President Jackson Uniden TX noise...

Sounds like audio oscillation. What mic are you using? What happens if you key the transmitter with no mic? Problem still there?

Telstar Electronics ha scritto:

i put an612 pin to ground ......so mic is cutted out .....or carrier oscillator or power control loop from tx power transistors.......

Giulia

On Thu, 05 Apr 2007 23:00:20 +0200, Giulia wrote in :

Ignore Brian (Telspam) -- he'll tell you that your ground plane needs to be nine square feet.

Refer to your schematic and use your scope to trace the oscillation backwards through the audio chain starting at the audio power amp. You'll find the problem stage eventually. Don't forget to carefully inspect the board for bad solder joints and loose pads.

OK... you have pin2 grounded for no input signal. Now use your scope and trace backward from the RF output collector... moving back through the audio chain... noting the origin of the 100Hz oscillation. Normally those radios have only a single transistor buffer for the mic... followed by a high power op amp (usually mounted to chassis wrapper) for the audio. Your signal tracing should take long at all. Hope that helps.

*************

100Hz?????

That is a subaudible frequency. How could anyone detect it without a scope?

james

Huh??

The range of "normal" human hearing is 20-20,000 Hz. How is 100 Hz "subaudible"?

If you can't hear 100Hz... you need to get your ears checked Jim.

********** Obvisously you lack the knowledge of what is the subtle difference between audible and audio frequency. Also I did forget that CB radios are not necessarily well designed radios. In a well designed radio, any audio frequency below 300Hz is considered subaudible.

Why? Because the power density in the human voice below 300Hz is negligible. Most well designed receivers for communications purposes do not have audio response below 300 Hz. Also human hearing threshold increases as the frequency decreases below 300Hz. That is the source of the audio freq uency needs to have a larger PSL in order for it to be heard. Sensitivity to audio frequency in Humans is not flat like that of speakers and some sound systems. Instead SPL, Sound Pressure Level, needs to increase as frequency decreases below 300Hz to gain the recognition. For most Humans the power of a 20Hz signal needs to be about 30dB higher than that of 100Hz.

james

****************

That maybe true but the Human ear does not perceive sounds with a flat response for that frequency range. Instead as frequency goes very low and also very high the power density for a given frequency may have to increase or decrease. The human ear does not have a flat range of sensitivity.

Also in communications the Human voice has very little power below

300Hz. Therefore most communications equiptment considered below 300 Hz as subaudible. A well designed receiver for voice communications should reject frequencies below 300Hz. In fact they should also reject frequencies above about 3KHz also.

The range of 300 to 3000 Hz is where the ear is most sensitive to frequencies. Also the Human voice contains most of its power between this range.

james

Changing in VR5 (some related to modulation) remove the noise , i put vr5 more near ground.

Noise seems generated from somewhere feedback at hight swr.... regulation of vr5 remove noise but what kind of effect can it have on modulation (in am all semms right RF evelope in the scope and audio received from another cb)?

Giulia

RF feedback into the microphone? If you feel it is due to high SWR - why not check and reduce it? Try your tests again. What kind of mic are you using? If you mentioned it, I didn't see it. I've seen D104s do that. On 2 meters - I've seen RF get into the power source and do all kinds of weird things to the signal.

Check your SWR then - go from there. IF it IS high, even if the mic/audio issue isn't cured, at least another problem will be - and save you from yet a future one.

Are you using an "Amplifier" with this set up?

I "believe" you said you're running the radio off a car battery. Is it "Actually" mobile or in the house? If in the house, are you running a trickle charger in the process? IF the modulation was cranked up all the way, were you running anything in the background maybe the mic picked up? Many items can cause a hum - you won't hear - that the mic will pick up. If it is mobile, any chance something in the vehicle is causing it? With the modulation cranked up - maybe it was enough to allow "that" to be heard. I've also seen a transmitter make some weird sounds on key - up - with a weaker battery. The antenna SWR as you alluded to - the antenna not grounded - could also create issues along those limes. I've seen lots of weird things happen to cause noises such as you describe - not always internal. Recheck your Antenna coax connections too.

You're backpedaling.

What you clearly said was that it could not be detected without a scope. Then you come up with a bunch of gobbledeegook about 'well designed receivers'. That's not the point, and that's not what you said.

Suffice to say that you were simply wrong. 100 Hz is clearly audible. It doesn't take a scope to detect. A pair of common ears is all that's necessary. Whether it should be heard in a CB receiver is debatable, but if it's there, there's no reason why one shouldn't be able to hear it.

jak

To be fair, "subaudible tone" is a fairly common euphemism for CTCSS or "PL" tones. As you point out, though, they're only "subaudible" because they get filtered out before the final audio stages.

On Sun, 08 Apr 2007 01:35:01 GMT, james wrote in :

Hogwash. The .3-3kHz limits were standardized (by the military) because researchers discovered, over half a century ago, that the bandwidth carried the majority of -intelligibility- in the human voice. IOW, by limiting the frequency range of the vocal spectrum, transmitted speech can be understood using much less power than when transmitting using the full voice bandwidth.

Hmmm..... my Onkyo is a well-designed receiver for communications purposes and it is flat down to 2 Hz.....

Huh? When was the last time you looked at a weighted equalization curve?

That depends on how much you bump up the boom on those 24" JBLs inside your sub-compact riceburner. The sad thing is that when those kids go deaf they will collect welfare for their self-inflicted disability.

mode)....

**************

I stated how could one detect it without a scope? please reread my post and comprehend better.

100 Hz is hearable or percieved. As I stated before there is a subtle difference bwetween audible and hearable. Yes you can "hear" 100Hz if increase the signal power compared to a 1000Hz tone. But at equal power desity, a 100 Hz tone by most people is not "hearable" or audible. The same goes for the higher end of the audio spectrum. Granted some have better sensitivity to a wider spectrum of audiio frequencies.

Besides this tangent has gone far beyond the scope of the original post and realy was not my intention. My intention was to discuss the detection of the offending signal with and without a scope. Not to discuss what is audible to your ear or the frequency response of your hearing.

thank you

james

On Fri, 13 Apr 2007 03:09:54 GMT, james wrote in :

I reread the original post as well. It's pretty obvious, even to the most casual observer, that you meant to say "subsonic".

I didn't see anything in his post about the loudness of this 100 Hz noise. Whether it was "hearable" or "audible", it was detected nonetheless. And apparently it was loud enough to cause a problem, so it's safe to assume that this "noise" (his word) was detected without a scope. Your damage-control explanation about subtle differences in definitions just doesn't resonate.

Not if their application is over the phone. ;-)

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida