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 easier?
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 though.

Q2:  Why didn't you use a TBA820 or a similar output stage chip?
A2:  You can't mount it on terminal strips and the workings are hidden.
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 Q13 below.

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 use BF199s?
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 that instead?
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 one. 

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 forever?
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. 

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Henry J. Walmsley 2017


29-MAR-2017 Edited Q5 and added Q13 to the effect of BF199s being obsolete.