I don't know of any good way to troubleshoot an oscillator without an
oscilloscope and signal generator, except to measure all the voltages.
However, the voltages can be right and the oscillator still not
oscillate. Or you can find that some voltages are different simply
because it's not oscillating. Perhaps someone else has some tips that
can help.
Your problems with oscillators are possibly caused by poor component
choices. Capacitors should be low loss types, like mica, NPO ceramic, or
polystyrene plastic, and should be physically small so they don't have
much inductance. NPO ceramic has the lowest temperature coefficient so
is the best choice for stability. If you have a poorly made inductor,
the temperature drift of a polystyrene capacitor might more-or-less
cancel it, but that's a poor way to get stability.
Inductors also have to be low loss, and that's probably the answer to
your question about the toroid. There are just one or two types of
ferrite that might be usable for an oscillator inductor, but they're
uncommon so you're unlikely to have them in your junk box. All other
kinds of ferrite cores are too lossy for this use. You should use only
powdered iron cores. In my experience, the best is type 6, which is
colored yellow and typically with a glossy finish. Type 7, colored
white, should be similar, although it's much less common. I wouldn't use
any other type of toroid core except these two types of powdered iron.
Ferrite is a good core choice for the RF choke, although you might have
trouble with core saturation if you're putting a fair amount of current
through it. Chances are you don't really need a choke with a value this
high, although you might in some applications.
Don't apologize for the questions! We all had to start somewhere, and
are glad to see you learning. And you never learn much from circuits
that work -- it's the ones that don't which are educational.
Roy Lewallen, W7EL
Jock Cooper wrote:
Hi everyone,
I've been interested in radios and how they work since I was a kid --
I had a crystal radio kit and a RF-2200. Recently I started building
radios and have been having a blast. Please keep in mind that I
am very newbie.
After scouring the web for am radio circuit schematics I found quite a
few and successfully built several of them (I'm using breadboards).
Then I wanted to get a little more sophisticated so I got some NE602s
and started to work on a superhet ish type design -- basically
cobbling together bits from other circuits. I had some decent success
using the NE602 internal oscillator (although getting the oscillator
to track the rf input is tricky), then I wanted to use an external
oscillator.
I tried numerous circuits and numerous types (Hartley, Colpitts etc) I
had found but I couldn't get any of them to start up. Finally somehow
I got one working reliably: it was a JFET Clapp with a JFET buffer
amp. I hooked up my Elenco LCR to it to measure the frequency. I was
using a 3 gang 10-500pf air cap and a coil I wound onto a pill bottle.
The oscillator was running from 980 khz to 2050 or so. I fed it into
the NE602 pin 6. Only problem was (again) I had a hard time getting
the main tuning (also on the 3 gang cap) to track the same range (well
the same minus 455). I guess I had plenty of stray capacitance or
something that was mucking up my calculations. So at say 540 they
would line up but tuning to the other end they'd be off by a few 100 (!)
khz.
So I decided to build a second oscillator to help align the antenna
tuning tank. I pulled out one of those little 'mini' breadboards and
basically dupicated the working oscillator circuit on it. Only this
time no startup. I also moved it to a second breadboard but still
no good.
Now I'm at a loss as to how to proceed. How do you diagnose a bad
oscillator circuit? I thought about measuring the voltages all various
points on the working osc while it was running and comparing those,
not sure how much that will tell me.
Also, a related question. I tried winding a toroid with the same
inductance as my pill bottle coil, and hooking it in place of the
pill bottle coil in the working oscillator. But it wouldn't start
then. Any clue why this might be the case?? Also I have seen some
circuits that call for a 1mH RF choke.. Can I wind a toroid for this?
I don't have any chokes that high.
thanks for any help and apologies for any dumb questions,
Jock Cooper
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http://www.fractal-recursions.com