FM Transmitter help

[DIY Guy]

So, I just received the first FM transmitter I've ever owned. Consequently, I have no idea what to expect out of this thing.

It is supposed to be a 50mW transmitter. It came with an expandable metal antenna that attaches to the bnc connector on the rear of the unit. It expands to about 2'.

If I tune my vehicle in, about 25' from the unit, I can barely hear anything and it is full of static.

I am not sure if this is just because the antenna isn't tuned for a certain frequency, or if the unit is defective.

I am not adverse to creating a dipole antenna, but I don't want to waste the time if the unit isn't working properly.

Thanks!

[Mactayl]

First of all what Brand is it and what is the model number so we can check the specs on the unit you purchased

[DIY Guy]

The unit I ordered: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260592002285

I know other members have dealt with this seller with no problems.

I guess I can't understand why any unit would be so weak as to only transmit 25' in free air. If it's the antenna, then I understand.

[Mactayl]

Did you ever turn the unit on without the antenna connected and did you try to tune it to a different frequency to see if others work, also check the pin on the antenna bnc and make sure it is not bent, if all looks good then it looks like it might be bad! 50 mw should go much more than 25 feet.

[DIY Guy]

Nope, never ran it without the antenna. I did check the pin and it's good. I checked a few different frequencies, same thing.

Thanks for the help.

[dirknerkle]

The power supply for those things is notoriously noisy. Try connecting it to a 12v battery for the "pure" test.

[Mactayl]

I do not know of anything else to check without opening the unit but that is not an option, I think the unit is bad so I would return it.

[DIY Guy]

Actually, I just opened the unit. THere was a piece of bare copper wire used to jump across some pads and it looked like it sagged and touched some other pads. I bent it up so it wasn't touching any pads.

Now the music is clear, but the hum is way louder than the music. I'm going to try a different power supply and see if it sounds acceptable.

Still sounds kind of weak, but it is definitely better. Hopefully the short didn't screw up anything.

[Mactayl]

Actually, I just opened the unit. THere was a piece of bare copper wire used to jump across some pads and it looked like it sagged and touched some other pads. I bent it up so it wasn't touching any pads.

Now the music is clear, but the hum is way louder than the music. I'm going to try a different power supply and see if it sounds acceptable.

Still sounds kind of weak, but it is definitely better. Hopefully the short didn't screw up anything.

Good Luck, I hope it works

[DIY Guy]

Would the +12v rail from a pc power supply be considered a good power source for this? Easier than taking the battery out of my lawnmower :)

Thanks for all the help.

[Mactayl]

Would the +12v rail from a pc power supply be considered a good power source for this? Easier than taking the battery out of my lawnmower :)

Thanks for all the help.

If you are connecting it up to the +12 rail of a power supply that has a load on it it should be ok, just do not try to connect it to a PC supply that does not have all the other voltages connected to a mother board on it or it may not power up, do ya know what I mean

[Jrd]

Would the +12v rail from a pc power supply be considered a good power source for this? Easier than taking the battery out of my lawnmower :)

Thanks for all the help.

That should work fine but if there are any case/cpu fans connected to it directly (IE. not through a motherboard) then you will probably get some hum from them. Another "pure" 12v power source would be the battery in your PC battery backup.(Oh, remember to unplug and turn off your battery backup before opening it and disconnecting the battery.) Putting a ferrite bead on your power supply line usually cleans it up pretty well.

[DIY Guy]

Ok, I used a pc power supply and it's a little better.

Now the problem is pretty much a weak signal. Off I go to build a dipole antenna.

I've already sent a message to the seller stating I don't think it is working properly. Maybe it's just a really crappy antenna, but I am leaning towards returning the unit.

[Mactayl]

Actually, I just opened the unit. THere was a piece of bare copper wire used to jump across some pads and it looked like it sagged and touched some other pads. I bent it up so it wasn't touching any pads.

Now the music is clear, but the hum is way louder than the music. I'm going to try a different power supply and see if it sounds acceptable.

Still sounds kind of weak, but it is definitely better. Hopefully the short didn't screw up anything.

Can you take picture of the PCB so I can see that bare wire you are talking about becuse sometimes those wires are used for tuning the transmitter and usually it is next to another copper wire and it is use for RF tuninig the output.

[DIY Guy]

Here are the pics.. the wire in question was the long one with 2 or 3 curls in it.

[Mactayl]

Those wires have a hard enamel coating on them and the are used for tuning so when you moved it you changed the RF signal, you can try moving it slightly to see if it increases the the signal, do not use a small screw driver, use a small peice of wood or plastic, if all fails send the unit back

[DIY Guy]

To be clear, the wire is from L17 to C47, not the tightly coiled wires.

It did get better when I lifted the center of it up of the board (ie lifted the center about 1/16").

It worked extremely poorly before I moved it-- now it works very poorly :)

[Mactayl]

To be clear, the wire is from L17 to C47, not the tightly coiled wires.

It did get better when I lifted the center of it up of the board (ie lifted the center about 1/16").

It worked extremely poorly before I moved it-- now it works very poorly :)


Did you check to see if there are any cold solder joints on the PCB ?

[DIY Guy]

Yeah, everything looks good. I can't get to particular on the tiny parts because it's difficult to tell, but with the naked eye everything appears soldered well and the traces look good.

The seller is going to ship me a new one.. only $8 in shipping to send this one back.. ugh.

[Jakeleg1969]

I have one of these transmitters, and it will go much farther than yours. I had a horrible hum when I first received mine. I opened the unit up and the filter cap had burst. I replaced it and everything was fine. Easier for me to do that than sending it back, considering the cheap cost of the capacitor. Not a bad unit for the price if it will work.

Jerry in Louisiana:cool:

[Mactayl]

I have one of these transmitters, and it will go much farther than yours. I had a horrible hum when I first received mine. I opened the unit up and the filter cap had burst. I replaced it and everything was fine. Easier for me to do that than sending it back, considering the cheap cost of the capacitor. Not a bad unit for the price if it will work.

Jerry in Louisiana:cool:

If that is the case check the caps and make sure they are installed with the correct polarity?

[Jakeleg1969]

If I remember correctly, the proper polarity is indicated on the board.. The unit works fine now.

Jerry

[DIY Guy]

I have one of these transmitters, and it will go much farther than yours. I had a horrible hum when I first received mine. I opened the unit up and the filter cap had burst. I replaced it and everything was fine. Easier for me to do that than sending it back, considering the cheap cost of the capacitor. Not a bad unit for the price if it will work.

Jerry in Louisiana:cool:

I wish it were something obvious, but I have looked the board over and can't find anything that looks out of place. Going to get a new one from the seller and keep my fingers crossed.

I have to say the response was immediate. The seller responded to all of my emails in less than 10 minutes. Only downer is paying return shipping for a unit that is defective.

FWIW, the unit was drop shipped from www.fmku.com

[Mactayl]

If that is the case check the caps and make sure they are installed with the correct polarity?

most of all the large blue cap, that should be the DC filter cap

[DIY Guy]

Yes, the silkscreen is marked with "+", which corresponds to the markings on the cap.

[Mactayl]

Well at least you are getting a new one and hopefully you do not have the same problem, I can't think of anything else for you to try.

[rkhanso]

I think you soldered that wire to the wrong location. If you look at the other wires in that section - and specifically the silkscreen on the board.

That should probably be a tightly wound coil just like the others near it and the end you soldered waaaaay over to the left is actually for a leg on a transistor (probably a stage removed to lower the power of the transmitter).

I think you should try to make that wire look like the other coiled wires near it and solder it onto the L17 inductor pad that has nothing on it right now. I'll bet if you do this, it'll work much better. One way to coil up that wire is to wrap it around a bolt (don't remember if it's a 7/16 or 1/2 inch bolt) and winding it around the threads will give you the right diameter and shape. You then 'unscrew' the bolt from the wire and solder it onto the board. Squeeze or separate the coils from each other to give the proper inductance. But the others look like they're tight to each other so that's probably how this one should work, too.

[Jack Stevens]

Actually, I just opened the unit. THere was a piece of bare copper wire used to jump across some pads and it looked like it sagged and touched some other pads. I bent it up so it wasn't touching any pads.

Now the music is clear, but the hum is way louder than the music. I'm going to try a different power supply and see if it sounds acceptable.

Still sounds kind of weak, but it is definitely better. Hopefully the short didn't screw up anything.

I have a similar unit, but with a bit more power. It did not come with a 12 volt power supply; I used a 12v power supply that was supposed to be for a monitor. I had a fair amount of hum in it until I put a couple of ferrite cores, one in the power line, another in the audio cable, and that seemed to clear it up.

The ferrite cores I used were Radio Shack catalog #273-104. This isn't an endorsement of Radio Shack, but they were the only local place that had them.

As far as your antenna - take your u nit outside for a test and use an mp3 player to run it. 50 mw isn't much; it may not be penetrating your house wall that well. I had a similar issue with the "Whole House Gold transmitter" I used last year. The signal was "OK" in the same room but I had issues just the other side of the outside wall. But then, the unit I had was for the upper FM band, there wasn't a frequency selection it could use that didn't have a station on it. Their current products will let you select from the entire band, the one I had only had 6 frequency selections.

[Aurbo99]

It appears you may have a 1mw version of this transmitter, which with your setup as purchased will get you about 25 feet of range.

They most likely shipped you the wrong Tx.

[DIY Guy]

Thanks for all the help guys. The seller gave me a choice of refund or a new unit. Given other members' satisfaction I am going to have him send me another unit.

This unit was preassembled; I didn't have to solder anything. There was a label over one of the screws that said 50mW, but who knows.

The fact that going from the 2' expandable antenna to a tuned dipole antenna had no effect seems to indicate something. What that is, I have no idea :)

[Mactayl]

Thanks for all the help guys. The seller gave me a choice of refund or a new unit. Given other members' satisfaction I am going to have him send me another unit.

This unit was preassembled; I didn't have to solder anything. There was a label over one of the screws that said 50mW, but who knows.

The fact that going from the 2' expandable antenna to a tuned dipole antenna had no effect seems to indicate something. What that is, I have no idea :)

I like you train of thought ! Get rid of it!