Thanks for the suggestions. I purchased a couple of snap-on rf chokes from Radio Shack (2 for $3) and using one on the usb cable seems to have made a difference when using the AC charger. I've yet to check out what happens when connected to the car power outlet.