"Radiosrfun" wrote in message
...
"Giulia" wrote in message
...

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.

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.