Project: HX3208 (CD9088-based) SMD “Micro” FM Radio Kit

The kit drawer is like the bottomless-pit of entertainment, except that it does have a bottom and I’m slowly reaching it. You can tell because these are the kits that I’ve left behind for a later day – perhaps out of fear that I would stuff it up.

This post looks at the HX3208 (CD9088-based) SMD “micro” FM radio kit which I purchased for AU$3.22 including postage a while back. Of course, I’m not new to CD9088-based radios – I’ve already built one, but that one was not as small as this one. The allure of small size leads to a new challenge – that of numerous SMD components to handle.

The Kit

This kit comes in a stapled plastic bag …

… and like a bad case of recursion, there is another stapled plastic bag within. Already, we can see the pair of rather painfully cheap earbuds and a slightly mottled grey enclosure which looks like it could have been made of recycled plastic. At that price, I’m not complaining …

The dimensions of the case don’t seem perfectly right – the batteries seem somehow offset from the side of the casing – but having a case is better than just having a bare board.

The full bag of components include the above, which is a copious number of SMD components on tape. Extra care is necessary to make sure they don’t fling across the room, especially because the capacitors are unmarked on the package – only on the rear of the tape.

The PCB is marked HX150523 134777 on the top, where there is silkscreening. It’s a fibreglass single-sided PCB with silkscreening on both sides.

The underside is hot-air solder levelled tin finish with green solder-mask but the design of the board leaves some to be desired – where adjacent SMD pads connect, they are not separated by solder resist which makes it hard to achieve a neat finish.

But the biggest issue by far is a lack of instructions combined with a lack of markings as to values on the silkscreening. As a result, I ended up relying on the schematics from this page to get it built – with some minor discrepancies worked out by a process of elimination.

Construction

Because this kit uses SMD parts, it makes sense to mount the SMD parts first. I decided to tin the pads of the target components, line them up and then use a hot-air gun to reflow the solder. This resulted in a number of instances of excessive solder resulting in blobby looking joints. It’s important to be patient and do components one value at a time, to avoid confusing the similar-looking capacitors. The lack of solder-mask between adjacent pads resulted in some rather strange looking solder joints too.

By the time I was done with the SMD resistors and capacitors, I had mildly overheated the area near R4 with the hot air gun causing the traces to slightly lift. As a result, I mounted the IC and transistors with a fine iron instead, again applying a little too much solder.

At this stage, only the through-hole components remain – including the jumpers made from component legs as indicated on the top-side.

While most components need to be mounted as close to the board as possible to ensure adequate clearance, the exception is the LED which should stand proud of the board slightly so that it can meet the hole in the front side of the casing. Once the board is fully assembled and the wires to the battery terminals (which can be shortened) are made, then it’s time to look at installing into the case.

That being said, it is important just to double-check L1 as the fine enamelled wire doesn’t want to make good connection with the solder until the enamel is properly burned off.

Start by populating the button holes with the supplied buttons, then install the board into the bottom of the case where one screw is used to secure it to the bottom case. This is the origin of the “extra” screw noted in the page linked to earlier – there are no spares in this kit.

Then the knob base can be installed with the smaller screw securing it to the combination trimpot. Then, affix the knob cover, taking alignment into account.

Install the terminals into the approrpiate slots in the case, then tuck the wires carefully under the board in free spaces before bringing the halves together and screwing together with the two remaining screws.

The unit should look as above – neatly closed up with freely-spinning knob, freely actuating buttons and an LED visible through the hole.

Testing

Install two AAA batteries in the correct orientation, connect the earbuds, rotate the volume knob clockwise and hit the reset button followed by scan.

If all goes well, then you should be hearing some local FM stations – albeit in very poor quality due to the lacking earbuds.

Conclusion

The HX3208 (CD9088-based) SMD “micro” FM radio kit is one I approached with a sense of trepidation. While it is rather inexpensive, it poses a significant challenge to the unaccustomed – significant amounts of SMD soldering with components that are tiny and look visually similar. As a result, it’s good practice for those who want to do some homebrew SMD, but probably not a great experience for the beginner as there are no spares – you better not make any mistakes. As a result, it pays to go slow and do one value at a time. The lack of instructions included, slightly flimsy poorly-fitting case and slight inconsistencies between schematics and silkscreening were also downsides. The price is cheap enough as to be good entertainment, although in some places you could probably buy a fully-assembled auto-scan radio for the same price if that’s what you wanted.

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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