FM radio channel memory - how does it work

[vwchico]

Hi

I'm trying to repair an FM radio, previously it stored the saved frequencies. When you unpluged it and plugged it back in the stored channels remained. Now when you unplug it they gone.

I want to repair this, but have no clue on what could be wrong, how does it store these channels. I looked for a battery / big cap but nothing. Any idea ?

 

[Nigel Goodwin]

Hi

I'm trying to repair an FM radio, previously it stored the saved frequencies. When you unpluged it and plugged it back in the stored channels remained. Now when you unplug it they gone.

I want to repair this, but have no clue on what could be wrong, how does it store these channels. I looked for a battery / big cap but nothing. Any idea ?

As you say, it's usually a battery - often a slim Lithium cell, soldered on the board somewhere.

Other options are a large capacitor (but only for short term memory), or an EEPROM - but these are fairly uncommon in FM radio.

Assuming you do find a lithium battery on the board?, be VERY careful removing it - I presume it's caused by leakage from the battery?, but the solder tends to 'explode' off the board when you apply the soldering iron, it can easily hit you in the eye (I wear glasses, and have had solder splashes on the lens more than once!).

 

[vwchico]

But i've looked all over the board top and bottom and cannot find any Lithium battaries? Any other suggestions ?

 

[Joel Rainville]

But i've looked all over the board top and bottom and cannot find any Lithium battaries? Any other suggestions ?

I know my little AM/FM radio only uses a relatively large cap to keep my 6 memorized frequencies on some sort of volatile memory. If my batteries go dead and don't replace them quickly I lose my settings. Might just be a faulty cap?

 

[Someone Electro]

No the cap gets drained down like a batery.It is recharged wen you put the baterys back in.

I know i lost all the setings and date and time wen i had the baterys out for a long time.

 

[Joel Rainville]

No the cap gets drained down like a batery.It is recharged wen you put the baterys back in.

I know i lost all the setings and date and time wen i had the baterys out for a long time.

Isn't it exactly what I said?

 

[ThermalRunaway]

The trouble is, finding the cap responsible for memory retension without a schematic is going to be difficult. What make/model receiver are you using?

Brian

 

[Someone Electro]

There are those "gold caps" that are rated up to 5,5V and are up to 5F.These capes are mainly desigend for memory back up.

 

[Someone Electro]

oh an joel you sead "Might just be a faulty cap?"

 

[vwchico]

5v 5f cap .... will this thing shock me with so many farads :lol:

I did see a few big caps like these on the baord but the problem was that they weren't located anywhere near the FM tuner modue / IC .

I'm trying to repair a micro HI-FI its a JVC FD7

 

[zachtheterrible]

Not sure if it's true or not, but in a computer class that I took, they said that some BIOS chips have batteries built into the chip, so when the computer starts forgetting, you have to replace the whole BIOS. Maybe it's the same for this?

 

[vwchico]

No Big caps

I had a relook at the borad no batteries and no big caps the biggest is like 100uf.

There are two IC's
TA2057N - This is the FM IC
LG72136N ( this helps search frequency i think)

But there are no big caps arount these chips except a 10v 100uf cap.

So how do i fix this thing so that channles are stored again when i unplug it ?

 

[Joel Rainville]

I opened my little portable Sony AM/FM radio with 5 channels memory in the hope that I could find something that could help you. Unfortunately, what I saw is mostly one big 1000uF 6V cap (the radio operates on 2x AA/LR6), one proprietary Sony IC (with the top painted black so no markings are showing) and the rest looks like the normal analog RF stuff. No other strange looking IC...

While I dismantled it with the batteries removed, it was flashing a "no power" logo on the LCD for about 2-3 minutes, and then it stoppped. When I put the batteries back, the preset channels were gone. So it looks like on my radio, the big cap and the unknown IC are doing the storage job... Sorry.