The building of a superhetrodyne
transistor radio from a kit of parts
17/12/2011
Recently whilst trawling through the pages of eBay looking for a suitable gift for a friend, I came across a small transistor radio kit. Long gone are the days when you could buy a kit of this type from a British manufacturer, or even a British retailer! I suppose that there simply isn't a market for such things anymore, which is a shame since Britain at one time were World leaders in the design and manufacture of transistor radios. Anybody remember Bush? Roberts Radio? Hacker? All world leaders in their field, although to my mind, Hacker always had the edge - build quality, sound quality - Hacker was always supreme, Roberts merely being excellent :)
This one was different from all the other kits I found on eBay, being a superhet built from 100% discrete components - not a single IC in sight. What could be a better way to introduce Jay to the world of electronics? I've found that no matter how many questions get asked of me, no matter how much I ramble on about the heterodyning process, about what an intermediate frequency is, how much I explain that a capacitor will block a DC signal but allow the passing of an AC one, the person asking the question simply looks at me, with a blank look - their question answered but clearly they've not understood most of what I've said.
Well, during the building of a real transistor radio, questions asked and answered would have a direct bearing on what was being built at the time, far easier to understand the answer to the questions being asked, and demonstrations of the answers given leading to a much better level of understanding. To this end, the kit served a multi-purpose role.
1) To serve as an introduction to what I hope will be a long standing hobby
2) to serve as an educational tool during its construction, in that:
a) An insight into the constructional methods used in the world of electronics
b) Learning to use a small Multimeter on Ohms, Volts and Current ranges
c) Learning how to identify various electronic components, and that they have different values
d) Learning the resistor colour code to establish the value of the resistor prior to testing on a meter
e) Learning about the importance of component polarity
f) Learning how to handle a soldering iron and basic tools
3) To serve as a suitable gift for a curious friend, wanting to know more about my hobby of electronics.
4) End result - a little transistor radio that will give hours of listening pleasure, self built, giving a sense of achievement. Proof that you CAN build something that works properly, first time.
Two kits were ordered - I wanted to have some fun too! I was dubious about buying from a seller that was half way around the world in Hong Kong - his feedback was excellent, so I had no doubts about the seller's honesty - it was just that Hong Hong is an awfully long way away, it was near to Christmas, Customs and Excise would delay it further, and being a radio kit, it could easily suffer damage in transit. My fears were groundless however, five days later, the kits arrived. I was impressed, since two of those five days were a weekend, so top marks to the Post Offices of both Hong Kong and the UK. Undamaged also since the seller had packed the kits very well indeed.
On inspecting the contents of one kit, I found all to be in order. The instruction sheet included in the kit contained a checklist and I emptied the bag of components into a tray, and put them back into the little plastic bag, one at a time after checking them off against the check list. Shame I only did this with one of the kits, because in the other kit, we later found there to be an extra capacitor and a missing LED. No big deal to me, since I have stocks of such items - only problem was, since I visited the home of my friend in order to talk him through the construction of the kit, I didn't actually have access to my stock - hence only one kit was actually built. We shall finish that when I return for another visit, armed with suitable spares. No doubt this page will be updated with more photographs at that time.
Even though the instruction sheet was in Chinese (the seller emailed a perfectly good translation of the sheet, so top marks again to the seller), it was easy enough to work out how to build the radio. The board layout, component overlay and circuit diagram was very easy to follow and after showing my friend how to solder and how to identify and test the various resistors etc, we both got on with the construction.
Construction continued smoothly until Jay held up a resistor and said it was wrong - his meter reading and the colour codes on the resistor didn't agree. He knew that a 5% tolerance resistor won't be perfect, but he was concerned enough about this resistor to mention it. I tested the resistor - it was supposed to be 120 Ohms - but it measured over 200, so that was put to one side and I gave him the one from my kit - which tested at 122 Ohms, as it should have. Proof indeed that he's understood the resistor colour coding, but also proof that you should always test your components before soldering them into place. A lesson well learned there, plus a bit of satisfaction that he'd used his test meter and found a dud component.
The resistor will of course be replaced from my stock. I informed him that in all honesty, I've never in 40 years found a new resistor to be so far away from its marked value - well done Jay!
Construction continued, Jay's soldering skills improving all the time, various questions about flux and fumes were asked and answered. The tinning of wires and various techniques were taught and learned, how to hold a component, the board, solder AND the iron using only two hands for example.
Finally, the board was finished, the battery contacts slid into place in the case, wires soldered, voltage and current readings taken from various test points as detailed on the circuit diagram were taken and approved. The speaker soldered into place and finally the big switch-on.
The little radio burst into life instantly, a quick tune across the dial showed that at least 7 stations were available including a foreign one, above the buzz and other electrical noise that was being picked up from the Wifi, fluorescent lighting etc - performance would be much better when the sources of EMI were removed. Obviously the IF transformers and the LO can were supplied pre-aligned, else we would have had even more fun since I'd left my RF signal generator at home. Apart from maybe a small amount of tweaking to the tuning capacitor's trimmers, there's nothing more to be done.
The little radio worked first time, tuning was sharp and defined, even without the final trimmer tweak. The small ferrite loopstick antenna showing it's directional properties to good effect, and the sound quality was perfectly adequate - even though the radio only has a small 2.5 inch speaker.
We were both very pleased with the result, Jay's wife even made coffee for us both to celebrate the successful building of his first electronic project. Thanks Tray :D
What a wonderful way to spend two evenings! It's hard to think of a better gift than this, we both learned so much from building them, and we have a nice little radio as a result. The radio was a pleasure to build, and despite the instructions being in Chinese, thanks to the board overlay details and the circuit diagram, construction was easy. As mentioned earlier, the seller did send me an English version of the instructions via email, but they were not needed. I would dearly love to write a detailed instruction manual for this little radio kit - alas I doubt it will be in production long enough for there to be any point - which is a great shame. If only there were a bigger market for little gems such as this.
If you have any interest at all in this kit, try eBay and search for "Seven Transistor AM Radio Kit". I won't mention the name of the seller on here for obvious reasons.
Maybe next time, we'll build an FM radio from a TDA7000 and send the output to a nice 4 inch speaker via a TBA820 audio amp :)
