Regen FM reciever not working

Thread Starter

Hamlet

Joined Jun 10, 2015
518
I've found and built 5x circuits to receive commercial FM broadcasts, but none of them work.
I saw the following circuit (below) that had two new twists, 1) regenerative, and 2) used a jfet.
I built it exactly as described, but used a 3-24pF variable capacitor in place of the 15p/80pF,
and am using my LM386 bench amp conected at C6 and ground. I couldn't even get
this to oscillate.

I can indeed receive FM broadcast at my location, I have a crappy transistor radio that does okay.

Am I expecting too much from a simple circuit?


https://www.circuitbasics.com/what-are-fm-receivers/
1643855776614.png
 

DickCappels

Joined Aug 21, 2008
10,124
It looks like it “should work”.

Check to see whether the MPF102 is biased into its active region. Some of them need a very high gate bias voltage. Do this by measuring the voltage across R2.

What is L2?
For that matter what is L1?

What are you using for an antenna?

Do you live near any FM stations?

The one I made using an NPN transistor could only detect a couple of strong stations and neede
 

Thread Starter

Hamlet

Joined Jun 10, 2015
518
It looks like it “should work”.

Check to see whether the MPF102 is biased into its active region. Some of them need a very high gate bias voltage. Do this by measuring the voltage across R2.

What is L2?
For that matter what is L1?

What are you using for an antenna?

Do you live near any FM stations?

The one I made using an NPN transistor could only detect a couple of strong stations and neede
L1 8mm x 6 turns, 0.8mm, tapped at second turn for antenna, which is 60cm, but I've also used a long wire.

L2 is 8mm x 30 turns, 0.8mm.

No nearby FM stations, but a crappy little handheld picks up a few strong stations.

Voltage across R2 is 1.82v, I'll try to increase it a little and see what happens. I also might try another
jfet, perhaps a 2sk117gr...
 

DickCappels

Joined Aug 21, 2008
10,124
Its not a superhet, it is a slope detecting regen and an experiment at that, not a consumer product.

The current sounds about right, but trying another JFET might solve the problem. There are many substandard parts out there. If you can't get it to work you might chalk it up to an absence of strong signal.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
518
Alright, thanks for looking it over. I took the apparatus outdoors today, and I think I might have got the barest fleeting tickle of FM rf reception. AM & SW is easy by comparison. I think I'll put it to bed for now. I'm hoping
The Radio Board forum will accept my membership request, as I'd like to ask them for a robust & proven FM circuit that doesn't need special test equipment to get up and running.
 

Audioguru again

Joined Oct 21, 2019
6,619
The Dollar Store sells a poor performance FM radio with a TDA7088 in it. You press a button and it scans the radio band until it finds a strong station. Since it comes with a battery then the "radio" is free.
It sounds awful.
 

bassbindevil

Joined Jan 23, 2014
818
Take a look at some old ham radio publications. They may have had plans for ultra simple FM receivers (for the 2 meter or 6 meter bands).
https://worldradiohistory.com/BOOKSHELF-ARH/Bookshelf_ARRL.htm
Your circuit looks similar to the one on page 32 of The Radio Amateur's V.H.F. Manual:
https://worldradiohistory.com/BOOKSHELF-ARH/Technology/The Radio Amateurs VHF Manual 1972.CV01.pdf

Also, other old hobby electronics books. There's regenerative receiver plans (though not for FM frequencies) in:
https://worldradiohistory.com/BOOKSHELF-ARH/Sams-Books/Sams-FET-Circuits-Turner.pdf
 

crutschow

Joined Mar 14, 2008
34,062
Ahh, takes me down memory lane. :)

I built this one-tube regenerative FM receiver when I was in school, and amazingly got it to work for a couple strong FM stations in the area.
Think I bought most of the parts from the old Lafayette Radio Catalog since they sold a lot of cheap (at that time) Japanese stuff
A little finicky to operate but the sound wasn't too bad.

I built that, since I could afford the cost of a standard tube FM radio at the time.

The only instrument I had was an EICO VTVM (below).
That thick voltmeter probe was rather interesting.
You could twist the tip, with one position measuring normal DC and low frequency AC voltage, and the other position measuring rectified RF signals (as from a radio IF stage) from a built-in small diode (likely germanium).

I still have the receiver in my old box of electronic stuff, but the last time it was fired up was likely in the '60's.

1644376662836.png
 

vu2nan

Joined Sep 11, 2014
341
Here's my simple home-made FM receiver.

2.png

It uses a dual-gate mosfet 3N187.

1.png

The power source is a 1.5 V button cell.

3.png

Ten stations rated 1 - 10 kW, situated 6 km away (as the crow flies), are received quite comfortably

with a pair of sensitive balanced-armature phones (also known as sound powered phones).

Nandu.
 
Last edited:

DickCappels

Joined Aug 21, 2008
10,124
I just noticed that the circuit to which Crutshow linked in post #11 is the same circuit as that which V2nan posted in post 12, except the later is the modern solid-state version. Funny how some things never change.
 

Audioguru again

Joined Oct 21, 2019
6,619
Isn't a simple regen "FM" radio actually an AM radio that picks up all the clicks and pops amplitude fluctuations caused by interference? Then you must tune the radio to one side of the station's radio frequency for it to slope-detect the FM but then it still picks up the interference.
Another problem with using an AM detector is adjacent stations interfere with each other. An FM detector has a good "capture ratio" that blocks adjacent station sounds.

My first FM radio used an FM ratio-detector. When tuned directly to the radio frequency it played sounds perfectly without AM clicks and pops.

FM radio stations transmit sounds with pre-emphasis (treble boosted) so that the matching de-emphasis in all FM radios cut the boost down to normal and also reduces any hiss. The simple "radios" in this thread do not have de-emphasis then the sound must be very shrill.
 

vu2nan

Joined Sep 11, 2014
341
Isn't a simple regen "FM" radio actually an AM radio that picks up all the clicks and pops amplitude fluctuations caused by interference? Then you must tune the radio to one side of the station's radio frequency for it to slope-detect the FM but then it still picks up the interference.
Another problem with using an AM detector is adjacent stations interfere with each other. An FM detector has a good "capture ratio" that blocks adjacent station sounds.

My first FM radio used an FM ratio-detector. When tuned directly to the radio frequency it played sounds perfectly without AM clicks and pops.

FM radio stations transmit sounds with pre-emphasis (treble boosted) so that the matching de-emphasis in all FM radios cut the boost down to normal and also reduces any hiss. The simple "radios" in this thread do not have de-emphasis then the sound must be very shrill.
Hi Audioguru again,

Yes, it's slope detection. I'm able to tune in to the stations on both the slopes.

It sounds quite okay and there is no interference.

Nandu.
 

DickCappels

Joined Aug 21, 2008
10,124
Thank you. Looking at the datasheet I see that it is a rare depletion mode MOSFET. That caught me off-guard!
Now I can understand how it is possible to oscillate.

Never use depletion MOSFETs, they seem to be unpopular but I can see their value.
 
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