FM Radio Receiver Problem

[Mahmoud Magdi]

I used this circuit to build a FM receiver , but when i completed all the connections , i listen only noise, i can't listen a clear voice.

Could you help me please ?

 

[Audioguru]

Re: FM Radio receiver problem

It is a simple "regen" radio. It must be TUNED to one side of an FM radio station and have its regen adjusted high enough to work as a radio.
If you built it on a breadboard then the high capacitance between the rows of contacts and between the jumper wires causes it to be tuning to frequencies too low. Try it when the 5pf capacitor (the one parallel to the tuning capacitor) is removed.

 

[chuckey]

Re: FM Radio receiver problem

Solder /fix a wire to point 11, when you touch it to point 9 , you should get a click in the ear piece, this tests, the last two transistors to a certain degree. Get you DVM/multi meter (on volts) and measure between Point 11 (the wire) and the junction of the 10K and 22K, feeding the first transistor, while doing this put your finger across the tuning capacitor, this should stop the first stage squegging and the current will rise so your voltage will fall. Once you have proved this the problem could be in the tuning of Vc and the coil. Try spreading the turns of the coil (12 turns air spaced about 15mm long?). Come and tell us how you got on.
Frank

 

[Mahmoud Magdi]

Re: FM Radio receiver problem

It is a simple "regen" radio. It must be TUNED to one side of an FM radio station and have its regen adjusted high enough to work as a radio.
If you built it on a breadboard then the high capacitance between the rows of contacts and between the jumper wires causes it to be tuning to frequencies too low. Try it when the 5pf capacitor (the one parallel to the tuning capacitor) is removed.

Thanks Audioguru ,
please could you tell me about the specific capacitor you mean ? between any two points ( numbers ) ? , with known that the left circuit is a ready board .

 

[Audioguru]

Re: FM Radio receiver problem

The FM regen receiver is built on a pcb, not a breadboard. I do not know if it needs more or less tuning capacitance.
The photo shows that a 0.8uH inductor is missing but it is not shown on the schematic.

 

[Mahmoud Magdi]

Re: FM Radio receiver problem

OK , Could you send to me another circuit that I can build it to make FM and AM Radio ?

- - - Updated - - -

Solder /fix a wire to point 11, when you touch it to point 9 , you should get a click in the ear piece, this tests, the last two transistors to a certain degree. Get you DVM/multi meter (on volts) and measure between Point 11 (the wire) and the junction of the 10K and 22K, feeding the first transistor, while doing this put your finger across the tuning capacitor, this should stop the first stage squegging and the current will rise so your voltage will fall. Once you have proved this the problem could be in the tuning of Vc and the coil. Try spreading the turns of the coil (12 turns air spaced about 15mm long?). Come and tell us how you got on.
Frank

OK , but it is only 5 turns 7mm long , what is your opinion about changing it ?

 

[Audioguru]

Re: FM Radio receiver problem

A regen circuit is too simple to make a real radio:
1) A real radio is very sensitive so it can pickup weak distant stations but is not overloaded by strong local stations. A regen radio is not sensitive and gets overloaded.
2) A real radio has good selectivity so it can receive one station that is beside another station. A regen radio receives both or more stations at the same time.
3) A real radio has low distortion and no interference. A regen radio has high distortion and lots of interference.
4) A real radio does not interfere with other radios. A regen radio causes interference.

 

[godfreyl]

Re: FM Radio receiver problem

The photo shows that a 0.8uH inductor is missing...

+1
The circuit won't work without that inductor.
(It's shown on the schematic in post 1)