Sorry, one more question, do I have any leaway with the Variable 0 - 50 pf Capacitors?

The variable cap could be something of greater value, say 10-100pf would still be able to get a peak, it's one of those try and see issues because the coil will play a large roll in determining the value... If you had a 0-50pf and couldn't get a peak, a 50pf paralleled across it would net a 50-100 value, much of this type work is open to experimentation...

The ground can be just a length of wire, or connected to a cold water pipe...

The 150uh I have is open air type used back in the 60s, but either a toroid or conventional form coil would be fine...

I mainly included the rectifiers to give a example of voltage drop, if the transformer dropped 2-2.5v would be better to use five 1.5v batteries to give 7.5v vs 9v and a couple rectifiers... Alternately when the resistance of the coil and current draw of the osc are known, you could use ohm's law to calculate a resistor to set voltage... Lastly voltage doesn't have to be exactly 5v at the osc, with some transformers(but I haven't tried what you are using) I've seen that a voltage between 4 & 4.5v gave better modulation...

Tom

Here's a previous thread that you might want to read.

viewtopic.php?f=12&t=167850

Doug

I'm kind of surprised the LM386 version works as well as it seems to because at 5V the brick osc power supply is nominally 2.5V. That makes me curious. Has anyone who's built one of these, whether LM386 or transformer driven, looked to see at how low a voltage they still oscillate? And what mod depth they're getting before serious distortion sets in?

Mike Toon built one and said it performed nicely but I can't report same results... Mine was badly distorted, if I remember correctly the output voltage from the LM386 was only 2.5v and didn't increase with additional B+...

Tom

The 2.5V I would expect because an LM386 self biases at ½ Vs. It should increase with Vs but perhaps your osc draws significantly more current than his did, overloading the output, which might also account for the severe distortion.

Frankly, when I saw the circuit I wondered about the DC coupling, because it's not intended to drive a DC load, but, based on the current peak at the spec'd 'power out', I'm guessing it should be able to handle around 17mA.

Up the Vcc to 6 volts, and use a pass transistor follower on the output of the LM-386 to increase
the maximum current. 2N3053, etc....

Pete

Yes. As long as your LC tank can tune 1MC you are fine. I just picked standard stuff out of my junk box. There's a handy on line calculator called "Professor Coyle". My transmitter pulled 6ma when I had it tuned so 4 AAA batteries worked fine.

http://www.crystalradio.net/professorco ... ecyl.shtml


Doug