More on Pantry Transmitters.

[Joe]

These videos look interesting:-

http://www.youtube.com/watch?v=Bd8H9J5B_cI

http://www.youtube.com/watch?v=2Zb_tKVi_ZQ

- Joe

[pwdrive]

Wot, no valves...

Lawrence.

[AlanBeckett]

Wot, no Tripod - idiot!

Alan

[Ian - g4jqt]

Very nice and it's the proper way to generate an AM signal. But unnecessarily complicated for a simple AM very low power transmitter.

I managed it with two transistors and one IC!

Ian

[Joe]

If you want simple try this:-

   

I've just knocked one up and it does work. There's no low pass filter and it can be picked up in two places on the dial but as the range is only a few inches I don't think that's a cause for concern.

It would be fine directly plugged into an aerial socket or built into a set with no Gram socket.

- Joe

[AlanBeckett]

What does the output look like on a 'scope young Joe.
Give it a Sine wave Input.

Alan

[Joe]

I've tried but can't make sense of it. The output just looks like noise and the trace looks the same whether or not the battery is connected. I can hear the tone OK on a radio, I just can't see it.

Working - Tone heard on radio:-

   

Battery removed:-

   

AF gen switched off, battery still disconnected:-

   

- Joe

[Ian - g4jqt]

To get a stable trace try this:

Remove modulation from cct
Connect scope probe earth to -ve of cct
Connect RF choke (value not critical) between scope probe centre and earth
Place sort bit of wire from from probe centre/choke junction and put near oscillator o/p
Turn up mod

It looks like the scope is picking up mains hum (since there is not much signal from the cct) and this arrangement mostly eliminates that.

Ian

[pwdrive]

What's the carrier frequency Joe, that circuit looks like a Colpitts, should be quite stable.

Lawrence.

[Joe]

I'll have a try at that, Ian. It must only be mains hum as there were no florescent lights on or SMPSUs anywhere near. Neither can it be denied that my oscilloscope is a darn sight cleverer than I am.

Lawrence: My Autocrat picks it up at about 540m.

- Joe