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FM Radio circuit problem

Discussion in 'General Electronics Discussion' started by quantumtangles, Dec 23, 2012.

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  1. quantumtangles


    Dec 19, 2012
    A portable transistor radio worked really well... before I boxed it and hot glued it.

    TDA 7010T FM Radio IC
    TBA820M Audio Amplifer IC

    Although It still works for nearby stations, there is a pulsing variable in pitch thwacking feedback sound (especially at higher volume levels), and it goes away when I touch the exterior of the rechargeable 9v P5 battery powering the radio.

    Tried putting a variety of caps between the battery source and the circuit board, Made no difference. Haphazardly tried resistors and diodes. Nothing. In desperation, made an inductor from a few turns of wire and connected it up, but no dice.

    I have done something retarded here but cant figure out what it is.

    The only thing that stops the interference noise is hand capacitance... when I touch the exterior of the 9v battery.

    I tried replacing the rechargeable 9v battery with a standard P5 but it made no difference. Still a thwacking sound at higher volume unless I touch the casing of the battery.

    I am about to learn something here... but only if you can shed light on my wtf moment.

    Any ideas at all would be most appreciated (I have run out of ideas...for the moment)
    Last edited: Dec 23, 2012
  2. Relayer


    Dec 15, 2012
    Sounds like you have a grounding problem.
    It seems something that's supposed to be connected to ground isn't.
    Try poking around the circuit with a multimeter and see if it corrects itself at certain points.
    Relayer :D
  3. quantumtangles


    Dec 19, 2012
    Makes sense. I did not think about grounding issues. :eek:

    Flipping the circuit over to check later today.

    Many thanks
  4. quantumtangles


    Dec 19, 2012
    Looking for the ground problem in my FM radio, the sound was so much better when the aerial wire was touched onto the negative terminal on the circuit board. I thought I had 'grounded' it to solve the problem.

    Then I noticed something out of context. The radio had only been on for about 5 minutes but the PP3 rechargeable 9v battery was hot. REALLY hot. It had started melting.

    Was the radio somehow drawing massive current from the little battery? Was this 'equivalent series resistance' in action? No. Much simpler answer.

    When I first got the radio working, I didn't bother to connect an on/off switch straight away. But after I got it working, obviously I needed to solder an on/off switch. So I added an LED power indicator/resistor plus the on/off switch, making a hole in the case for the switch and LED (to turn it on and off without having to open the case and also to make the power indicator visible from outside the unit.

    I used black wire to hook up the LED and switch because I ran out of red wire.

    But when soldering the switch and LED, I forgot I had used black wire.

    What I ended up doing was connecting the positive and negative terminals of the battery together whenever the switch was in the on position :eek:

    Problem solved.
    Last edited: Jan 9, 2013
  5. quantumtangles


    Dec 19, 2012
    Problem was not entirely solved. It was a grounding problem but it was a second grounding problem.

    By connecting a wire to the negative terminal of the 9v battery (a second negative terminal), I tested the connections on the PCB, until I got a good signal. Then I soldered the second ground connection to the relevant node. Not exactly high tech but finally the radio works rather well :D
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