Wednesday, September 03, 2008

Simple Belkin Tunecast 3 mod to boost signal

In my Belkin Tunecast 3 FM transmitter disassembly article I noted that "The radio signal is attenuated before being passed to the antenna". Attenuator may be seen on the back of the circuit board between the FM transmitter IC and the antenna. Here is a closeup:
The pin at the top left corner of the IC in the bottom right is the RF output and the wire connection near the L5 marking near the top left is the antenna. Here is a schematic:
It would be simple to remove C38 and connect the internal antenna directly to pin 12. This would result in a more powerful signal. One could also connect an external antenna to pin 12. It might be better to put a capacitor between pin 12 and the external antenna; one circuit diagram uses 10pF. Note that some modifications may increase the signal beyond permitted limits.

34 comments:

David said...

Would it be a good idea to add a variable resistor to C38 to adjust it's transmission range?

Boris Gjenero said...

As long as you're using the internal antenna, you probably won't need to decrease the range. If you want to decrease the range, the simplest solution would be a potentiometer with one end connected to the output and the other connected to ground.

Nerv said...

So to enhance the transmitting power, it's best to connect an external antenna to IC pin that is connected to C38? Antenna length something about 25 inches?

Boris Gjenero said...

Yes, the internal antenna is very short and an external antenna would help a lot. You might want to connect a 10 pF capacitor between the pin and the antenna. 25 inches seems reasonable. If you want to know more about antennas, there's a lot of information online.

Anonymous said...

Around 760mm (or 30 inches) would be almost optimal for the antenna. 1/4 wavelength at 99MHz.

Where you say "remove C38 and connect the internal antenna directly to pin 12" do you mean simply remove C38 and bridge the pads or remove it and bridge pin 12 to the the output of L4?

Boris Gjenero said...

I mean remove C38 (breaking the circuit there, not shorting across C38), and take the output directly from the chip. The resistors are the real problem; they decrease signal amplitude. The point is to disconnect those from the signal path.

I'm not actually experienced with RF circuit design, and someone with such experience may have some better ideas.

Anonymous said...

How did you know about the attenuation of output and what does the adding a 10PF cap. do?

Boris Gjenero said...

The attenuation is mentioned in the operational description which was was submitted to the FCC. I found it by searching for FCC K7SF8M010. This was mentioned in my disassembly post. Even without that information, the attenuator is quite obvious; just trace connections from the antenna.

I've seen schematics which use FM transmitter ICs from the same family and just have a 10 pF capacitor between the RF output pin and antenna. I guess it helps filter out low frequencies and protect the IC.

Boris Gjenero said...

Another idea: Remove R36 and R39 (the resistors which connect to ground), short across R37 and just leave C38 and L4 between the output and the antenna.

mark said...

I just have some sensible advice for everyone on this and other sites about mods to transmitters. Just get a "real" transmitter!, instead of getting these "toys" that were only made to go across a room or get your MP3 to car and they don't even do that well!!. This is like getting a basic economy car and modding it to try and make it work like a corvette!!
Check out...Ramsey, mobile blackbox, hobbytron, canakit, decade, and the best of all HLLY ELECTRONICS based in China. they have a fantastic selection. They have one 1/2 watt the size of a cigarette package that with the stock telescoping antenna goes 1/2 to 1 km!!!. with a BNC ant. connection for any antenna.This costs $70 US and free shipping! How much did you pay for your tunecast? See how much the ccrane one is(30ft) wholehouse transmitter $100(150ft 7channels).
Its a nobrainer!! just get a more powerful transmitter. Just how much more will you acheive trying to "mod" the tuncast 3.

Boris Gjenero said...

I paid $5 for my Tunecast 3. Yes, it's a piece of crap, but at that price I cannot complain.

TC3fiddler said...

I've found that a simple antenna to boost the paltry signal from the TC3 is to fit it with a loop antenna about 1 Metre long (wire length of 2 Metres), and replace the internal antenna, with one end going to Pin 12 of the chip and the other to the Ground, now my TC3 has a decent range, of more then the original 10 centimetres, which makes it far more useful, now if only I could get rid of that annoying auto shut-off thing when I'm listening to music with quiet sections...

Boris Gjenero said...

I found that when I carefully adjusted input volume, going to the point where I can hear distortion and then backing off a bit, I didn't have auto shutoff problems.

TC3fiddler said...

Well, the music I have trouble with is quite loud in places and I adjust out the distortion then, but even so, the quiet parts (I'm talking of orchestral music) are still below the TC3's threshold, resulting in the beeping (which I can't seem to override) and then cut out, a pain in the backside!!!

Anyway, I was testing out my TC3 today, with a radio at one end of the house and my TC3 & laptop at the other, a fair distance which the original antenna couldn't get through, and my music was coming through loud and clear, all I need to do now is find my walkman and go for a walk to see how far I'm transmitting, in order to trim the antenna down to within "legal" limits. :)

Boris Gjenero said...

Shutdown is probably controlled by the small square chip, the Cypress CY8C21434 mixed signal array.

TC3fiddler said...

Hmm, flash chips are something I'm good at breaking, at least in the sense when I have a go at programming and screw it up!!! :S

Unless there's a way to bypass the volume threshold through a blob of solder, I'll have a look at that chip datasheet...

Anyway, I had to trim the antenna down as I was getting a little too much range on it, I wanted it to get through the walls, but not reach the railway station across the way!!! :S

But anyway, it works, I'm using the connect directly to the Pin12 method (though I left the original circuit in place incase I get collared by OfCom (I'm in the UK, and they don't like any FM transmitters in the hands of joe public!!!) and I can return it to original), and all it took was a longer antenna to get it to work properly!!! :D

TC3fiddler said...

Here's one for you, is it possible to integrate or at least connect an RDS encoder to the TC3? It would be nice to have something other than the frequency on the display of my receiver, even just a simple name like "Tunecast" or whatever would be preferable to "107.80". :)

Anonymous said...

Newb here, I'm using the crappy TC3 at work; 50km from the nearest town. Is the best way to run an external antenna from pin12 to the ground? Straight off pin 12? Off the internal antenna? Ino this is obvious to you guys but thanks for any help.

TC3fiddler said...

Connecting the red antenna wire very marginally improves the signal strength, the best way to improve it is to use a wire outside of the casing (I personally think the batteries restrict the signal through reflection which would cancel out a lot of the signal), use about 50CM of thin 2-core wire, solder one end to Pin 12 (or to the resistor on the chip-side), and the other to the Ground connection point, then solder the other ends of the wire together and cover with tape or heat-shrink tubing, tie a knot in the wire inside the casing (with enough room as not to pull off the soldered ends), and finally cut a hole in the casing and pass the wire through then power up and enjoy your tunes on the radio. :)

Anonymous said...

Thanks for the help, I was using the wrong ground point so initially I just extended the antenna about 3 feet. It work alright most of the time except for some occasional static. Works alot better now!

TC3fiddler said...

No problem, it's great once it works properly, as it's a clever little device, it's just poorly designed (or overly restricted due to legal legislation), but as long as you don't broadcast too far, you should be fine... :)

I have actually made an instructable based on this mod (with a link to here as this is where I found it originally), it shows how I modified my TC3 and what I did, I'm sure it'll be useful to others. :)

http://www.instructables.com/id/Belkin_TuneCast_3_Antenna_mod/

Anonymous said...

Heres a thought... hook that little bugger to a tv signal booster and connect that to a tuned antenna on your roof. I did this with impressive results.
Good luck... Halfpint

Anonymous said...

I've done the tv booster trick with the older tunecasters and got remarkable rusults, upwards of .5-1.0 miles. depending on how much gain the booster has.

Anonymous said...

Isn`t short circuiting the inductor L4 a better and cleaner solution?

Boris Gjenero said...

The resistor network also limits signal strength. If I was just going to short-circuit one component, I would short circuit R37.

duane said...

Hi. Thanks to the info found here,
I have just removed c38 and soldered about 6" of wire direct to the pin. i drilled a small hole through to the battery compartement and looped it around the batteries. It now works great. V-Strong signal and good sound quality.

ryan said...

Hey guys I have the tunecast IIII. Does anyone know where I could get a Guide for this one?

Boris Gjenero said...

Open it up, find the FM transmitter IC, find its datasheet and examine the circuit.

If it has an FCC ID on it, look that up. You can find internal pictures and other information there

ryan said...

I searched Both FCC ID and IC numbers om the that site and they both gave me nothing. The numbers are FCC ID: K7SF8Z343 and IC: 3623A-F8Z343. I did find two wires that look to be antennas labeled RX_ANT and TX_ANT.

Boris Gjenero said...

The FCC ID search worked for me. I see the link I posted earlier is 404, so here's a new link to the search page. There I find a form with two fields. I put the first 3 characters in the first field and the rest in the 2nd field and got this result.

Based on the internal photos, I wonder if the square chip near the antenna connections is doing the FM modulation. It seems to have a Belkin label, which would mean it's a custom chip for which you cannot find a datasheet. It's probably best to just examine the circuit that connects to the TX ANT wire. Check for attenuation like I described with Tunecast 3 and/or hook up a longer antenna.

ryan said...

First off, thank you for helping. secondly I don't know if you saw this but my circuit bored is all blacked out with some type of paint. There small lines to witch i can follow to a point. But in addition the blackness there is glue and crap on there that are inhibiting me from seeing where every thing goes. So it looks like my best option is making the antenna longer.

Ray890 said...

I don't own the Belkin Tunecast 3 transmitter as of yet, but I'm thinking of MAYBE getting this one as a temporary low cost FM Transmitter, until I save up enough for the C.Crane 2 FM transmitter w/ the transmitting power mod (It seems to be twice the price in Canada).

My question is, what is the range before and after applying the circuit and antenna-extension mod? (I'm hoping for about enough range to get good signal in other rooms in my house)

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