Choccy Block Transistor Radio
Frequently Asked Questions
Q1: Why didn't you use the ZN414, MK414 or TA7642 AM radio
chips? They have AGC and would have made the whole thing
A1: Yes, those chips work pretty well and the performance is
about the same or better than this radio. It would have been
simpler to build, but the workings of the chip are hidden.
You can't learn about transistor biasing and demodulation from
it. I'll probably do a design using the TA7642 elsewhere
Q2: Why didn't you use a TBA820 or a similar output stage
A2: You can't mount it on terminal strips and the workings
Q3: I can make a loudspeaker radio using just two
transistors. Why didn't you do that?
A3: Because those designs use an archaic output transformer,
use fiddly regeneration, don't have any AGC, use awkward reflex
action which makes LW operation hard without changing the RFC,
aren't as loud, and are more sensitive to transistor
variation. Despite their apparent simplicity, it's very
difficult to fully explain the reflex action in those
circuits. This design has an extra stage but it's actually
simpler and more consistent.
Q4: What is a "funny electronic trick?"
A4: A funny electronic trick is where you use some
secondary, unspecified, unintended property of a device or circuit
to make it work. Secondary properties are usually not well
controlled by the manufacturer and indeed might be "improved away"
to make the primary specified properties of the device
better. The result is that over time, funny electronic
tricks tend to be inconsistent and can fail to work with newer
components that are supposed to be the same.
Q5: Can I use different transistors?
A5: Yes you can, but I don't guarantee that they will work
unless I've listed than as an alternative. The ones that
I've used are common and should remain so for many years, but note
Q6: I've found a way of arranging the components in
different holes so that it's easier to put together.
A6: Great. Some of the arrangement is deliberate from
an electrical viewpoint to avoid signal currents going around big
physical loops and possibly causing instability. But some of
the positioning was just what seemed most convenient at the time.
Q7: What is 100R, 100 Ohms and 100Ω?
A7: They all mean exactly the same thing, 100 Ohms.
100Ω is strictly correct but when computer aided design systems
came into being they could not easily print Ω on the screen, so R
was used instead. It is still quite convenient to use that
today and it is widely accepted as a standard.
Q8: I can make this with BC547s, for Q1 and Q2. Why
A8: BF199s have a higher specified operating frequency and
are intended for this kind of application. Despite having a
lower gain at low frequencies, they will be more consistent when
used at the MHz frequencies seen in this circuit.
Q9: I have a different variable capacitor. Can I use
A9: Yes. You can use pretty much any tuning
capacitor. A lower value will get less tuning range at the
bottom end of the band.
Q10: Why don't you supply a small screwdriver and some wire
cutters in the kit?
A10: I intend to do more kits, and you don't want to end up
with a spare pair of cutters and a screwdriver each time you buy
Q11: Why not use a 1K preset pot for R13 in order to adjust
the bias current?
A11: You can, but depending on the size of the part you will
need to be clever or connect some wires onto it if you're building
on terminal strip.
Q12: Where's the FM radio kit?
A12: That's a bit harder to do in a simple design, but not
impossible. It's not available yet.
Q13: I've tried to find BF199s recently and they appear to
be completely obsolete. I thought you said they'd be around
A13: In 2011 it looked like several manufacturers were
happily making BF199s in full production and indeed they
were. Since then Fairchild, NXP, and On Semi have ceased
production and regular supplies have dried up. Obsolescence
is the bane of the electronics engineer's life. I've got a
few thousand BF199s to supply with the kits so that will remain
the same for now. If you're making your own you can try
BC549Cs and they do work. A better part would be the SS9018
which I've tried and is specifically intended for such
applications. Unfortunately neither part have the same
pin-out so you would have to either contort the legs into place or
re-arrange the circuit layout.
© Henry J. Walmsley 2017
29-MAR-2017 Edited Q5 and added Q13 to the effect of BF199s