It’s been a bit quiet around here lately – I’m just swamped with work, and it never quite goes away, but neither does the desire to do something for my own enrichment and enjoyment … and the desire to blog about it.
Seeing as I enjoyed reviving a few of Dick Smith’s Funway 2 Pocket Transistor Radio kits, I wondered if there was anything out there I could build which would be a little more attractive and perhaps slightly more sophisticated. I was aware of the Tecsun 2P3 which looks to be a very nice kit, but a bit on the pricey side for an AM only radio. So, I rummaged on eBay looking for radio kits, and surprisingly, there were quite a few! Sadly, a good number of them are very opaque in the sense that they rely on a digital auto-scan FM receiver chip and there really isn’t much to them aside from that and an amplifier chip.
I settled on one which was listed as CF210SP AM FM Radio Kit, with a few other “related” words thrown in. As expected, it’s a dual band (AM/FM) radio, with the whole kit coming in at a reasonable AU$7.99 including postage and comes complete with exterior casing. The big downside? Everything is in Chinese. The only English you will find is in the listing description, which isn’t particularly easy to read nor informative:
CF210SP is a new type of radio FM, AM two band radio, the FM band using the CD9088 chip, which uses patch element SMT package, the receive frequency range of 76-108HMZ, not only to accept the FM radio, can also receive campus radio and part of the audio signal of the television stations; AM band using direct IC the TA7642, receive frequency range is 525-1605KHZ circuit is simple, assembly success rate, a better selectivity having a little;. amplifier circuit using TD2822 special function ICs having loud. Sound quality is good.
While I am of Chinese decent, I’ll have to admit that I can’t read, nor write in Chinese. Even though the instructions are in Chinese, I didn’t feel that to be a big impediment because when it comes to components and schematics, the drawings and markings transcend the language barrier. Just another way engineering solves the day! Luckily for everyone, the board also has very good silkscreening to judge by the listing.
As a result, I accepted the challenge and put in an order for two kits. I suppose it’s just a habit – if you order kits, you know they come with just the right number of parts, so there’s no room for mistakes. Ordering two ensures that you have some spares in case things do go wrong …
This post will be a little review of the kit, along with tips and hints especially for English speaking constructors.
Rather unimpressively, each kit comes in a bubble wrap package. Trouble was already spotted when a loose screw was seen rattling inside the bubble wrap – the components have broken loose and the only thing preventing them from going everywhere was a thin and somewhat torn bubble wrap.
The case for this was soon found – the components are actually wrapped inside a cellophane bag, but that sort of material has a tendency to tear when punctured by the sharp component legs – quickly the smaller components work their way out.
As you can see, inside the package, you get the case halves without anything populated, a front tuning label with the absolutely bogus Paeansonic band, the main PCB, the speaker, a telescopic whip, and a bag with the “rest” of the parts. It’s nice to see that the plastic shell has a captive battery door, which makes it a nice touch. The downside is that the moulding of the case is a little slack so the halves don’t fit together seamlessly with a few rough edges, and there are areas where the plastic was so thin that it cracked in transit and a little super-glue was needed. The components being bagged inside a cellophane bag may be cheap, but doesn’t respect the ESD sensitive nature of the ICs used, and is a recipe for bent pins.
Rather nicely, the PCB is a single sided paper-type board with silkscreening on both sides which is very descriptive, and solder mask on the rear. It seems to have a lacquer finish to stop the pads from oxidising, but is not otherwise tinned nor plated. There are some points on the rear for “tack soldering” wires, and a slot near the top to slot in a ferrite rod holder.
As mentioned, the instructions were all in Chinese. Unfortunately, the instructions are only a single double-sided sheet, so the educational value of the kit is somewhat limited. The schematic as listed is shown above. The CD9088 is responsible for FM reception, with the only datasheet I could find being in Chinese. That’s okay, because it’s a clone of the TDA7088 which is a mono FM receiver circuit. The unit has an IF of about 70kHz, and features a frequency locked loop with internal muting of weak signals. Filtering is achieved with the external components – mainly R-C filters.
The AM reception is being achieved with the CD7642. If you’ve had a feeling you’ve seen this before – you’re right. It’s exactly the same chip as used by the Funway kit mentioned earlier – one of them had a TA7642 but it is otherwise identical to the MK484 in being a tuned radio-frequency receiver, basically operating at the carrier frequency amplifying and pushing the signal through a bank of filters until it is finally detected into audio.
That leaves the TDA2822, which is a stereo audio amplifier chip to drive speakers and headphones. It’s not particularly fancy and is probably quiet hissy, and offers about 20mW into a set of 32 ohm headphones when powered from 3V. It is used to drive the headphone jack which disconnects the internal speaker once a headphone plug is plugged in.
Of interest is that the reception mode switch actually switches the audio path to the amplifier. Whenever the radio is switched on (by the volume switch that has an on/off detent), both AM and FM demodulation are taking place but just one of the signals is sent to the amplifier. I suppose this is a cheap way to do things, and the low-ish current requirement of the cells means that “wasting” half the power isn’t seen to be a major issue.
The rear has a “duotone” print image with drawn components overlaid, which makes it a handy reference in case the silkscreen is covered up by components.
There is also a list of parts included – 56 different “parts” in the table, but some values are doubled up – e.g. the 0.1uF ceramic disc capacitors on line 23, 24 and 25 with quantities of 6, 6 and 1 respectively.
Construction and Tune-Up Process
As an experienced kit builder, I wasted no time in getting started. I didn’t even bother to count all the parts as some people might do – I just started construction on the first kit right away and “learnt” a few tips along the way, which I will share.
Things You Need
No kit is entirely complete – there will be things you will need that don’t come in the kit. In the case of this kit, this specifically includes:
- About one to two hours of time to complete the kit.
- A container of some sort to empty out the components and sort through them, so they don’t get lost.
- An appropriate (~30-50w) soldering iron with a fine tip – the SMD chip absolutely demands it.
- Some solder – 60/40 is the “good stuff”, lead free is not as fun.
- Sidecutters to cut off component legs.
- Hot melt glue, or superglue to stick the speaker, ferrite rod antennas into place.
- Desoldering braid (optional) to fix mistakes.
- Magnifying glass and light to read small component values and markings (also optional).
- Small Philips head screwdriver to assemble the case and tuning/volume knobs.
- Small flat-blade screwdriver to perform the variable-cap tuning procedure.
- Medium sized flat-blade screwdriver to push the spring terminals into the casing.
- Two AA batteries to power the receiver.
- Bench-top supply with crocodile clips, 3V output to power receiver during tuning (recommended, but not absolutely necessary).
- Another radio that is in tune to verify received stations (also recommended, but not absolutely necessary).
- A multimeter of some sort, preferably with capacitance measurement ability and/or an LCR meter for troubleshooting and component identification.
I’ve also prepared some helpful hints in regards to construction which will be presented in indented blocks.
We start by first emptying all components into a plastic bowl so they can be sorted through. If you’re really diligent, you can sort through the components to check there are no missing parts – but given the Chinese parts listings, it’s a bit difficult so I prefer to just start constructing and see what’s left over or missing.
For traditional kits, the recommendations usually stipulated that “heavy” passive components go on first, and “delicate” ICs go on last. For this kit, I preferred to do things the opposite way. I started with the CD9088 surface mount chip on the rationale that if it couldn’t be soldered into place decently, then the rest of the kit is pretty much a write-off. Start by identifying pin 1 and straightening bent pins. Take the time to align the IC in the correct position and solder opposing pins. Then solder each successive pin taking care not to short out adjacent pins. A little excessive solder or skew to the chip is not fatal – as long as each pin makes good contact, you’ll be fine.
Note the temptation to use hot-air reflow techniques. I actually tried this on my second kit and it was a disaster. As I didn’t have solder paste/flux, I just tinned the pads lightly, sat the chip on and waved the gun over the top until things melted into place. Unfortunately, while the chip did appear to solder into place, the tarnished legs didn’t get wetted by the solder sufficiently, so I had to touch it up with an iron anyway … but the biggest issue was the heat burnt off the lacquer on all the pads in the area, causing the pads to immediately oxidise, making construction difficult. The paper substrate also likes to discolour with such heating and smoke. As a result, I’d have to advise you resist the temptation to use hot-air.
Once that is soldered into place, you can breathe a sigh of relief as the worst part is over. The next chip to go in was the TDA2822, a DIP-8 chip. Note the orientation, unbend the pins, insert and solder away. Repeat for the UTC7642 as well.
At this stage, I decided to mount the electrolytic capacitors, since there were only a few of them and their values are easy to read. The silkscreen uses the “shaded” half-circle to represent the negative pin. Take note of the orientation, insert, bend leads, solder and clip leads until all the electrolytics are mounted.
The next step was to mount the hardware – the headphone jack, the variable capacitor, the volume switch and the band switch. These are fairly straightforward as well, although you will note that some of the pads have been “cut” through so the solder might not take as “cleanly” as you might expect. This is normal. Take care with the orientation of the variable capacitor, as it’s a dual capacitor and orientation is important. The thin tab faces the edge of the PCB, and the “fat” tab sits inside the slot cut in the PCB. Reversing this is probably going to give you a lot of grief as they don’t like the heat of being desoldered … luckily I noticed this during construction.
Some of the hardware has a habit of shifting in place during soldering. To combat this, I find it’s useful to have a dab of solder on the tip of the iron while holding the component in place, and just touch that to the edge pins to hold it in place. Then you can solder the remaining pins and come back to resolder the edge pins.
From here, we can mount the two copper air-coiled inductors. The PCB marks them as 7T5 and 8T5. Technically speaking, you should count the turns and check that they have the right turns – mine didn’t, but I didn’t care too much and just soldered them in anyway. Note that the inductance can be changed later by “massaging” the coils and spreading/compacting them. They are made of enamelled copper wire, and need a fair amount of heat to “burn” away the coating – if you don’t do this, you won’t have a good connection so take your time with these.
From there, I decided to move to the LED, which only needs to be inserted as far as the “studs” on the legs are. Note the polarity. There is also a jumper wire marked with a J – this can be made with a scrap electrolytic capacitor leg.
By now, most of the remaining components are resistors or capacitors. Because there were more capacitors, I decided to tackle them first. Take care with the ceramic disc capacitors not to lose them as some are extremely small. They are non-polarized and can go in either direction, but for component dressing and ease of troubleshooting, it’s best to keep the writing facing in one direction. Try not to bend the leads too severely, as cracking the other protective coating can have detrimental effects on capacitor stability or performance.
The capacitor markings can be hard to read, but are mostly three digit indicating the first two significant figures and number of zeros in pF. A 104 capacitor is hence a 10 0000 pF capacitor, or 0.1uF. Sometimes, because of production reasons (in both my kits) the capacitors can become unmarked for some reason. The best approach is to populate all capacitors you can and work out the remaining capacitor’s value by process of elimination. If this doesn’t work, your other option is to measure it with an appropriate meter, or take a guess.
In rare cases, you might find that you’re genuinely missing a component despite checking all the packaging. That happened with me, and I contacted the seller and received an AU$1.00 partial refund. In my case, it was a 104 ceramic capacitor, which I can get from element14 for AU$0.68 (if I pick it up). In my case, I ended up substituting a 104 salvaged from another piece of broken equipment … but this is why having two kits with the intention of making just one might be useful.
Getting to this stage takes about an hour. My board looks like this – ready for the population of resistors.
The soldering on the underside has gone mostly well – some pads were slightly obscured by silkscreening almost deliberately, and the SMD IC could have seen a slightly better alignment and less solder – but if yours looks like this, you really can’t complain.
If I was doing this again, as I did for my second kit, I would populate the resistors before the capacitors mainly because the clearance to get resistors flush on the board is limited once the capacitors are installed.
Continuing on, the board is completed with the population of resistors. They’re non-polarized, but it’s nice for component dressing reasons to keep the bands going in a particular direction for easier troubleshooting. The values can either be read or checked with a meter.
Note that some resistors go in a vertical orientation for space reasons, and these are marked with a circle around one pad indicating where the “body” of the resistor should be for best space optimization. I’ve mounted all components flush to the board to avoid potential clearance issues in the case.
Now it’s important to pay attention to population of the case hardware. I began by installing the “series shorting terminal” into the rear casing – this is the plate with a spring and raised flat. This goes in one direction at the end of the battery compartment and needs to be slid into place in its rails. A flat bladed screwdriver helps.
The other two battery terminals form the positive and negative. Keep the terminals loose on your bench and solder a red wire to the positive and black wire to the negative. Do not fit them into the casing until they are cool, otherwise you will melt the plastic casing! Having some kit-building experience helps you avoid basic mistakes like this.
Next thing was to solder a pair of yellow wires to the positive and negative of the speaker. The polarity isn’t important as there is no other speaker to worry about phase with. Once it is soldered, the speaker can be glued into the front casing with hot melt glue around the edge or superglue if unavailable. Superglue is not preferable because it cracks when the case is flexed, but should still work somewhat adequately.
The FM antenna also needs to have a wire connected to it. In other radios, a solder ‘tag’ is often used with screw compression, but this radio doesn’t have this. Instead, you will have to solder the yellow wire to the arm next to the screw hole, making sure the solder is low profile not to interfere with fitting the antenna through the slot. Do the soldering with the antenna on the bench, so not to melt the casing, and ensure adequate heat. Do not touch the antenna or you may be burnt! Afterwards, give the screw on the antenna which adjusts the pivot tension a bit of a twist to make it “firm” as the case obstructs this screw once assembled.
Assembling the AM antenna takes a little bit of work as well. The AM antenna comprises of the ferrite rod, a wire coil, and a plastic “holder”. Check the orientation of the plastic holder and the PCB slot, and glue it into place so it holds the ferrite rod parallel to the PCB. Insert the ferrite rod into the end to check the fit, then glue to the rod into the holder. The coil of wire may have been crushed in transit, so “round” it out gently by inserting a screwdriver shaft into the hollow and rolling the coil. Massage it over the end of the rod, and glue into place (optional).
Now, the screws need to be sorted and the dials installed. There should be:
- three long tapping screws (pointed end)
- two short tapping screws (pointed end)
- two identical machine screws (flat end)
- one slightly different machine screw (flat end, threads not all the way to the head)
First, obtain the smaller volume dial and check the indentation for the rectangular brass stud of the switch. Align the switch and use one of the two identical machine screws to fasten. Next, obtain the larger tuning dial and align the indentation. Then, take the tuning indicator plastic and place over the rear of the dial with the “tail” pointing outward, checking the direction of rotation will “coil” the indicator upwards. Then secure this with the other identical machine screw.
You can remove the self-adhesive backing from the front window and apply it to the front of the casing, checking the orientation.
At this stage, we are ready to make the final connections and assemble the unit. First, slide the battery terminals into place – positive is the top terminal on the front case, negative is the lower terminal. Tack-solder the negative wire to GB-, and positive wire to GB+. Next, solder the speaker wires with both yellow wires going to one SP terminal respectively. The AM rod antenna connections can be made by looping out the enamelled wire (over the top of the PCB is best) and tack soldering the ends to AM terminals. The order doesn’t matter, although if you’ve trimmed the wire, you’ll need to ensure you apply enough heat to burn off the enamel.
The final connection is the FM antenna connection. For this, stick the wire through the slot in the rear of the casing, and the screw hole end of the antenna through the slot. Lead the yellow wire to the ANT terminal and tack solder. Use the slightly different machine screw to secure the antenna to the rear casing.
Now that all the connections are “made”, it’s time to thread the tuning indicator into the front of the case. Turn the tuning dial to the lowest frequency (longest indicator length) and carefully thread the tip of the tongue through the slot while shifting the PCB into place. It shouldn’t require force. Once roughly in place, you can take the AM-FM mode switch, deburr the plastic and push it through the exterior casing over the switch.
Don’t worry if the wires are a little over the place, because we’re not ready to go yet.
Alignment and Testing
If you’ve jumped the gun and closed the case up, as I did when I first built the first unit, you will be sorely disappointed. You might get static, or nothing. Maybe one radio station at the most. The reason? The radio is out of tune!
At this point, it’s best to have a benchtop supply and crocodile clips. If you don’t, alignment will be a long and tedious process. Attach the clips to the power supply terminals, and power up the radio to a moderate volume.
Start alignment with the FM band. Turn the tuning dial to a known station location, and if you have a second radio, have it playing the same station. To adjust the FM alignment, you need to use a flat blade screwdriver and adjust the right brass screw on the rear of the variable capacitor (i.e. the lower one). See if turning it slowly, you can find the station you’re after. There’s a good chance you still can’t and instead will be finding stations below the frequency you expect to be hearing. To overcome this, you will need to adjust the inductor near the variable capacitor by “fanning out” the coils slightly which raises the frequency of the filter. Once you’ve done this to some degree, you should be rewarded with hearing the right station.
The next step is to align the AM band. Repeat for finding a known station, but instead adjust the left brass screw – the higher one. Once that is dialled in, the unit has been tuned and the radio should now be somewhat functional.
However, you might encounter some critical issues here. If you don’t hear anything on the AM band, it’s a good indication the connections to the coil are not well soldered. Try re-soldering as the enamel on the wire can be tough to break through especially if you’ve cut the wire. Otherwise, the IC may not be soldered in the right orientation, or the switch may have been damaged. If you don’t hear anything on the FM band, try shifting the tuning dial a bit since the IC has an auto-mute feature for weak signals – also try extending the telescopic whip turning alignment to make sure you have sufficient signal.
At this stage, you’re basically ready to close it up for good. Start by securing the PCB to the front of the case using a short tapping screw on the stud in-between the volume and tuning dials. Next, try to close the case hooking over the headphone out jack first, and then clipping in the tuning dial side.
You might find the case is hard to close and doesn’t make up at the seams. This might be the case especially near the headphone jack. Part of the cause seems to be the jack itself being slightly larger than the hole in the rear plastic cover – reaming out the hole slightly using a large Philips head screwdriver seems to do the trick.
Secure the rear of the case to the front using three long tapping screws on the exposed holes, and a short tapping screw on the hole inside the battery compartment.
Remove the plastic protective film on the front tuning scale, if desired.
Once that’s done, you’ve basically got a pocket radio which most people probably can’t tell you’ve built yourself. This really is a kit that looks (almost) store bought!
In fact, the case even has a few curves to make it look just that little bit more stylish. The beige-white isn’t particularly trendy though.
The reamed hole first the jack a lot better.
Unfortunately, the skew of the volume dial is something I couldn’t fix and happened on both kits. I suspect the dial itself just isn’t that well made, but the clearance in the case is enough that it doesn’t actually rub when in use.
Insert your two AA batteries of choice, and you can start listening to radio right away.
In Practical Use
As a practical pocket radio, it’s not a particularly noteworthy radio. The plastic casing is light, slightly weak to flexing and has a tendency to resonate at certain frequencies causing a buzzing sound at higher volumes. The speaker itself is not particularly “deep”, and has a tinny frequency response. It’s surprisingly loud for its size, but it also runs into “clipping” when the volume is turned past about halfway.
The FM reception isn’t particularly noteworthy – it’s sensitive enough but its selectivity is problematic in the crowded FM bands in Sydney. On one of the two samples, it can occasionally (during a signal dip) end up getting pulled into an adjacent station over time. On the other sample, it’s fairly difficult to get a weaker station that’s inbetween two stronger ones, requiring careful rocking of the tuning dial gently back and forth. Its monaural reception is very “basic” at best.
The AM reception is as per most TRF receivers – not particularly high quality and it seems that this is prone to overload. With the strong AM signals in the Sydney basin, it sounds like it’s clipping on peaks as if the AGC is not operating well enough. It’s intelligible for voice, but hardly optimal for music.
The headphone out is available, but using it is really at your own risk because it’s drive seems to be insanely loud such that any movement of the volume dial risks blowing out your ears, and the TDA chip used is very hissy indeed. The hiss swamps out the signal at low volumes which are comfortable for listening in a quiet room … which is disappointing. But at least they did provide the jack.
Samples of the audio as recorded by my Zoom H1 recorder are here.
For the price and complexity, I don’t think I can complain – most pre-made pocket radios retail at a very similar price, and kits normally come with a sizeable mark-up. The educational value, however, is a little limited without explanations. However, it does force the constructor to be a little bit “clever” in their techniques of deduction, precise in their soldering, and patient in populating the large number of capacitors. However, as a practical pocket radio, I think this one might only get a few uses before being “shelved” for good.
After enjoying repairing a few AM radio kits, I embarked on a challenge to build a cheap Chinese dual-band radio kit. It wasn’t without challenges, involving some fine soldering work, persistence and patience in tuning and component population and troubleshooting/problem-solving techniques. However, it was successful, and ultimately, I have two pocket radios that I’ve hand-built which is quite amazing to think about.
The kit itself is not entirely trouble-free, and you do need to be suitably equipped to build it. However, if you’ve read this far, you’re probably well prepared to do it. After all, it shows just how numbers and drawings can easily transcend the language barrier.