Mini AM Transmitter

[awc]

I wanted to transmit to my AM only valve sets using something simple and easy to knock up and found the attached circuit in a Tandy 50-in-one kit.

Although it "works" using the output from a portable CD player, reception is like that from a distant Transmitter. I suspect an impedance mismatch as the original circuit utilised input from a high impedance crystal earpiece.

As my technical knowledge stops there, can any one suggest a circuit modification that might improve things?

Thanks

Alex

[paulsherwin]

It's difficult to understand how that circuit is expected to work at all.

There are lots of AM microtransmitter designs if you search the forum or the wider net, though even the simplest ones are more complex than that one.

[Nuvistor]

Buy or build one of these, they work well, I use mine daily.

https://www.vintage-radio.net/forum/...d.php?p=755588

https://www.vintage-radio.net/forum/...hlight=minimod


Frank

[G8HQP Dave]

The circuit shown in post 1 lacks a clear 'ground' i.e. a reference point from which other node voltages can be considered. Potentially everything is live with RF, including the piezo (which is part of the oscillator feedback to the transistor base). Connecting a 'line level' audio source will do at least two things:
1. add a 'ground' - possibly in the wrong place
2. overload the transistor base

It is a wonder that it works at all!

[ukcol]

Originally Posted by paulsherwin

It's difficult to understand how that circuit is expected to work at all.

Yes it's pretty horrible Paul. Redrawn the European way it sort of makes sense but an earpiece as a microphone! The 0.01 and 0.05 is series looks odd too; does it need an 8000pF capacitor?

Yes, I'll go along with the consensus, get something decent.

EDIT: Yes I had missed that Dave, a ground connect might be nice.

[Nuvistor]

The circuit is what it is, with the components recommend it will work, not well, but good enough for a 12 year old child to experiment with.
Not good enough for the purpose Alex requires.

Frank

[paulsherwin]

A crystal earpiece will work quite well as a crystal microphone, but as has been said the circuit isn't suitable as a general purpose pantry transmitter. The redrawn circuit does make more sense though.

[awc]

Many thanks to all who have responded, much food for thought.

I do have a Gizmo for use as a pantry transmitter, but I was hoping to get the Tandy thing working for 2 reasons;

1. I wanted something portable and

2 I have lots and lots of the bits it uses to try and find a use for!

However, it would seem from the comments received that I would be trying to make the proverbial silk purse if I pursued the Tandy option so I'll give careful consideration to pursuing one of the other options unless anyone has any further thoughts.

Thanks again

Alex

[Nuvistor]

The MinMod will fit in a box 12cmx6.5cmx3.5cm and include a PP3 battery.

[ThePillenwerfer]

If you want something simple you can make out of bits you already have this works.

— Joe

[awc]

Well, I constructed (if that's the word!) the revised transmitter as per Colin's design in Post 5 which of course uses the same parts.

This does indeed perform differently to the original, being noticeable for being somewhat quieter, but suffers the same problem of a poor signal to noise/distortion issue.

I found a note I made somewhere in the dim and distant of a potential divider to attenuate the input from the headphone socket of a CD player consisting of 50k and 1k resistors in series put across the earphone terminals with the input from the CD player connected to the junction. I found that although it needs quite a high input level from the source to overcome the poor signal to noise ratio it is indeed possible with a strong enough input and some tweaking of the ferrite rod in the coil etc to get a listenable result.

[paulsherwin]

There is a bare minimum of components that you need to construct a practical pantry transmitter. You need an oscillator, either tunable or crystal controlled, and a modulator. The oscillator may have an additional PA stage, which makes modulation easier. The modulator is typically an LM386 chip in modern designs. Some designs use a dedicated crystal oscillator to provide the RF.

The drawback with any crystal controlled design is that it is unlikely to oscillate on a valid European MW frequency, and the crystals are relatively expensive unless 1MHz is free in your location.

Joe's suggested circuit is probably the simplest way of getting decent performance. It doesn't use a modulator but relies on a line level signal to provide modulation. While it's more complex than the Radio Shack circuit, it's still relatively simple. I may try to build it over the next few days - any general purpose transistors should work.

[raditechman]

Paulsherwin said:

Joe's suggested circuit is probably the simplest way of getting decent performance. It doesn't use a modulator but relies on a line level signal to provide modulation. While it's more complex than the Radio Shack circuit, it's still relatively simple. I may try to build it over the next few days - any general purpose transistors should work.

I am also tempted to try Joe,s circuit, (when I have finished my current restoration, a time consuming Murphy A50). It would be interesting to know how it performs if you build yours first Paul.

John

[Goldieoldie]

I guess you could use a scrap tranny radio and use the local osc coil and cap with a simple modulation circuit and output stage .Even using the aerial side in the output at stage ?
That would give you the ability to tune it easily and have it cased for free !
Cheers Pete

[julie_m]

If you do build any sort of transmitter, it's well worth having a good wander around with a battery portable receiver just to see how far your signal is getting. And if you have a short-wave receiver, have a listen out also for any unwanted harmonics. The line between "clutter" and "pollution" is the line between your space and other people's space .....

If, while on your travels, you bump into someone else wandering around with a battery radio, don't go straight home!