I am continuing with the Chinese radio builds. This time, I will be building the Sheja JH-188 radio kit.
About the Sheja JH-188 Radio
The Sheja JH-188 is an FM only digital mono receiver kit that is commonly found on sites such as Amazon, eBay, Banggood, and Alibaba e-commerce web sites. I purchased mine from eBay for $7.69.
It is built around the CD9088 integrated circuit. The CD9088 is a Chinese clone of the TDA7088T, which was developed by Philips Corporation in the early 1990s.
The TDA7088T, and Chinese CD9088 clone, is a mono FM integrated circuit intended for use in battery operated pocket radios. With the CD9088, no specialized parts such as IF coils or ceramic filters are required, making the parts count built around this integrated circuit low. It has builtin tuning capability if used with an external Varicap BB910 diode. The CD9088 has builtin FM station scan and reset functions. In addition, it has all the necessary stages to process RF input from the antenna to audio output. The CD9088 can be operated with as little as 1.8 Volts DC.
SHEJA JH-188 Circuit Theory
Unfortunately, my JH-188 didn't come with assembly instructions or a schematic. If I would have inventoried this kit upon arrival, I would have noticed the missing assembly/schematic documentation. I was, however, able to find a picture of a schematic on a listing in Alibaba, and included it below. Note: The bottom schematic is an extension of the top one.
Based on my years of electronic experience, I'll try to extrapolate what the discrete components do in this circuit.
C3 - Possibly used to debounce scan reset switch SW2.
C5 - Used to debounce scan switch SW1.
BB910 and L1 - Varicap diode and coil used for the LC tank circuit used for tuning.
C4 - Decoupling or Bypass capacitor, shunts noise caused by other circuit elements to ground.
CD9088 - Integrated circuit used to convert a Radio Frequency(RF) modulated signal to Audio Frequency(AF).
SW3 and LH2 - Built in flashlight circuit.
SW2 - Switch clears all radio stations from memory.
SW1 - Switch used for radio station scan.
PL1 - Audio jack for ear buds. The ear bud wiring is also used as an antenna.
KW1A/B - Audio volume control and main radio power switch.
Q1 - Transistor used to amplify audio signal.
L2 - Possible RF choke, preventing Radio Frequency (RF) signal from entering the audio amplifier stage.
My JH-188 came in an international mail envelope, direct from mainland China. The mail envelope contained a plastic bag with all of the radio's discrete components.
It is important to inventory all parts, prior to starting the assembly process. Immediately reach out to the vendor if any parts are missing. If I had taken my own advice, I would have noticed that assembly instructions and schematic documentation were missing. I use an old plastic food container that contained a salad mix to contain all of the radio's parts so they don't get lost. A mistake in this picture is that I should have stuck the CD9088 integrated circuit and transistor, circled below, in anti-static foam to protect them from damage. You can wrap these components in tin foil if anti-static foam is unavailable. Granted, the supplier did not protect these components by sticking these component's metal leads into anti-static foam, but it is a good practice to get into.
You will need a soldering iron with accurate temperature control and an ultra fine tip, in order to solder the surface mount CD9088 integrated circuit to the printed circuit board. I use the ZENY 862D+, which is a soldering / hot air surface mount rework station, available from many e-commerce vendors.
First, I soldered the surface mount CD9088 integrated circuit to the foil side of the printed circuit board. I started with this component as it was the hardest component to solder in place. It is easier to mount when no other components are already soldered to the printed circuit board.
I then tested all other components, using the Mega 328 Component Tester, before soldering to the printed circuit board. The Mega 328 can test resistors, capacitors, inductors, transistors, and diodes. It cannot, however, test integrated circuits such as the CD9088. I find that these inexpensive radio kits often have discrete components that are either out of tolerance of just simply defective. I replaced any defective or out of tolerance components with spare parts I had on hand.
Next, I soldered all ceramic capacitors and inductors in place. The rest of the components are of the thru-hole type, where the component is inserted from the component side and soldered on the foil side.
I then soldered the single resistor in place.
Next, I soldered the transistor and Varicap diode in place. These components are polarized, meaning they are have to be installed in the proper direction.
All three switches were then soldered in place.
The audio jack, volume control/power switch, and the incandescent light bulb were soldered in place. I placed plastic insulators around leads to the light bulb.
Finally, the wires that connect to the batteries were soldered to the foil side of the printed circuit board.
It is important to test the printed circuit board assembly before mounting it into the radio's case. I connected two AAA batteries to the power connection leads and the supplied ear buds to the audio jack. I then turned the volume control clockwise until I heard the click of the power switch then adjusted the volume control to mid point. Push the SW3 button should turn on the light bulb. Push SW2 should reset scan and then push the SW1 scan button until you hear a radio station. If you don't hear a radio station, check component placement and orientation.
Defluxing the circuit board
Solder contains flux, an agent which aids in the soldering process, but leaves a sticky brown residue behind. Before mounting the printed circuit board in the radio's case, I spray defluxer on the foil side, use a tooth brush to free any stubborn flux, then blow it off with compressed air to remove any residue and to completely dry it.
Look how clean the foil side of the printed circuit board looks after defluxing. At this point, I also soldered the battery connectors to the end of the wires. The spring terminal goes on the black wire while the plate terminal goes on the white one.
Place the radio's front cover face down on a smooth surface and then install the Light, Reset, and Scan buttons.
With the radio's front cover still face down, place the printed circuit board foil side up in the top area of the radio case. Route the wires inside the case so they don't get crimped when the back cover is installed. Install the AAA battery terminals into the battery compartment. The battery terminals just slide in place into the provided slots.
Install the back cover and belt clip, secure with the single provided Philips head screw.
Now turn the radio over and install the silver plastic plate labeled "SHEJA JH-188" it just clips in place.
The knob for the power switch/volume control installs with a single Philips screw in the center. The knob is installed correctly if you hear the click of the power switch opening its contact when at the 8 O'clock position.
A chrome plastic cover clips in place over the top of the screw hole of the power switch/volume control knob.
Place two AAA batteries into the battery compartment then install the battery cover over it.
Plug the supplied ear buds into the audio jack. Turn the volume control knob clockwise until you hear a click of the power switch then move until about mid point. Click on the Reset button, then click the scan button several times until you hear a station. Your radio is now complete!
Station Reception Issue
I noticed that during the testing phase I was picking up a lot more stations than after final assembly. I had determined that I was touching the positive battery lead during testing and my body was acting like a natural antenna. In order to improve radio reception, I connected a 2 ft long blue wire to the positive battery connection then routed the antenna through a small hole I drilled at the bottom of the radio cover.
Below is a picture of the finished SHEJA JH-188. At a price point of under $8, the JH-188 is a great little kit for a hobbyist or a STEM program. It can be used to teach soldering and assembly skills. However, because the JH-188 performs all signal processing using a single integrated circuit, this kit really does teach radio principals. In addition, the JH-188, even with the addition of an external antenna, lacks the sensitivity of most standard FM radios. The low sensitivity of JH-188 only make it suitable for serious radio listening in metropolitan areas were strong radio signals are present.
Who Writes This Blog?
John is an IT professional from Cleveland, OH who enjoys amateur radio, ham radio, metal detecting,
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