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Old 9th Feb 2020, 1:14 am   #1
correiadesa
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Default National Panasonic 702 B AM/FM clock radio static when off

Hi.
I've just bought an old National Panasonic clock radio. It was not working at all. After a quick cleaning with contact cleaner spray (except for the flip clock) everything started functioning but with a strange issue. When i put the volume down you can hear a weak and permanent static white noise sound coming from the speaker. When you turn the radio off that noise continues. It seems that this a permanent noise in the background. When you tune it to a radio station with the volume up you cant hear but i think its there. Ive looked carefully inside but everything seems to be ok. Can anyone help me or give some clues to repair this?
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Old 9th Feb 2020, 2:07 am   #2
rambo1152
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Hi correiadesa and welcome.

Have you got a cordless telephone close to the radio?
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Old 9th Feb 2020, 4:49 am   #3
correiadesa
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Thank you Graham.
I've tried in diferent places and nothing changes. The static noise doesnt seem to be an external interference and is a very stable noise and not scrachy.
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Old 9th Feb 2020, 10:34 am   #4
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Hi correiadesa, are you certain the sound is coming from the speaker, and not from the clock mechanism? Just a thought, as the clock will continued to be powered when the radio is off. Tony.
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Old 9th Feb 2020, 1:53 pm   #5
correiadesa
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Thank you Tony.
It's definitely coming from the speaker and its loud enough to ear it clearly trough a silent room. I wonder if it could be a faulty on off switch.
Best regards.
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Old 10th Feb 2020, 10:43 am   #6
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

When the radio is turned on and with the volume control at minimum do you still hear the noise?
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Old 10th Feb 2020, 11:08 am   #7
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Correiadesa


What type of display does the clock have?

If it's LCD, that can produce the problem.
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Old 10th Feb 2020, 11:39 am   #8
Herald1360
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Chances are when the radio is "off" it isn't OFF. Just muted. The noise could just be residual hiss from noisy transistors/resistors etc. It's easier for the radio/alarm design if the alarm doesn't have to physically turn on the radio power, just unmute the sound.

Does it sound the same with the radio on and volume turned right down?
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Old 10th Feb 2020, 1:43 pm   #9
correiadesa
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Thank you Simon.

Yes, when the radio is turned with the volume at minimum level you can ear the same background noise.
Best regards.
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Old 10th Feb 2020, 1:48 pm   #10
correiadesa
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Thank you "camtechman".
This is a little1971 flip clock transistor radio.
Best regards.
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Old 10th Feb 2020, 2:10 pm   #11
correiadesa
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

Thank you Herald.
I hope that is not the case because in a silent room that sound get's too loud. You couldnt use it in a bedroom at night
That sound( no hum noise, just a "shhhh" sound) is present when the volume is down. If you turn off the sound continues.
Best regards.
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Old 16th Feb 2020, 6:12 pm   #12
correiadesa
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Default Re: National Panasonic 702 B AM/FM clock radio static when off

I found out that if you unplug the radio from ac current for some hours, when you plug it again to ac current, there is no noise. After 20 or 30 minute the noise returns even with the radio in "off" position. I 've tried to see if any of components were warmer than normal but didnt find any. There is one 1uf capacitor that when you touch makes a loud "hum" noise but i dont know if thats normal.
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