Batboard

hey all,

as most of the day i am at my desk, I use a Motorola Portable radio. whenever i TX with the computer speakers switched on, a really really loud buzzing noise comes thru my speakers, even if the volume is switched right down. this happens anywhere in the room, not nessacerily right beside the speakers. RX is fine. its annoying how i hav to keep switching off my speakers when im trying to TX, and i wud rather not have to install a base station or give up my music any suggestions, or do i need to buy better speakers???

thanks ppl.

Your speakers are unshielded and are picking up
the radio frequency energy from the radio. Shielded
speakers are available.

I used to hear a faint "change" in the sound from a cheap pair of computer speakers on a pretty regular basis. Not really bad, just kind of . . . noticeable. And just for a second or two.

One day I had my MT2000 sitting on the desk with me scanning. Whenever it stopped, the speakers did their weird thing. So I started thinking "why would the HT scan stopping do that?" Then I realised it had been happening all along so I knew it wasn't the radio, but what an odd coincidence. Right?

Wrong.

I lived about a quarter mile from the county dispatch center with a 200 foot tower and about ten high band transmitters, a low band transmitter and two UHF repeaters. Some research with the scanner showed my speakers reacted with most (not all) of the high band stuff and one of the UHF repeaters. The high band stuff was all in the 50 watt category. Low band was 300w and the UHF were 100w.

I imagine folks who live right under that tower spend more money on computer speakers to find good ones.

well i certainly dont have any dispatch centers, radio transmitters or repeaters near my house at all, so i guess i will have to invest in some shielded speakers???

I bet you notice you hear a dit-dit-da everytime your cell phone gets a call or text message when it's sitting near the speakers or the wire as well

My two bits worth:

The problem is that the input of the amplfier in the speaker is detecting the RF signal.

Computer manufacturers provide shielding and RF filtering so that they can comply with FCC part 15, keeping RF within the computer, they are not overly concerned with external RF.

A "Shielded" speaker may refer to the magnetic field generated by the magnet, not RF shielding. (An un shielded speaker will cause purity problems if placed near a CRT monitor.)

The easy fix would be a Ferrite choke on the speaker wire close to the speaker
Next level by pass caps (.001Uf) on the Base-Emitter junction of the transistors, or audio IC as well as on the power and speaker leads.
Replace the speaker cable with shielded wire.

There is a carbon spray shield which could be applied to the inside of the speaker case.
Past experience shows that by passing is more effective than shielding alone, which may not be practical.

Aluminum foil is useful in temporarily shielding components in the process of understanding exactly how the interference is entering the equipment.

It is often necessary to by pass speaker leads in mobile installations.

Two way radios can cause all sorts of interference in other equipment from falsing Fire Alarms to causing Elevator controllers to malfunction.

I have seen this problem all too often. Normally adding some
.001 disc caps on the amp circuit board will solve or lower the
problem level. I have done it a number of times to many
speakers. Like Bernie said the caps do wonders.

For a start I add the caps across the input cable where it hits
the board from the computer.

I also bypass the power input wire.

Then add the caps to the speaker output wires.

Try to find the common ground plane on the circuit board.

Keep the cap wires short.

Make sure you don't short out any other points on the board
while your adding the caps.

Jim

My two bits worth:
As Jim says be careful you dont cause shorts!
The RF is detected at the Base-Emitter junction of the amplifier.
The trick is to mount the cap as close as posible

Back when I had to work on a lot of the old mobile nextel units I used to use the computer speakers I have hooked up to a CD player on my bench as a test.. if I hooked a nextel up and didn't hear that thumping noise in my speakers I knew it wasn't transmitting.

wa2zdy - We have a 200' tower site at our shop, it has a number of UHF transmitters, at least one high band paging transmitter, a low band, some 900 mhz stuff, a Nextel site, cell phone, and who knows what else, and it doesn't affect any of our computer speakers that I've ever noticed.. maybe being right under it all the signals overshoot us.

escomm wrote:I bet you notice you hear a dit-dit-da everytime your cell phone gets a call or text message

u bet i do!!