K7ITM wrote:
Something very fishy there with respect to the voltages at Q3. Notice
that Q3-b is fed from Q3-c, so the potential there should be lower than
Q3-c in any event. And if Q3 is OK and wired in correctly, since Q3-e
is grounded, you can't reasonably have Q3-b much more than 0.7V above
ground. It's just a forward-biased junction. So as a start, look for
problems around Q3! Remove Q3 from the circuit, and check the voltage
at the bottom of the 10k resistor...should be nearly the same as the
battery voltage. And at the junction of the 560k and the 100nF, should
be just slightly lower. Then if you ground the bottom of the 10k, the
junction of the 560k and the 100nF should be zero volts. If it's not,
perhaps the 100nF is bad (leaky or shorted). I'd say the 100nF (which
is indeed 0.1uF) is way larger than needed in this circuit--100pF would
likely be large enough.

The description says it's a "regenerative" circuit, but I'm not seeing
any significant regeneration! Looks like a simple RF amplifier
followed by detection in Q3's base-emitter junction, and Q3 amplifying
the audio.

100uH and 10pF would tune about 5MHz, though stray capacitance and
parasitic capacitance in the coil will make the net capacitance
somewhat higher. 100uH and 120pF would tune about 1.45MHz, just the
upper end of the medium wave broadcast band. I'd suggest you try for
more inductance and/or more capacitance in the tuning system. For a
quick trial, just parallel in 100pF or so; the tuning range will be
limited but at least you'll be in the MW broadcast band more solidly.

Cheers,
Tom

Tom,
Fantastic, thanks for the help. I realized that one of the transistors
was in backward, so that was one problem. The voltages are closer to
those reported, although off by a couple tenths of a volt. I also put
another 120pF variable cap in parallel with the first, so I have a
range of 20 to 240pF. After some tinkering, I've managed to pick up a
station. However, it's pretty poor quality (I can make out the words
despite a constant, high pitched whine). I also used a 12k resistor in
place of the 10k (as I'm out of 10k). So I'm led to a couple more
questions:

1. The current circuit doesn't have an antenna. How could I attach
one to this circuit? And what would be the best way to do so? (single
wire, x" long wire, etc?)
2. How much would attaching the circuit ground to a fence (or
something) help?
3. What might be the cause of the nearly constant background whine?
Is there anything I can try to fix it?

Thanks again for the help!
Dave