It was one of the simplest for radio enthusiasts wishing to build their own FM radio because the external component count was astonishingly minimal. An advantage of this radio solution IC is that you will not need any specialized IF coils or ceramic filters. You also have the advantage of using the built-in search tuning facility using a varicap diode BB , or connecting a mechanical variable capacitor if required, and there are nice clear circuits for both options. It has all the necessary stages built-in, from antenna to audio output, and it all operates from as little as 1.
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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!
The big downside? Everything is in Chinese. Sound quality is good. 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.
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.
Kit Contents 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. 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. 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 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 CD That leaves the TDA , which is a stereo audio amplifier chip to drive speakers and headphones. 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. In the case of this kit, this specifically includes: About one to two hours of time to complete the kit.
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 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.
Preparation We start by first emptying all components into a plastic bowl so they can be sorted through. For this kit, I preferred to do things the opposite way. 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. Note the temptation to use hot-air reflow techniques.
I actually tried this on my second kit and it was a disaster. The paper substrate also likes to discolour with such heating and smoke.
Once that is soldered into place, you can breathe a sigh of relief as the worst part is over. Note the orientation, unbend the pins, insert and solder away. Repeat for the UTC 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.
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. This is normal. Some of the hardware has a habit of shifting in place during soldering. 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. 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. 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 capacitor is hence a 10 pF capacitor, or 0. Sometimes, because of production reasons in both my kits the capacitors can become unmarked for some reason.
In my case, I ended up substituting a 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.
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. The values can either be read or checked with a meter. 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. 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. 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! Assembling the AM antenna takes a little bit of work as well. 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.
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 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. 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 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. 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.
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.
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