New AM Transmitter on ebay

How did you test and measure?  With oscilloscope  or with DVM?

If you have schematic of circuit you can send it to me and I will try to explain operation.

Mike

The schematic is posted on the link above from ebay.

Modulator Q5 varies the Amplitude of final RF stage Q3, a type circuit seen before in other transmitters.

Why it gives all the characteristics described in the opening post I am unable to guess.

There is a DC path from the base of Q4 to the collector of Q3 so the DC bias conditions on Q4 set via R9 will set the power input to the final amp (Q3).  R9 is more likely to control the final power input than the audio gain though it can do so by changing the input Z of Q4 as seen by the audio source.

Q5 amplifies the DC and audio with negative feedback to Q4 through R13 and frequency response set by the capacitors C13 and C14.  Negative feedback is usually used to improve linearity but can also be used to establish the gain.

The audio modulates by varying the voltage applied to Q3 collector.

To measure the final input power you need to measure the DC voltage from Q3 C to E and the collector current.  I see no easy way to measure current without breaking the circuit preferably between Q5 collector and L2 and inserting the current meter here.

Neil

This is what I tought was happening, basically the RF power is being lowered to bring up the percentage of modulation, From using the little transmitter.I have found it is a very nice low price kit if you want a LC oscilliator and basically one frequency... but sometimes thats all a person needs!  I am going to break the power to the final, and keep the inductor to keep RF out of the supply voltage, and insert a modulation transformer at that point, and drive the transformer with a seperate audio amplifier and see what happens! 

Radio Joe,

For the price this does seem like good deal.

I noted that the kit ships from Asia. Did you encounter any delays or problems with delivery?

Any comments about the quality of kit/parts?

Is the audio quality satisfactory?  I'll be curious to learn if adding the transform improves the performance.

Mike

Hi Mike, Far as shipping, this took about a month to get to Minnesota, the kit comes with no instructions but all documents are on the eBay site where the listing is posted. Instructions there include the numbers stamped on the caps so you can identify the correct values. I feel this is a quality kit, I used an MP 3 player for audio and this produced a  great over all sound and  nice low end which needs to be mentioned.

The circuit provides a balanced sound meaning good audio response to the ear. I haven’t run any tests to check audio response on test equipment. The only bitter sweet disappointment was without any modulation the transmitter produces about 300 Mw. That in its self was not a disappointment, but when you modulated the system meaning just turning the modulation control up, the RF power was was graduadualy reduced, meaning turning the control for modulation simply reduced the RF power, this is without audio going in or not, so that is why I’m going to see if I can modulate the carrier with a modulation transformer in the DC to the final. This shouldn’t be much of a problem but depending on what I find in the parts box for a transformer and how well it works with a given transformer remains to be seen.

To modulate this  unit to 100% you will need only about a 150 Mw audio amplifier if the RF output is 300 Mw. As it sets, out of the box, for $13 no way you can go wrong if you just want to put your tunes on the air around the house to your favorite radio.  

I need to point out that the schematic shown on eBay is not 100% accurate, if you look at the power connector and the LED, it looks like the positive side of the power should go to ground; I connected it that way the first time after assembly and burnt up the transistors and regulator! I contacted the seller and they said positive goes to center pin on the power. There is no reference to that in the instructions and based on the direction of the LED, it looked like the circuit had a positive ground but then I was anxious to get it up and running and didn’t look at the other active comports to see if a positive ground was needed there, which obviously that was not the case.

I replaced all active components and repowered and the transmitter worked great. I think I also reversed the polarity on the LED to get it to light after that.

To get maximum signal out of ten feet “or so” of antenna you will need to run the RF output into a loading coil, this is a simple job, I used an crystal radio loop stick with tunable core in series with the antenna, this worked perfectly for this low power application. I used a simple field strength meter to check he output when peaking.  Fun little circuit with a lot of potential, It has a stable oscillator, great mini power amp section. Like the add says, “Transmitter for learning!”

Would be interesting to combine the frequency selection circuitry of the SSTAN 3000 with this kit's modulator and RF section.

I tried a few transformers here to use as a modulation transformer, dirven by a small amplifier then using the secondary of the transformer (500 ohms) in series with the power to the final amp on the transmitter. It all worked but it seems like the orginal circuit worked just as well minus some RF power because of the settings. Over all nice little transitter but if you add the cost of a cabenit and loading coil and few extras your better off with a TH 5. At anyrate this is still a good package. Thats all I have to report. 

If one does decide to purchase a TH-5 new spend the extra bucks for the "enhanced" version. Audio is much better.

Joe,

I  ordered and received 2 of these kits. Before I  get to building them, I want to confirm the  power connection issue that you mentioned. In the schematic posted on eBay, it looks like the LED is connected to indicate ground being negative. Did the schematic get corrected I  wonder?

Mike