Tech Flashback: Dick Smith’s Funway 2 Project 7: Pocket Transistor Radio

Since the positive reception to my posts about Dick Smith’s Fun Way into Electronics book series (Part 1, Part 2 & 3), I’ve always been meaning to put up more posts about the series and its projects. It was amazing to see that my initial posts caught the eye of the legend himself, and I was granted special permission to reproduce parts of the books for my postings.

Unfortunately, in the recent past, the name Dick Smith was taking a bit of a battering with the bankruptcy and closure of all their electronics stores and the “handover” of the website and brand to Kogan. Even though Dick Smith himself was not directly involved with the stores for a very long time, I can’t help to think that many misinformed people probably did blame him for the mess, and the goodwill that was built up in the brand seemingly vanished overnight in a series of headlines from disgruntled purchasers who had outstanding gift vouchers which would not be honoured.

But that was not the real Dick Smith Electronics. The one we continue to reminisce about sold us components, kits and “real” electronics rather than consumer electronics. It bought us an education through the Fun Way series, and was responsible for inspiring many to take up electronics both as a hobby and as a career.

Another Story Begins

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Imagine my delight at seeing this container of goods being handed to me by a friend at the university. Apparently, these bits were saved by a teacher during a clean-out at a (presumably) high-school in the hopes they would be interesting to a student. This was then subsequently handed from person to person until it eventually reached me.

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As soon as I saw the cards at the top of the box, I knew it was a Dick Smith Fun Way kit. The quality control card and red disclaimer cards are both classic features of the kits. There was also a note about different equivalent radio ICs supplied, along with the labelling of pin numbers. This likely happened as the original metal-canned ZN414 may have been discontinued or a cheaper alternative exists.

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Inside, there were four sets of monaural earbuds, of a very classic sort which are normally used with crystal sets. They measured about 8.9 ohms when tested, which is lower impedance than I was expecting, and had a thin lead of about 3.6m in length.

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There were also various bits and pieces, including an SPST panel toggle switch (on-off), a bit of solder which (presumably) an idle student had contorted into a spiral, a variable capacitor with its tuning knob fitted, but with the legs snapped off, a ferrite loop-stick antenna, a mono panel jack, a pair of hook-up wires, a spare tuning knob, two carbon film resistors, an over-stripped 9v battery snap and a brass rod.

Already, we can tell by the antenna and tuning capacitor that there’s an AM radio involved. The switch may be just a “spare” part from another project. The brass rod? I’m almost certain this was a case that the school had also constructed the multimeter kit, as our school did, as the brass rod formed the 10A current shunt for the meter.

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The first board out of the box looked like this. Not an encouraging start, but I guessed this was a higher stability/power FM transmitter bug of sorts, due to the use of NPO capacitors, two transistors, a tuning slug and the printed inductor.

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Unfortunately, this board smelled badly of corrosion and was, indeed, heavily corroded. I wonder if someone was a bit too generous with flux that needed to be cleaned off, or more likely, it was somehow stored with a battery that ended up leaking and destroying the board. Nonetheless, the PCB had the code ZA1431 which indicated to me this was a “kit” not from the Funway series, and a look up at Silicon Chip showed that this was November 1989: Project: Super Sensitive FM Bug by Greg Swain.

Given the state of this board, I didn’t end up doing anything with it and just binned it. Attempting to reflow the solder gave out some acrid fumes, and I really didn’t need that …

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The other three kits were Fun Way 2 kits, namely, Project Number Seven: Pocket Transistor Radio. As you can see, the three kits seem to be in various states of completion, with the top left board a fair way away from completion. The other two boards looked close to completion but with complications of their own.

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The least completed board actually had a decent looking underside.

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However, the variable tuning capacitor wasn’t soldered in as well as it should have, and a lot of the joints had too much solder and were almost bridging.

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The second board had all the holes practically populated, but seemed to have all its legs untrimmed. The coil wires were not trimmed off and had questionable connections.

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The last board had its loop-stick wires threaded through, but not soldered and didn’t have its legs trimmed either. It was likely that these boards never worked properly, because the students responsible for constructing them didn’t do a proper job and never finished, didn’t follow instructions carefully, and didn’t debug their circuit after construction.

Project Number Seven: Pocket Transistor Radio

In case you’d like to follow along, I’ve scanned in the four pages (two spreads) from Dick Smith’s Fun Way into Electronics Volume 2 book which address this project.

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This shows the typical project structure for the series, including a monochrome photograph of the built project itself, a parts list, a step by step “brief” set of instructions, the physical layout diagram (no silkscreening on the PCB to help you), the circuit diagram and some steps to help inspire you to take the project just that little bit further. Also, there is a little section towards the end dedicated to historical figures.

While actually reading through the instructions, it seemed that in this printing, the capacitor C4 was drawn with its polarity the wrong way around. This probably won’t cause an explosion due to the low voltage involved, and would just mean no power supply bypassing which might affect audio quality during transients.

Another thing that was noticed was the use of the original ZN414 in the print, which is a metal-cased TO-18 IC. As the kits supplied were later kits, they used the MK484, a later version of the same IC, as well as the TA7642, both of which are TO-92 plastic packages. A look at the example circuitry for the MK484 datasheet shows that the circuit used for this project is a typical example of how the MK484 would be employed as an AM broadcast band receiver.

Unfortunately, this kit doesn’t quite take you down to the individual transistor level, because of the use of the IC, but I suppose that’s a decent compromise since RF-level filters often mean tuning due to the sensitivity to component values and construction. The IC in use is a tuned radio frequency receiver, TRF for short, which operates at the RF frequency in amplifying and filtering the input. This works best for low frequencies, less than 3Mhz in the case of the MK484. This type of design is simple and pre-dates the superheterodyne receiver.

As noted in the text, the IC and the whole project could operate from a single 1.5v cell, if you can find a holder for it. The use of a 9v cell was to simplify the cell connection and fit into the small sized jiffy box, however, most of the energy is wasted by dropping 6.8v over a Zener diode. Because of the low current draw, and the fact this isn’t likely to be used seriously, this is somewhat acceptable. There was another project for a crystal set, which relies on the received AM radio signal energy alone to power the receiver, thus obviating the need for a battery.

Repairing the Kits

Ultimately, seeing as the boards had made it all the way to me, and I’ve never actually constructed such a kit before, I wanted to relive the nostalgia by repairing the kits so that they actually functioned. I normally dislike working on things which other people have started, as it’s often quite a time sink to try and debug their work, in this case I had little choice.

Kit #1

To start things off, I decided to go with the least-constructed kit, as that should make it easier for me to construct and test. I figured if I was able to complete a single working kit, this could form a “golden sample” which could be used to “debug” the others by comparing readings and component placement.

It seems that the kits were constructed with guidance from a teacher which included advice such as “raising” the legs off the PCB to avoid overheating the component, etc. Because I’m well advanced past that stage, I prefer the cleaner look of “near” flush mounting, I decided to re-mount and check all the components already on the board.

It was interesting to see that there already was a construction error on this board – an electrolytic capacitor was soldered in the connections for the tuning coil. This needed some de-soldering to get out. Construction continued, only to find that the output series resistor was the incorrect value, and a few missing values existed so I had to rummage my junk box for it. The tuning wheel for this one was missing, so the black tuning wheel and screw were salvaged from the snapped tuning capacitor along with hook-up wire and super-glue to secure the loop-stick to complete the unit.

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Actually getting the loop-stick antenna coils the right way around was a little challenge, as the colour coding wasn’t quite right. Maybe the colours had faded over time – but with an ohmmeter, it was easy to find a 10 ohm winding (tuning circuit winding, connects to the capacitor pins) and a 1 ohm aerial winding (connects to the outer pads). Because of the twisted construction of the leads with support fibres and enamel coating, it took a decent amount of heat to burn through and make a good connection. I can easily see how these final steps could prove to be the “undoing” of many constructors who may be beginners at soldering.

The underside looked a little messy with darkened flux residue from the desoldering braid, but the connections are otherwise bright and shiny. Luckily for me, no tracks lifted during the whole operation – probably because I was using my Tenma rework station that has a decent temperature controlled iron. Prior to that, lifting tracks was a fairly common occurrence for me when doing such repair.

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This proved to be the golden sample, and worked right off the bat. You can hear it in action receiving the local stations in Sydney, such as News Radio 576khz, Radio National 630khz, ABC Sydney 702khz all the way through to 2KY 1017khz. Unfortunately, as built, the tuning range does not span the whole of the AM band, and starts slightly below. It wasn’t sensitive enough to receive Bankstown ATIS/NDB on 416khz. The solution to this would have been to alter the windings on the ferrite rod, but because of the way the windings are manufactured, this wasn’t possible without very delicate surgery, so I left it at that.

You also get to hear the very spooky noises of an untuned AM receiver which is rare to hear nowadays especially with digital tuning and muting of noise.

Kit #2

Interestingly, the second kit was mostly constructed but did have some critical errors. I’m not sure if it’s a sanctioned modification or an error, but both Kit #2 and #3 had the capacitor C3 replaced with a resistor. As this was not the design in my copy of the book, I decided to follow the book. Unfortunately, as Kit #3 was short of a capacitor, I decided to pinch the capacitor from this kit to fill in Kit #3.

Instead, I looked for the best substitute I could find – high quality Nichicon low-ESR 47uF capacitors. Sure, they’re half the value of the units they replaced, but I don’t think this is critical as they are better capacitors overall and they only serve to bypass the power supply.

This kit had one critical construction mistake – namely the Zener diode was installed like a regular diode, and thus wasn’t going to perform its voltage regulation duty. If power had been applied, it is possible that the MK484 would have immediately self-destructed.

The value of R3 was also incorrect, and was replaced. The loopstick antenna also had its coils incorrectly connected such that the coils were not connected across the capacitor, but moreso shorting the capacitor to the antenna pads. The retention loop over the stick antenna wasn’t doing well either and appeared to be digging into the windings itself.

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A similar amount of work was done, and the nasty flux residue is visible, but the soldering is now much better. After the remediation, this kit was found to be functioning just as well as the first kit.

Kit #3

The final kit shared the same problems with the Zener diode connection being reversed, and the coil connections being incorrect – they were threaded but not soldered down, and that’s a good thing, because they were incorrectly threaded.

This kit had a slightly different complication to the other two – namely the ferrite rod was “loose” and slid out of the coils. As a result, it needed to be glued back into place to avoid any chance of the rod sliding out, hitting the ground and shattering into pieces.

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Of course, the solder connections initially made were quite terrible and blobby, so it too got a lot of desoldering love. It was found that this one worked almost as well as the first two kits, but has a slightly more restricted tuning range, maybe because of the ad-hoc placement and gluing of the coil to the ferrite rod.

Ultimately, I probably spent more time and desoldering braid than what the kits were worth, but it was a good time spent reliving my early days of electronics, and good practice in deconstructing and reconstructing boards. Now, I can pass some of the working kits back to my friend in a functional state, so that they’re more useful than a random bag of bits.

Conclusion

The Fun Way books had lots of projects, but as a child, it was rarely possible to afford to purchase and build all of them. Often, you would look through and find the ones most interesting and affordable to you, and build them while reading about the others and perhaps wishing that one-day, you might be able to build them too. The projects are necessarily simple and aren’t “great” performers by any measure, but they are instructive and achievable.

I had never built this particular project before, and it’s been a while since I’ve had to follow kit instructions. Even though it is a simple project, actually troubleshooting someone else’s build brings a whole new element of fault-diagnosis and repair into the equation, requiring some careful desoldering to avoid lifting tracks and damaging components.

While the principles of tuned radio-frequency (TRF) receivers are simple, they’re really only applicable to the lower-end of the radio spectrum and their applicability to everyday life is diminishing. Fewer and fewer people listen to AM radio, and even FM is taking a hit from the proliferation of “on demand” and streaming entertainment. But it’s still good to know the simple principles behind a cheap pocket AM radio – namely, a ferrite-rod loopstick antenna, using a coil as part of an L-C filter where the variable capacitor forms the tuning wheel, chained with a stack of amplifiers and filters pushing into a detector (the MK484 IC) and a single-transistor amplifier for driving the earpiece (or Darlington-pair in examples with a speaker).

As a result of a few hours spent with a soldering iron and four pages of instruction, we progressed from three kits from a partial state of construction with mistakes, to three workable AM receivers with a limited tuning range. It was an enjoyable afternoon, even if I was totally sick with the flu …

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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