FM Transmitter...Please Help Me!!!

[vne147]

Hello everyone. A friend of mine asked me to build him a small FM transmitter so he could broadcast an mp3 player to his car stereo. I found plans online and built the circuit on this page:

**broken link removed**

I believe I didn't make any errors in laying out the schematic or board and I'm pretty sure there are no problems with the board itself (i.e. broken traces, shorts, etc.) The only problem is, it doesn't work.

When I try to perform the steps at the bottom of the page labeled "Test & adjustment" I'm running into problems. The voltage at TP2 is 5V and adjusting the slug of L1 makes absolutely NO difference. The type of adjustable coil I'm using is this:

I've done some poking around trying to troubleshoot the problem and I've discovered that the voltage at the end of L1 that's not connected to ground is 0V. Also, the voltage at pin 9 is about 2.9V.

Lastly, and I'm not sure if this is normal but the BH1417F IC gets hot when the circuit it powered. It's not so hot that it burns my finger but it's noticalby warm.

Anyway, if anyone has any ideas about what might be wrong or how I can further troubleshoot please, please, please, let me know. If you need more information to help me just let me know and I'll provide whatever I can.

I also posted a screen shot of my .sch file. I didn't want to post the file itself because it uses a lot of modified libraries but if anyone wants it I'll zip the .sch, .brd, and all the necessary .lbr files.

As always, thanks in advance for any help you can offer me.

 

[Mike - K8LH]

You can **broken link removed** for $5.87 (free shipping)?

**broken link removed**

 

[Mikebits]

I've done some poking around trying to troubleshoot the problem and I've discovered that the voltage at the end of L1 that's not connected to ground is 0V.

That is okay, there is a cap on each side of L1 so no DC.

Your coil L1 is most likely your problem. You need to build it exactly as in the article. You should use same ferrite core as recommended in article. Your coil seems to be higher up on the coil former than in the article, this will add inductance and change your osc frequency.

 

[vne147]

You can **broken link removed** for $5.87 (free shipping)?

Mike,

I know they are readily available and cheap but what fun would buying one have been?

That is okay, there is a cap on each side of L1 so no DC.

Your coil L1 is most likely your problem. You need to build it exactly as in the article. You should use same ferrite core as recommended in article. Your coil seems to be higher up on the coil former than in the article, this will add inductance and change your osc frequency.

Mike,

I looked around online a lot for a coil like the one in the article but couldn't find anything. Got any suggestions or know of any sites where I could purchase a suitable coil or parts to make one? Also, if anyone here has what I need, I could buy it directly from them. Lastly, in your opinion would the difference in L1 between what I used and what's in the article be responsible for the voltage at TP2 not changing at all. Even if the inductance was different than what the circuit was designed for, I would expect the voltage at TP2 to change at least a little. What do you think?


Thanks for the help.

EDIT: I forgot to ask one question. When I'm measuring the voltage at TP2, I have my multimeter set on DC voltage. Is that correct?

 

[Mikebits]

I also see you have a different varactor than in the article, this could be a problem as well. I think futurelec has the right diode. Compare the data sheets, the specs are different.
https://www.electro-tech-online.com/custompdfs/2010/11/ZC834BTA.pdf
https://www.electro-tech-online.com/custompdfs/2010/11/MMBV2101LT1-DPDF.pdf
ZETEX | ZMV833BTA | ZMV833 Series 330 mW 25 V 33 pF Silicon Hyperabrupt Varactor Diode - SOD-323 - Future Electronics

 

[Mikebits]

Mike,

I know they are readily available and cheap but what fun would buying one have been?



Mike,

I looked around online a lot for a coil like the one in the article but couldn't find anything. Got any suggestions or know of any sites where I could purchase a suitable coil or parts to make one? Also, if anyone here has what I need, I could buy it directly from them. Lastly, in your opinion would the difference in L1 between what I used and what's in the article be responsible for the voltage at TP2 not changing at all. Even if the inductance was different than what the circuit was designed for, I would expect the voltage at TP2 to change at least a little. What do you think?


Thanks for the help.

EDIT: I forgot to ask one question. When I'm measuring the voltage at TP2, I have my multimeter set on DC voltage. Is that correct?

Yes DC voltage is correct for TP1.
The reason the voltage at TP1 is at 5V is because the frequency is too far off. The TP1 is going to 5v rail in attempt to lower the varactor capacitance, but can't go beyond 5v. This is indicative of your using a different varactor.
Make sense?

 

[vne147]

Yes DC voltage is correct for TP1.
The reason the voltage at TP1 is at 5V is because the frequency is too far off. The TP1 is going to 5v rail in attempt to lower the varactor capacitance, but can't go beyond 5v. This is indicative of your using a different varactor.
Make sense?

I think I understand that but the varactor I used was listed as an alternate in the parts list. Do you still think I should go with the other one?

 

[Mikebits]

I think I understand that but the varactor I used was listed as an alternate in the parts list. Do you still think I should go with the other one?

Was it? hmmm I will have to go back and recheck. Correct me if I am wrong, you are using MV2109 in place of ZMV833?
I want to verify I am looking at correct data sheets. I think AG has some experience with the ROHM part, maybe he will chime in, but in the mean time I will recheck the varactors.

 

[vne147]

I just discovered that when I test for continuity between the bandpass filter input and ground it's continuos. Not sure if this is normal or not without knowing the specific internals of the filter. Does anyone know if this is OK? Here's the datasheet:

GFWB3 FM Bandpass filter

 

[vne147]

Was it? hmmm I will have to go back and recheck. Correct me if I am wrong, you are using MV2109 in place of ZMV833?
I want to verify I am looking at correct data sheets. I think AG has some experience with the ROHM part, maybe he will chime in, but in the mean time I will recheck the varactors.

Correct, I am using the MV2109 in place of the ZMV833. In the parts list at the bottom of the article in the semiconductors section it says "ZMV833ATA or MV2109 (VC1)". Thanks for double checking.

And if Audioguru is reading this thread, please help me sir.

 

[Mikebits]

Well... You are right, the parts list does say the 2109 is an alternate, but if you compare the graphs from the data sheets below you will see they have very different specs, so now I am confused

From what I can see, the 2109 is around 40pf @ 5v, the 833 is 20pf @ 5v.

 

[vne147]

so now I am confused

That makes two of us.

From what I can see, the 2109 is around 40pf @ 5v, the 833 is 20pf @ 5v.

I'm seeing about 22pf for the 833 and 35pf for the 2109. How can we tell if that difference is significant?

I'm going to try to modify the existing coil to closer match what is described in the article. I also don't know what type of ferrite material the slug I'm using is made from. The site I bought it from had no specs at all. I usually don't buy from any site that doesn't thoroughly describe their componenst but like I said, I had trouble finding a coil.

Please keep brainstorming and let me know if you think of anything else. I'll report back after I've modified the coil. Thanks Mike for your help.

Anyone else out there have any ideas?

 

[Mikebits]

Lets analyze this. Since TP1 is at 5v we can assume the osc freq is too low because if it was too high TP1 would be at 0v. This is true because if you look at varactor graph capacitance decreases as voltage increases. So if it is to low, we need less cap or less inductor. Thus we need less length in L1. So try playing with that. Does that make sense?

 

[vne147]

Lets analyze this. Since TP1 is at 5v we can assume the osc freq is too low because if it was too high TP1 would be at 0v. This is true because if you look at varactor graph capacitance decreases as voltage increases. So if it is to low, we need less cap or less inductor. Thus we need less length in L1. So try playing with that. Does that make sense?

Yeah, it makes sense. I'll fiddle with L1 since I don't want to wait for a futurlec shipment from China. I'll let you know if I'm able to get it working. Thanks!

 

[4pyros]

Vne147; This is the same Circuit the belkin tunecast II uses minis the RF amp. Thay used air coils and tuned them by seperating the coils and gluing them. Andy

 

[vne147]

Vne147; This is the same Circuit the belkin tunecast II uses minis the RF amp. Thay used air coils and tuned them by seperating the coils and gluing them. Andy

OK, I'm making progress at least. I switched out the adjustable coil for an air coil and slowly adjusted the length of the leads that were soldered into the PCB until TP2 was around 2V. So it appears L1 was at least part of the problem. Now, it is transmitting on the expected frequency but I don't hear music. Instead, there is this annoying repeatative warbling tone. It is obvious that it's coming from my transmitter because when I switch it off, the radio goes to static.

So guys, where do I go from here?

Thanks!

 

[mvs sarma]

Yeah, it makes sense. I'll fiddle with L1 since I don't want to wait for a futurlec shipment from China. I'll let you know if I'm able to get it working. Thanks!

i suppose that your regulator is wired correctly as the sch screen dump showed reverse connection of input and output. the best appears to use air cored coil. you can make a turn extra, tune Rx to mid frequency like 98 or 100MHz. feed the input.
amd with slight compression or expansion of coil length, manage to Receive the signal, how ever weak, initially. it lets you improve other things.
there were some moderation on this circuit, and I have a word file captured from another site few years back, i can mail
you may please send the eagle files tome at mvssarma(at)gmail
i have BH1417 ready with me , can make one and share the experience.

 

[vne147]

i suppose that your regulator is wired correctly as the sch screen dump showed reverse connection of input and output. the best appears to use air cored coil. you can make a turn extra, tune Rx to mid frequency like 98 or 100MHz. feed the input.
amd with slight compression or expansion of coil length, manage to Receive the signal, how ever weak, initially. it lets you improve other things.
there were some moderation on this circuit, and I have a word file captured from another site few years back, i can mail
you may please send the eagle files tome at mvssarma(at)gmail
i have BH1417 ready with me , can make one and share the experience.

Good catch on the regulator. I did make the mistake of connecting it backwards in the schematic. Fortunately, I realized this before I soldered it in the board and just flipped it around. So, the 7805 is soldered in correctly and at least that part of the circuit is working.

I'll zip all the Eagle files and post them here tomorrow.

Any ideas about my latest problem?

 

[mvs sarma]

you may reduce the audio input level by managing the Volume control.of your mp3 player.thsignal strength or streo lamp would be an indication of signal tuned. the attached mods have specific ref to siliconchip's micromitter schematic. I thanks the originator of these mods.

 

[Mikebits]

Glad your making progress. One thing to consider, going to an air core inductor may reduce your Q of the inductor, this could result in instability in your VCO. You might need a scope to see this on the VCO control voltage TP1. One other thing you might try is to remove one winding on the cored inductor. Few questions:

• Does the stereo receiver pilot light turn on when your testing?
• Have you tried the unit with no audio in. Should get silence
• Do you have access to a scope?

That's about all I can think of at the moment. Your getting close