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Problem in autoscan FM radio

Dear group,

I have this one-chip china-made auto-scan FM radio which has a problem. It has a "SCAN" and a "RESET" buttons on it. The scan button is used to tune and lock to a station. Another push on the button would step it to the next higher station. The reset button is used for reseting it back to the lowest band again. It was able to receive frequencies from 88MHz up to 103.3 MHz but not able to detect anything above 103.3 MHz. I know there were at least 2 stations above 103.3 MHz but I could never receive them.

The fault was there when I bought it new. As the set is very simple and it was made almost identically to the datasheet as shown below. The block diagram and the surrounding components are on Page 2 of the datasheet. The audio part is just 2 NPN transistors cascaded together to drive an earphone.

The datasheet is as below, just click on "data sheet view"

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Would someone be kind enough to just take a look and teach me which component might have turned bad and caused the above problem.

Thanks in advance and best regards.

Allen

Reply to
Allen Bong
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There doesn't seem to be any sensitivity adjustment, so maybe the stations you are looking for are too weak with the antenna you are using. The only other possibility I can see is that the VCO circuit won't go high enough, maybe because the varicap diode is out of limits, or the adjustable inductor (the one labeled 78 uH) is set to too high an inductance. If it's actually adjustable in your radio, you could try changing it by backing the slug out of the coil; if it's just a coil of wire in air, you could try spreading the turns to get to a higher frequency. Or the other coil, on the RF input (70 uH), might be so misadjusted that it weakens the highest part of the band, too.

Or, since you just bought it, you could return it for exchange.

-- John

Reply to
John O'Flaherty

Thanks for your suggestion. I will try to pull the 2 coils a little far apart and test the one by one to see if it helps.

I will report back here once I got the results.

Allen

Reply to
Allen Bong

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