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Cobra 148 gtl dx mk2 pb010aa receive fm distorted


Using a RF signal generator with a built in audio generator input a very strong RF signal, say 100uV/-65dBm with a 1kHz +/-1.5kHz tone. With an oscilloscope monitoring the audio output from the radio and the volume level on the radio set not too high - say 10 o'clock position, tweak for max gain with minimal distortion and a nice sine wave shape. You'll find if you go too high you end up with a sinewave that has a shallow rise but steeper decline instead of being uniform.
 
Alignment of the FM detector usually helps this problem however, the root of the problem is deeper. Very few of these export radios can properly demodulate FM with the full + / - 5 KHz. deviation without noticeable distortion. That's because they use the 6 KHz. AM IF filter to pass the 10 KHz. wide FM signal. Don't be surprised if you can only make it sound good with low levels of deviation. Better performance with higher deviation will only be possible when installing the missing 10 KHz. FM filter.
 
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Looking at it closer only loud strong signals seem to make a noise from the speaker, and it's doing It on all modes I was wrong thinking it was just fm.
So far changed the 10.695 filter, the 1000uf cap in audio circuit that was swollen a little on the top, still nasty broken received audio.
 
nomadradio said:
Hmm. Clicking on 'Abikerlad's avatar reveals his last check in to be 2014. Don't hold your breath waiting for a reply.

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Yeah, its still relevant but obviously dormant. Interesting that since the noise turned out to be on loud signals on all modes that the speaker was never mentioned.
 

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