Audioguru's FM Tx Troubleshooting

[Menticol]

Hello!

I'm building the famous Audiguru's FM transmitter.

After lots of reading on this and another forums, I got the following "Construction Rules":

1) Spacing between components: < 2 cm
2) Veroboard only - unused tracks must be removed
3) 30" antenna
4) L1 and L2 - see attached file (inductores)
5) Try to make GND track as thick as possible

_____________________________

I had some trouble getting the parts.

a) I got abnormally huge and ugly polyester capacitors
b) 5-35 pF was not available. I've used 0-70 pF instead
c) The voltage regulator was not available. I've used a lousy LM7805 instead.
d) 160K resistor was not available. Built using two separate resistors in series.

As you already guessed at the beginning of the post, the transmitter is not working. Any suggestion would be very, very appreciated.

PS: The awful ceramic capacitor flying over the board was only a test, trying to find the problem (frustrating). It was placed on a more decent way before taking the picture.
PS2: The microphone was removed, as you can see

 

[Menticol]

Please, can any moderator move this topic to Electronic Projects? Radio and communications section doesn't move very much

 

[colin55]

You haven't got the 4p7 connected correctly

 

[audioguru]

My coils are insulated with enamel paint, yours look like bare copper so they are shorted.
I used two 1000pF (0.001uf) ceramic capacitors to filter the power supply voltages at RF frequencies but you used something different (yellow tantalum?).

My coils and tuning capacitors are spaced a little away from the circuit board and away from other parts.

Tuning capacitors never go to zero pF. Maybe yours go too high so then the number of turns on your coils should be reduced.

 

[Menticol]

You haven't got the 4p7 connected correctly That's true!

My coils are insulated with enamel paint, yours look like bare copper so they are shorted. Haha no Audioguru, it's yellow enamel. They are not shorted.

I used two 1000pF (0.001uf) ceramic capacitors to filter the power supply voltages at RF frequencies but you used something different (yellow tantalum?).Yes they are. But I will replace them if you advice that. - By the way, what do you think about the big and ugly green capacitors? Are they adequate?

My coils and tuning capacitors are spaced a little away from the circuit board and away from other parts. A total fail, my spacing is zero

Tuning capacitors never go to zero pF. Maybe yours go too high so then the number of turns on your coils should be reduced. Looks like the old lady on the shop tricked me again. I'll try with one turn less

Thank you very much by your suggestions Colin55 and Audioguru!!!! I'll fix the issues and post the results

 

[audioguru]

I have not used tantalum capacitors for 35 years so I can't remember if they are any good at 100MHz. I know that 1000pF ceramic capacitors with short leads are good so I used them.
Green caps are Oriental film capacitors. Yours are huge but they are fine. I use much smaller European film capacitors.

Try two turns less on your coils since your trimmer capacitors have a capacitance of double what is needed.

 

[transistor495]

Menticol, I'm very much disappointed on seeing your version of the transmitter. You've spent lots of time on the pcb for removing unwanted copper tracks, but still it's not working. It'll be fairly easy to build it using this kind of boards:

..and you can use the component leads itself underneath to bridge between the terminals.

Your component selection and layout/placing looks very bad especially that ceramic cap with much spreaded legs -finally that telescopic antenna looks funny there on the unworking circuit. You can place it once the circuit comes alive!

That was a much open criticism

 

[audioguru]

I used stripboard where the strips form half of a pcb and parts and a few jumper wires form the other half.

 

[Menticol]

Your opinion is very valuable for me Transistor495!

That board looks much better, but I didn't have that type available on my workbench. Anyway, removing the tracks took only seconds because I have used a dremel: It's a very good technique, just hold your breath and keep your hand steady to avoid removing useful tracks.

You are absolutely right about the badly placed capacitor legs. It was placed on a decent way using a jumper wire, but during my angry search I removed the jumper and placed the capacitor in that way. The fat green capacitors messed up the component placing (Sticking to the <2 cm spacing rule was pretty hard)

I don't find anything strange about the antenna, I think it's better than a chunk of wire and keeps the morale high

As Colin55 said, the problem may be the 4p7 capacitor. I haven't fixed it yet, the University takes all my free time I'll fix and post the circuit as soon as possible

 

[mvs sarma]

They are also enameled but lighter color enamel but with more transparent enamel. So they appear as coils with bare copper.

 

[transistor495]

Hello!

I'm building the famous Audiguru's FM transmitter.

After lots of reading on this and another forums, I got the following "Construction Rules":

1) Spacing between components: < 2 cm
2) Veroboard only - unused tracks must be removed
3) 30" antenna
4) L1 and L2 - see attached file (inductores)
5) Try to make GND track as thick as possible

_____________________________

Let me look onto the 5 construction rules what you've collected so far:

1. Components should be placed and spacing has to be adjusted in an artistic and convenient way keeping in mind that the critical RF parts should be as close as possible unless otherwise stated in the article. Leave '<2cm'.
2. I don't get 'veroboard only'. A designed and etched pcb is the best one!
3. Antenna length is afterwards matter.
4. In the original construction you can see there's nothing components placed in between L1 and L2. The regulator unit is kept independently at the top corner. You've placed the components exactly as shown like in the schematic -there's the mistake and there you loss the RF touch.
5. Again this is not much a critical matter!

I think it'll be better to go for a fresh assembly by keeping in mind all these points. This unit will hardly deliver the range of a 50mW single stage fm TX if it ever get works

 

[audioguru]

I made my FM transmitter on Veroboard (stripboard) and did not remove any unused tracks because I planned the layout so that all tracks are used.
In fact, most tracks are used for a few different functions with a cut in between.
A few tracks have unused sections of tracks with cuts separating them from used parts of the tracks.

 

[sanjubluerock3]

hii Menticol please construct using pcb mvs sarma has the pcb layout....and he is really helpful...

 

[mvs sarma]

Thanks Sanjubluerock3

here are the PDF files that can help Menticol make the PCB and test and use it. If , on the otherhand he is interested to make his own artwork, perhaps i can share the eagle files and he can mod them to what suits him.

 

[audioguru]

Hi sarma,
I used small 1000pf ceramic capacitors for C10 and C11 in your circuit because they work well at 100MHz and do not cause the low dropout regulator to oscillate.
Your 0.1uF capacitors are 1000 times more which are probably very poor at 100MHz and probably cause the low dropout regulator to oscillate.

 

[mvs sarma]

Thanks AudioGURU,
the OP can mount the specific components as per your recommendation. I shall also be changing the sch as per your suggestion.

Ps: Fresh PDF files, adopting the suggested capacitor values attached.

 

[ckchew8]

hello there everyone,
I've seen countless FM transmitter circuits and happen to stumble upon MOD4. I am interested in building it but before doing so is there a site by audioguru or anywhere i can get the explanation on the circuit operation, how the values were chosen and etc? Thanx in advance and great job audio guru!

 

[audioguru]

hello there everyone,
I've seen countless FM transmitter circuits and happen to stumble upon MOD4. I am interested in building it but before doing so is there a site by audioguru or anywhere i can get the explanation on the circuit operation, how the values were chosen and etc? Thanx in advance and great job audio guru!

On another forum somebody posted an FM transmitter schematic that did not work:
1) Its mic preamp transistor was biased so that it was saturated when the battery was 8.0V or more and was cutoff when the battery was 7.0V or less.
2) The frequency changed as the battery voltage ran down.
I fixed both by re-biasing the preamp transistor and adding a low dropout voltage regulator.

3) The frequency changed when something moved towards or away from the antenna.
I added an RF amplifier transistor to isolate the oscillator from the antenna and it gave much more range.

4) The sound quality was horribly muffled without any high audio frequencies.
I added pre-emphasis (treble boost) like all FM radio stations have. All FM radios have de-emphasis to reduce the boosted high audio frequencies back to normal level and at the same time reduce hiss.

I made 4 modifications and Mod4 is the last one.

The parts values were calculated with simple arithmatic.

 

[ckchew8]

Hi audioguru!
it's a great honor to receive a message from the master so thank you for replying my mail! i've been wanting to build one for years and hopes this will work well...just to confirm, the circuit by mvs sarma is the final version you mentioned? Btw, in mvs sarma PCB design it does not have any pour copper to reduce high frequency noise in it.(leaving all unused copper behind and grounding it) Is it advisable to do so? Again, thank you for your reply! have a great day!

 

[audioguru]

I didn't look at Sarma's pcb design and I think nobody has built it.
I made mine on stripboard.

It is illegal because it might cause interference to a real radio station. I used mine for only one hour testing its range and its excellent sound quality. It is old because it is not stereo and does not use a crystal-controlled frequency synthesizer.