FM Radio

[audioguru]

we tried doing the same project, there are hummings but no radio stations. we were not able to fine polyester.. we used mylar instead...,, is there any way it will work?

Your capacitors are fine. The little ceramic ones must be ceramic.

Humming indicates mains interference pickup. Maybe the wires in your circuit are too long. Radio circuits won't work on a breadboard because the long wires and rows of connections have too much stray capacidance and inductance. Therefore a pcb must be used with very short printed wiring.
The coil must be exactly like it says (if you can understand the poor translation). The 5+5 turns of enamel-insulated wire in the coil must be touching I think. Spread the turns a little and tune the pot and maybe it will work.
Use a pot for tuning it before trying to tune it from your computer.

[someshwar_ms]

ur discussion was of great help to me n thanx to everyone :)

[mvs sarma]

hai all

plsee carefully the FM radio schemtic of the project and you will appeciate that

THE VARYICAP IS SHOWN CONNECTED WRONGLY-- IT SHOULD BE IN THE REVERSE DIRCTION TO THE BATTERY POLARITY, ON THEN, THE DIODE WOKS AS A VARYCAP

PL TRY OUT YOU ARE SURE TO START RECEIVING STATIONS   

anxious to get feedback

sarma

[MP]

sarma,
variable caps are not directional like an electrolytic.It does not matter which direction you connect.
However, I do not see one in the project. Are you discussing the project in our project section here?:
http://www.electronics-lab.com/projects/rf/020/index.html

MP

[audioguru]

Sarma is talking about the BB329 varicap tuning diode that is backwards in our project. It is supposed to be reverse-biased but is shown connected with forward bias.

Nice going, Sarma, you have good eyes. ;D

[MP]

Good. I have notified Kyriakos to correct his schematic and re-post.

Sarma, thanks for the correction. Since I was not following this thread, I read your post as variable cap and not varicap.
If this schematic does not get corrected in the next month or so, please remind me and we will correct it without the original author's intervention.

MP

[AN920]

The cathode side of D1 is connected to +5V through L2 as it should be or is this already modified?

[Guest nanop]

Like the project says, steal a varicap diode from an old TV tuner.
Do you know where to get an old TDA7000 radio IC? It also hasn't been made for years.

Is it true that u can descramble cable using a variable cap?  ;D

[MP]

The cathode side of D1 is connected to +5V through L2 as it should be or is this already modified?

I haven't looked at the schematic for the problem yet. No it has not been changed from the original. I have only asked the original author to get involved.

MP

[AN920]

I see the drawing has been modified now. The anode is connected to +5V through the inductor. This is wrong! The diode will now be conducting when the pot is set low enough or less than 4.5V on the slider. It will not function as a varicap anymore. The original diaram was correct.

[audioguru]

I think the varicap diode was backwards, then it got reversed recently, then it got reversed again recently. It is odd that the schematic doesn't say anything about the "correction".

[audioguru]

Is it true that u can descramble cable using a variable cap?

My cable-TV company changed all pay-per-view to the digital channels years ago.

In 1980 I modified my TV to unscramble the scrambled analog signals. My circuit was a lot more than just a variable capacitor since the sync was suppressed at certain times and needed to be detected, made then re-inserted when needed. The picture also went negative at times, which needed to be detected, made in reverse then re-inserted. Making the picture reversed wasn't easy because the colour burst must be left the same as original, or the colours would be reversed.

[MP]

The diode was not originally backwards. I should have looked closer after you the two of you told me it was wrong. Datasheet shows that it was in the correct placement.
This is why I strongly press on members that we do not make changes unless you are actively bench testing a project. This is the mess that results from it.
I think the original author might have re-submitted the schematic since I inquired about it to him.

MP

[AVRFreakMan]

I haven't done any changes in this circuit.

This is my reply in MP's email notification that I got yesterday:

Hello Mike.

The circuit is correct. I had to sit back and think of the theory behide the
varicaps for a while and check the real PCB. The result is that the circuit
is correct. Every varicap, like zeners, has to be biased in reverse order,
otherwise it acts as ordinary diodes. If you see, the cathode is DC
connected to the +5V line, via the coil, and the anode is allways connected
to a lower voltage, via the potensiometer.

The PCB is verifying this. The black line in diode's body goes to +5V. I
made it severals years ago and was hard to find it in the pile, but still
works! Here is a foto that I took yesterday with my mobile phone camera,
while I was listening some random radio station.

I used the BB329 varicap in this prototype and works very well.

However, I should say a few more words that may not be obvious.

1) The operation range of the varicap in this circuit is 0..5V. The BB329 works up to 32V but here is used up to 5V. This results in about 40 to 15pF.(The datasheet is wrong when it says VR=3V -> ~3pF. It should say VR=30V -> ~3pF. Check the "Capacitance versus reverse Voltage daigram".) Use a similar varicap.

2) If you decide to use the LPT option, cut the line in point "A", delete the potensiometer and connect the "B" point, to wiper's line. If you leave the pot in, you may use it for manual micro-tuning but it will introduce some unwanted voltage drop of the R2-R LADDER circuit.

3) In my prototype I used the LPT option, without any potensiometer.
Furthermore, I added an op amp connected as a buffer x1 from "B" to "A", just to be sure that the R2-R LADDER circuit will work with my LPT port. However if the pot is deleted, the buffer is not necessary.

P.S.
Dont's ask me for the PCB layout. I tranfered the circuit by hand directly into the PCB. One time job.

Kyriakos

[MP]

To All:
The project schematic is back in it's original form and is correct. Anyone making this circuit should have no problems with it's design.

Thanks Kyr, for joining the discussion and adding the additional comments about your project.

MP

[AVRFreakMan]

Found another scan image of the PCB.

[MP]

Excellent!
Thanks for posting.

MP

[Andy815]

Hi am having trouble getting the parallel port to work and i dont understand why. The radio is working purfect with the potenteomoter but i cant tune the radio with the parallel port. If anybody knws why i would really be happy

[AVRFreakMan]

Hi am having trouble getting the parallel port to work and i dont understand why. The radio is working purfect with the potenteomoter but i cant tune the radio with the parallel port. If anybody knws why i would really be happy

Explain more!
Did you use a program? What program?
What OS? My program runs  only in plain old DOS environment. That means DOS, Win95, Win98(SE) but not any NT based Windows.
Have you measured the voltage in B point with a multimeter? Does it change in regular steps?(0 to 5V, @ 5V/255 steps)

If you think everything else is correct, check parallel port's settings in BIOS. Alter SPP/ECP/EPP settings and you'll see.

Report your findings and if anyone has any photos, it whould be nice to see them.

Kyriakos

[mvs sarma]

Fine-- there was a time lapse in my reverting back-- as i was out of station attending on a relation who was sick, thus not verymuch on net.

i went thro all the letters  and also the philips application note an192

PIn 5 of TDA7000 sits on =ve voltage thus the Varicap positioning is RIGHT

the circuit is also checked working by andy815 with potentiometer.

  i stand corrected.

sarma

[mvs sarma]

Fine-- there was a time lapse in my reverting back-- as i was out of station attending on a relation who was sick, thus not verymuch on net.

i went thro all the letters

[MP]

Thanks sarma,
I think we are already past this. It is good to have you back. Hope all is well.

MP

[Andy815]

in the schematic i connected the parallel port to point "A" and removed the potentieometer. I connected r4 to c18 and r3. The program i am using is Java. I am sorry i dont understand what you mean when you want me to check the voltage. This whole time i was checking resistance and the resistance is changing. will attempt to figure out the voltage.

[AVRFreakMan]

in the schematic i connected the parallel port to point "A" and removed the potentieometer. I connected r4 to c18 and r3. The program i am using is Java. I am sorry i dont understand what you mean when you want me to check the voltage. This whole time i was checking resistance and the resistance is changing. will attempt to figure out the voltage.

You should NEVER attempt to measure resistance in a powered circuit.

Anyway, remove R4 and it should work. You didn't see the scissors?