Joe McElvenney wrote:

Hi,

Further to my e-mail, having seen the schematic now, it
is probably the antenna which is screwing things up.

Firstly, take the antenna off the collector of the
oscillator and either link-couple it (a couple of turns
with one end grounded) or tap it down the tank coil till
the oscillator starts reliably.

Okay, I've not had the experience with this, but my father has. I
normally would ask him for help, but lately, he has been irritated when I
have asked, I suppose because he thinks I already know a lot of these
ideas. Anyway, having used your reply, I was able to get him to explain
what you meant.

In my discussion with him, he mentioned that because of the simplicity of
the radio, I would have what amounts to line loss by link-coupling the
antenna. He suggested the tapping bit. After explaining that to me, I
recall now having seen schematics that used that technique. I think
that's what I'll try.

You are not looking for an exact match here but a way of coupling
energy more efficiently into the antenna without the oscillator thinking
that it's his mother.

If I get the image you're trying to send, I must say that's one effective
way of getting your point across ;-)

Secondly, the base biassing you have on the oscillator
is too dependent on individual transistor parameters. I
suggest that you connect the base to ground through another
10K and then adjust the emitter resistor until you have 5
to 10 mA flowing through it. You can tell when you have it
about right by measuring the voltage across it. Touching
the circuit near the coil with your finger should also show
some variation in voltage if the circuit is oscillating.

Sounds easy enough. I assume you mean the base of Q2?

Finally, you appear to be relying on the battery for an
RF return circuit which is not good. Try placing a
capacitor of about 0.01uF between the supply and ground,
close in to the oscillator.

And this seems to be standard practice, the capacitor. I've seen
reference to that on just about every circuit dealing with oscillators (I
have a small collection of "Engineer's Mini Notebooks" from Radio Snap,
along with a book from an old electronics lab kit I got about 14 years
ago. Shouldn't be a problem to implement.

Cheers - Joe

Thanks for the help, Joe - I'll let you know how it goes when I get the
chance. Right now my father's needing my help loading a trailer with some
carpet and then we're off to our old house to see if the carpet can fit in
anywhere. After that I should have a few hours to tinker, I'll try this
then.

Cheers to you, and 73 de KE4EDD
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