• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

I can't believe my FM radio is going haywire again!

Status
Not open for further replies.

mstechca

New Member
Look at the schematic. This is my FM radio. The values that are highlighted have been modified. Sometimes, I can barely pick up radio announcers at 108Mhz Sometimes I change the highlighted diodes to resistors. When I touch these resistors, I get an AM station in loudly. The values I used for the resistors in yellow boxes were in between 50 ohms and 500 ohms. I rarely used 1k as it provided the poorest results. The diodes highlighted in teal were used because I wanted to force a one-way operation and I wanted to pass as much of the signal as possible. I got similar results replacing these diodes with resistors. The resistor in purple had the biggest impact. It seemed that the lower the value, the more clear and quiet the station is.

What I want to know is what values should I use for the highlighted components, and do I need a resistor in the red area? if so, what value?
I also want to know why you chose these values? I want some math!

I want to be able to pick up ANY station clearly.

Note that changing the value of the components in green did not make things better.
 

Attachments

mstechca

New Member
I have been tweaking. I added a resistor between the cathode of the first diode and ground. and I noticed the following:

The hum is reduced when the resistance between +ve and the cathode of the first diode is more than the resistance between the -ve and the cathode of the same diode.

The quality is increased, and the hum is reduced when I increased the resistance of the resistors highlighted in yellow.

I added a resistor between the collector of the right-most PNP transistor and ground. This had made the most noticeable difference. I managed to make the resistance big enough (around 500 ohms) to hear galactic sounds. (sounds you hear between two AM radio stations)

In the circuit there is a resistor and a capacitor connected in parallel. I removed the capacitor. This made another noticeable change. I figured out that the resistor (where the capacitor was in parallel with) controls bandwidth. The bigger the resistance, the more bandwidth I have.

I managed to barely pick up two fuzzy FM stations, and I managed to pick up 1150khz AM!

I want to be able to improve the signal to noise ratio (if that is what they call it), plus I want to get rid of hums.

I am using the same circuit shown above with the exceptions explained in this post.
 

mstechca

New Member
Look at the picture at http://www.arbornet.org/~stak/wave.bmp. If I am right, one capacitor and resistor pair (of the multivibrator) is controlling area X, and the other capacitor and resistor pair is controlling area Y. Area Z is the center.

Am I on the right track with properly receiving an FM signal?
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top