Problem in autoscan FM radio

[Allen Bong]

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"
http://www.datasheet4u.com/html/S/C/1/SC1088_ETC.pdf.html

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

 

[John O'Flaherty]

On Mar 31, 4:12 pm, Allen Bong <allenbsf6...@gmail.com> wrote:

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"http://www.datasheet4u.com/html/S/C/1/SC1088_ETC.pdf.html

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.

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

 

[Allen Bong]

On Apr 1, 5:52 am, "John O'Flaherty" <quias...@yahoo.com> wrote:

On Mar 31, 4:12 pm, Allen Bong <allenbsf6...@gmail.com> wrote:





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"http://www.datasheet4u.com/html/S/C/1/SC1088_ETC.pdf.html

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.

 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- Hide quoted text -

- Show quoted text -

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