tiistai 2. huhtikuuta 2019

Modifying an FM radio

TL;DR I tried to modify an FM radio that only goes up to 108 MHz to receive weather satellites at 138 MHz. That failed, but I learned thing or two on the way.

Introduction

I read about receiving weather satellites with PC sound card and a modified radio many years ago, and since that time I've always wanted to do that. Today it is rather simple to do with very inexpensive software defined radios (SDRs), but using pre-built components is only half the fun.

The weather satellites I talk about transmit at 137 to 138 MHz using FM modulation. The easy way to receive that is to use a radio scanner and connect it to PC using the sound card line input and the software in previous link decodes to images. A powerful antenna may be needed, but that is just details ;)

Radio scanners tend to be quite expensive compared to normal FM radio receivers. Where I live, the FM radio band goes from about 88 MHz to 108 MHz, so quite close to what I needed. I decided to try and modify an existing radio to reach the required band.


Selecting a radio

I looked at the most inexpensive receivers and especially one that looked to have an analog tuning mechanism. I thought that one with analog tuning would have some LC tuning circuit, which I could modify to an upper frequency to extend the band upwards. I assumed that ones with digital tuning (e.g. buttons to tune up and down) might be hard to modify.

I ended up buying a Prego FM/AM radio which had analog potentiometer style tuning and volume. Below is a picture of what the radio looks like.


 

Opening it up

I opened the radio and looked at the PCB. There was no obvious tuning circuit (coils and capacitors) or even crystal oscillators visible on the board.

On the top side there was few IC circuits and on the bottom side just one. The bottom side one was a power amplifier driving the speaker, so what I wanted was one on the top side.

On the top side there was some regulators and interestingly a EEPROM of type L24C02B. This turned out to be important but I didn't know it yet.

The radio chip was under some plastic, protecting it from the mains transformer. The type of the radio receiver is KTmicro KT0922. Luckily I found datasheet for it.