Low-Power AM Broadcast Band Transmitter

The modulation control is just a series variable resistor instead of a voltage divider volume control. If it re-wired as a volume control then it needs another capacitor in series to prevent shorting the output of the preamp transistor to 0V.

Hi Audioguru;

Perhaps I gave the wrong impression about the modulation-related control.
It appears to be wired as a series variable resistor. However; the article by Andy Collinson refers to it as controlling the level of modulation. You seem to have something in mind. Could you elaborate?

VA6GDG said:

Hi Audioguru;

Perhaps I gave the wrong impression about the modulation-related control.
It appears to be wired as a series variable resistor. However; the article by Andy Collinson refers to it as controlling the level of modulation. You seem to have something in mind. Could you elaborate?

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A series resistor is a poor way to adjust the level of modulation which is a volume control. I don't know why he didn't use a voltage divider that is a real volume control or modulation level adjuster.

Potentiometer Connections

Appreciate your suggestions. I stand to learn something here.

With regard to the current circuit...

Actually, I think it's wired incorrectly even for a variable resistor. The input should come in one end of the potentiometer resistance, the output should be taken from the other, and the wiper should be connected to the output terminal.

But, if it were a volume control, the input would go to one end of the potentiometer resistance, the other end would go to ground (common to both input and output) and the output would be taken from the wiper. Which end is input and which is grounded would determine whether it operates CW or CCW. (And ... I guess I should double-check the taper. That could be a problem for the apparent sensitivity in itself.)

I've attached a small diagram of typical potentiometer connections (from my store of notes and diagrams) for reference.

So, on to what you suggested...

I can imagine that this might upset the current version of the circuit unless proper capacitors were in place as you suggest. Where would the new capacitor be inserted and where would the existing capacitor connected to the wiper? Would the value of the new capacitor be the same as the existing one or would both be replaced with ones of a new value?

The circuit and the modulation level control should be like this:

Edit: Another coupling capacitor was added to the modulation level control.

OK. So, there isn't an additional capacitor as in the previous post on the modulation control. I just need to rewire the existing one per your diagram and change pot to 25k.

By the way, does the 4.7uF need to be non-electrolytic as shown? A 4.7uF ceramic would be a bit difficult to get.

(I'll still put in the 10 pF cap around the battery as previously suggested.)

By the way; thanks so much for the diagram. I really appreciate the trouble you took to make it.

I also note that R1 is only 4.7k, rather than 10k as originally thought. That may help a bit too.

I forgot to add the extra capacitor. It is added to my previous schematic.
Use two non-polarized metallized film 1uf capacitors instead of a 4.7uF capacitor.

Found a good explanation of how the transmitter works (with calcs) by Harry Lythall
at his page: **broken link removed**

Well worth a look!

Hi Audioguru;

You were very specific is the use of non-polarized metallized film 1uf capacitors instead of a 4.7uF capacitor. Is there a reason for using non-polarized and this type in particular? Should I be considering the use of this type for all audio-frequency coupling capacitors?

I'm not questioning this choice at all. I'd just like to learn something from this and hopefully ask fewer questions as I get better at circuit design.

VA6GDG said:

Hi Audioguru;

You were very specific is the use of non-polarized metallized film 1uf capacitors instead of a 4.7uF capacitor. Is there a reason for using non-polarized and this type in particular? Should I be considering the use of this type for all audio-frequency coupling capacitors?

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You can use a polarized electrolytic capacitor for each 1uF capacitor. I always use film capacitors for coupling audio.

If the location of C7 makes it work or not, then the resistance of the battery is an issue. A cap across the +9 and 0V rails, as near as possible to the oscillator section, should help.

hi please am designing a low power am broadcast transmitter could u have a circuit other than the one provided to do the job. specifically incorporating a crystal oscillator, a buffer amplifier, audio amplifier, modulator and rf amplifier.please

papa said:

hi please am designing a low power am broadcast transmitter could u have a circuit other than the one provided to do the job. specifically incorporating a crystal oscillator, a buffer amplifier, audio amplifier, modulator and rf amplifier.please

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Yes.

You are designing it. We are waiting.

I think he is designing a box for the AM transmitter.
He wants the circuit to be designed by somebody else.

what is the meaning of 200uH coil?

pepsi said:

what is the meaning of 200uH coil?

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The value of the coil is 200uH in inductance.

what is the meaning of 200uH coil?

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Its part of your tank.

is there any circuit to control voice modulation and operate the different electrical equipments.

Mr. Audioguru,
This is a very old post and the original schematic was one of the most common search result when using google to search for AM Transmitter Circuit. I had my doubt with that circuit when comparing it to others cause they look similar but had a difference with the modulation level part. Your discussion as to what are the problems has really helped me understand this circuit.
In order to make the range longer, does changing the value of R2 will make it really work?

ladyphoebe15 said:

In order to make the range longer, does changing the value of R2 will make it really work?

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A high value of R2 as shown reduces the current and voltage swing of the output transistor which reduces the output power which reduces the range.