My little “mission” to build random Chinese radio-kits from eBay continues with this rather odd and difficult kit. Unlike the other kits, this is the first to come with an LCD display, but is advertised strangely as FM Radio Kit Electronic DIY Hobbies Learning Suite Frequency range:72-108.6Mhz. The kit cost AU$13.59 including postage, so I thought I’d give it a whirl. It’s more challenging, but it’s also potentially more rewarding.
As far as kits go, this one is just like the rest, starting life off inside a plastic bag of sorts. This one has all of its components encased inside the plastic shell, with the shell taped together. On the shell, T2 is written in pencil on the rear.
Inside, the components were somewhat loose and rattling, and I feared the LCD may have broken in transit. Luckily, it proved to be intact. There are a few carbonized adhesive tape connectors for use with connecting the LCD – it seems about five or six are provided, which is good because they can be quite tricky to get on. The case is moulded, and while relatively plain, it’s still an attractive enclosure for a DIY kit. The PCB for the front panel and LCD is seen here, along with the speaker, bag of components, telescopic antenna, a brass threaded rod and the tuning knob.
The unit comes with a single double-sided A4 sheet of details, mostly in Chinese. A major section of one side is consumed by the trace pattern and silkscreen component guide, with the other part of this side of the page consumed by the schematic.
The unit is based around the CD2003GP, which is an integrated AM/FM radio IC. All of its AM capabilities are not used in this design, which is a bit of a shame. It also uses an SC3610D as an LCD display driver with AM/FM frequency display, clock and alarm functions. An TDA2822 is used as the audio amplifier, as is common with many of these Chinese kit designs. Rather interestingly, this kit utilizes two ceramic filter components which implies a 10.7Mhz IF is being used. This can be an attractive feature in case you want to use the IC as a downconverter of sorts, and should mean better FM selectivity than designs relying on passive-component filters. The output of the detector is on Pin 11 which should have the full FM baseband. No multiplex (stereo) decoding is offered, which is a little disappointing.
The display itself is basically a frequency counter which measures the frequency and displays when the radio is running. If the radio is not running (Vfm low), then it displays the clock and allows alarm setting. If the alarm is triggered, the 8550 transistor is turned on to turn the radio on (i.e. connect the batteries to the CD2003). It’s a pretty simple design, and the display is not strictly necessary for the operation of the radio at all.
The second (main) PCB is pictured above. It has a fibreglass style substrate with silkscreening on the top. Component values are directly silkscreened on, making it easy for construction.
Unlike the other board, however, it is not tinned on the underside and is merely lacquered. It does have a solder mask, although the regularity and sizing of the mask doesn’t always match the intended copper pad shape. It still feels like a good quality board to solder with, even though a few scuffs can be seen near one mounting hole. Interestingly. the antenna connection point seems to show a trace at the top of the board being used as a counterpoise (ground) for the antenna, which should improve reception performance.
At the time I decided to construct the unit, I was a bit sick from the flu, so I was a little lacking in energy and filled with frustration. As a result, I didn’t end up taking many photos of the construction process, but I will comment on some of the things I discovered along the way.
The first is that the kit absolutely lacks any instruction as to how to put it together. The constructor must work out what to do from the silkscreening. It’s not that hard to do, but it can be more daunting for the absolute beginner.
Because the LCD was a pain-point, I decided to start with assembling the LCD board first. I decided to mount the LCD first, which was a big mistake as it made the chip-on-board module more difficult to solder. As a result, I did make a bit of a mess of the soldering and had to come back and redo it because of a solder bridge or two.
I would recommend starting by soldering the chip-on-board to the PCB. Unfortunately, the board didn’t solder as easily as I would have hoped – a fine iron and fine solder wire is recommended, but even then, the solder often refused to flow freely possibly due to oxidation of the board or bad “vias” on the sides. Regardless, if you can’t get each pin soldered down to the board (starting at the corners first), you won’t have the LCD display and alarm features! This is quite challenging, even for me, so I don’t think it’s a good project for an absolute beginner.
Following that, some ceramic capacitors and a crystal oscillator need to be mounted on the rear of the board. The umbilical cable can also be attached, noting your choice of colours for the different connections.
To assemble the LCD requires some directional light (to see the metallization on the glass), steady hands, some fingernails and the assembly of the “stamping tool”. The tool can be assembled by taking the brass head, putting the silicone insert in, and screwing the rod into the rear. Then, with a careful fingernail, separate one of the carbonized self-adhesive labels from its backing. Take care to align it with the metallization on the glass – you need to get it within about 0.3mm or so. Then press down on the back of the self-adhesive label where it is on the glass with the stamping tool to make sure the label is well-adhered.
Then, remove the backing on the self-adhesive foam labels on the PCB and align the LCD to the PCB. Lower it down onto the self-adhesive then push down on the carbonized tape and get that to adhere to the traces on the PCB, again using the stamping tool to ensure good solid contact. If you didn’t fold the tape, crinkle it or break it, then you’re probably okay as long as the alignment is right. If it’s not, you can peel it off and try with a fresh piece. It’s good that there’s a few pieces, so you can give it a few goes.
The front panel buttons uses “snap” type buttons. The snaps are included, and I tried to solder the edges into place, but they’re of a metal which solder doesn’t take to. Instead, you should probably tape them into place with adhesive tape as other manufacturers often do. The power button is not a soft button, and instead relies on a hardware power switch which threads through the hole in the PCB. As a result, the power button can be “cut” from the front panel button set.
The main PCB is populated as per the silkscreen, with the push button assembled with a cap and the tuning dial assembled with a small non-tapping screw. One thing to watch out for is the variable capacitor, which had very tarnished pins which would not take solder. Having some sandpaper to file off the legs seems to be a good idea. The resistors supplied are high quality five-band resistors, so if you’ve been used to reading the four-band codes, you’ll have a nice time rethinking your multipliers.
Wires to the speakers can be soldered, along with the wires to the board. A wire can be soldered to the tag for attachment to the antenna. I soldered the wires for the battery terminals, but you shouldn’t do this as one of the terminals needs to be soldered after it has been fitted to the casing.
The assembly looks a little like this – don’t be fooled by the inductor printing on the PCB either – the orientation depends on the winding of the inductor – one of them was not wound in the expected way. Another thing worth noting is that the capacitors in the speaker “circle” need to be bent flat against the board to assemble the kit correctly. Note that there are a few extra holes on the board where no components are fitted and no silkscreen print exists – no components are intended to be fitted into these places.
At this stage, it’s also good to consider bending the capacitors on the back of PCB2 flat as well, so that it will assemble more nicely.
To fit it into the case involves sliding the rear antenna in with the tag in-between the case and the antenna and using the larger threaded screw to sandwich it into place. You should have the antenna with the screw pointing outside in case you need to tension it in the future. The tuning knob has to be fitted with the small non-tapping screw. The battery terminals need to have the positive fitted into the bay and then soldered after insertion – note that the terminal order is opposite to what I would expect (namely the negative spring in the door). Don’t lose the door either – it’s not captive.
Hot glue is necessary to secure the speaker into place. Two screws are used with PCB2 (top two), four with PCB1 (corners only). The rear of the case shuts over, with a long tapping screw in the centre, two short tapping screws in the battery bay and two behind the antenna.
Tuning and Troubleshooting
At this stage, I do not advise securing or closing the kit until tuning and troubleshooting has been completed. Partial assembly is necessary to test the unit, however. Have the battery bay assembled so that power can be fed from the AA batteries and then apply power and observe the LCD.
If all is well, the LCD should show the time (if off) or frequency (if on). First turn it on and run the tuning dial through all the frequencies. Then turn it off and run it through all the hours and minutes (AM and PM) then alarm. Check that there are no missing segments or “shorted” segments – if you have a corrupted display, chances are, you have either some solder bridges on the LCD side of the COB or you might have a flaky carbonized tape connection. Check the connection by pressing down with the stamping tool – if the segments flicker, you might have an air bubble under your tape. If nothing changes, then it’s time to grab the desoldering wick. I needed a few attempts to get this right.
Once the LCD is working, tuning is easy as on the two right-most screws need to be adjusted on the rear of the potentiometer. The bottom one affects the tuning range – the LCD will read out the frequency presently tuned to and so tuning is a breeze – you merely need to adjust the screw until you can cover the whole FM band. The other screw controls selectivity, so you will need to find closely spaced stations and adjust for best audio from the tuned station.
Rather interestingly, the tuning range does exceed the FM band – initially I had it set to cover this range, but later I decided to change it to 70 to 108.5Mhz, which partially covers the OIRT band. It’s relatively flexible although the tuning range does depend on the exact start-and-end frequencies. The indicated frequency is correct to within 0.1Mhz (it seems). Leaving a little slack at each end seems desirable as thermal drift can change the tuning capacitor range slightly.
If all succeeds, then you can continue to close the case entirely and screw it all together.
Don’t be alarmed if you have quite a few spare parts. It seems that this kit was packed by someone who knows better and provides about one spare component of each value (or more) so that in case you drop one and can’t find it, you’re not out of luck! That’s a nice touch!
The plastic casing is a bland white colour with a few scratches due to shipping. However, it is a rather nifty unit. The sound quality from it is passable – not great but not bad either. The mechanical power button is “inverse” – in is powered off, out is powered on. The alarm functions perfectly, and the unit keeps time well, allowing you to set an alarm time and the radio will come on. However, you must ensure the volume is set to an adequate level and the radio is “tuned in”, otherwise you’ll be waking up to static or silence. The choice of 74Mhz as the beginning range is a little odd – I tend to see 76Mhz as a more common value for Japanese and CCIR band radios.
The alignment of the casing isn’t perfect – it scrubs against the volume dial a bit and the headphone jack seems a little out of place.
The tuning wheel sits in place well, and with the number of screws involved, the unit feels relatively solid and sturdy. Unfortunately, the design doesn’t have any “feet” or lanyard, so it does feel a little jittery on a table, and could potentially dropped while carried. The case doesn’t resonate as badly as some of the lighter cases.
A little bit of scrubbing on the rear removes the pencil marks. The non-captive battery cover is less than ideal, but as a DIY kit, it’s not really easy to distinguish this from a commercial product when it’s properly built. I think it’s pretty good looking for a DIY effort.
As this radio is not a real digital tuning radio, it offers a halfway compromise in the form of a frequency counter on the front which shows what frequency you are tuned to. Because of drift in the components due to voltage or temperature, the tuned frequency can shift (e.g. if you’re tuned to 106.5Mhz, it could flicker and display 106.6Mhz from time to time). This doesn’t really affect reception so much as potentially cause annoyance if you insist that the display has to show the “right” number.
The LCD contrast was a little disappointing. It appears this type of LCD is best viewed from below – so if you sit it on a shelf above you, the clock is much more easily read. Unfortunately there doesn’t seem to be a contrast adjustment.
Because it’s a mono receiver, it doesn’t decode the multiplexed signal and the audio output has the same on the left and right channel. However, in order to provide compatibility for mono and stereo headphones, it is wired using the outer pins only, so stereo headphones will have “phase reversed” audio on the two channels, which makes for an awful listening experience.
The filtering doesn’t seem to be too great on the output, so we can actually see the pilot tone and the stereo multiplex information on the output audio itself.
The audio recording at 96khz is here in case you’d like to examine it.
Even while I’m sick, I managed to conquer yet another cheap Chinese radio kit. This one differs from the other kits by providing a decent enclosure with an LCD screen that provides a frequency counter and an alarm feature. This makes it more functional and practical compared to the other kits. It also adds an additional dimension of difficulty to the kit, which is compounded by the lack of circuit theory and kit construction guidance. As a result, I don’t recommend the kit for absolute beginners, especially due to the tricky SMD COB soldering. Luckily, it seems a few solder bridges on the output drive pins to the LCD has no permanent adverse effects.
Its educational value is limited because of the lack of information, however, it does engage some logic skills in deducing how best to assemble the unit. It does redeem itself as well in providing extra components to compensate for potentially dropped/lost/damaged components during assembly, avoiding any last-minute scrambles to find a particular lost component. It’s also not particularly expensive, although, it doesn’t seem to be a highly popular or available kit. The lack of stereo decoding and AM functionality seemed a little disappointing though. Still, it was good soldering exercise, and kept me entertained for an evening.