HX-6B Chinese Radio Kit Build

Today on Radio Boat Anchor, I am going to build the HX-6B Chinese Radio Kit, otherwise known as "the little blue one" by many electronic hobbyists. This kit can usually be found for a few dollars on many e-commerce sites such as Amazon, eBay, Banggood, and Alibaba. I purchased my HX-6B on a "buy it now" auction for $4.59. It shipped directly from China and three weeks later it arrived with my mail.

About the HX-6B Radio

The HX-6B is of standard Superheterodyne or "Superhet" radio receiver design. The Superhet radio design was invented by Edward Armstrong, an Electrical Engineer from the United States, in 1918. It has been used widely in analog AM and FM radio receivers for most of the 20th century until it was supplanted by SDR (Software Defined Radio) in which all single processing is done through code on a one or more integrated circuits. The HX-6B would make a great kit for an educational STEM program as it has all of the building blocks of a basic AM radio receiver. This kit is great for education purposes but will probably lack sensitivity as most AM radios of the Superhet type have two IF (Intermediate Frequency) stages where this kit is only designed with one IF stage. Station reception may not be a problem if you live in a metropolitan area were AM station signal strength is strong. ​

HX-6B Circuit Theory

The instructions you get with the HX-6B radio kit is primarily in Chinese. However, the schematic and parts list in the instructions are printed in english. In addition, the component side of the included printed circuit board has english destinations for the components. You can easily assemble this kit without the need to read Chinese. All of the circuit theory is written in Chinese, so I am going to extrapolate the circuit theory of the HX-6B, based on my 30 years of electronic experience. 

• Transistor V1 acts as a mixer between the incoming AM (Amplitude Modulation) modulated RF (Radio Frequency) signal and the LO (Local Oscillator) circuit. This producers a constant 455kHz AM modulated signal that is fed into transistor V2. Capacitor CA is the Tuning control and forms a tank circuit with T1, allowing the radio to pass a certain RF frequency while rejecting all others.
• Transistor V2 is an IF (Intermediate Frequency) amplifier circuit. Transformers T3 and T4 "pass" the 455kHz signal and reject all others. 
• Transistor V3 is what is known as a "Detector", it strips the RF carrier signal from the AF (Audio Frequency) signal.
• Transistor V4 is the AF preamp. Its purpose is to amplify the audio signal. Potentiometer RP is the volume control. 
• Transistors V5 and V6 form a Class A push pull amplifier,  providing enough power to drive a 8 Ohm speaker or external headphones.
• Typically radios have an AGC (Automatic Gain Control) circuit, this prevents radio stations from fading in and out during listening and giving a fairly stable audio level. I believe R3 and C5 might be an AGC circuit, but am not %100 sure. 

Unpacking:

The HX-6B radio kit comes in a clear plastic wrapper. All of the components, including printed circuit board and instructions, are housed in the blue plastic shell of the AM radio.  

As mentioned, the instructions are primary in Chinese. However, the schematic and parts lists are in English. 

It's a good idea to inventory an electronic kit, such as the HX-6B Radio Kit, before assembly. Make sure all required parts are present and accounted for, before assembly. If any parts are missing, reach out to the vendor, before starting the assembly process, to acquire any missing parts.

I like to put all small discrete parts in a plastic container before assembly. That way they are all contained and it is less likely that I might lose one. 

Assembly

When assembling radio kits, I usually do ​it in a certain order. First, I solder the resistors to the printed circuit board. Before soldering a resistor to the printed circuit board, I check its resistance value with my Mega 328 component tester. This handy device can check resistor, capacitors, diodes, transistors, and inductors. I often find resistor, capacitors, and inductors either defective or out of tolerance in these little kits. I don't believe they use "top shelf" components. I replace any components that are either defective or out of tolerance.

I solder each resistor, one at a time, to printed circuit board. This is a fun activity and I am in no real hurry. 

Once a component is soldered to the printed circuit board, I cross it off of the parts list.

Finally, all of the resistors have been installed in the printed circuit board.

Next, I install the ceramic disk capacitors. Checking each one with my Mega 328 component tester before soldering to the printed circuit board.

Time to install the electrolytic capacitors. Once verified with the component tester, I solder them to the printed circuit board. Electrolytic capacitors are polarized. The negative lead side is denoted on the printed circuit board by a side half of the circle colored in white.

There are three types of NPN transistors included in this kit: 

• 9013H (Quantity 2)
• 9014
• 9018 (Quantity 3)

Transistor type is printed on the flat side of each transistor. 

​As with other components, I check each one with my component tester before soldering to the printed circuit board. Note, transistors have three different connections, Collector, Base, and Emitter. It is important that you install the proper transistor into the proper holes in the printed circuit board. The locations are labeled on the component side. In addition, you must install them in the proper orientation. You must match the flat side of the transistor with the flat side of the  label printed on the component side of the printed circuit board. See areas circled in the picture below:

All of the transistors are now installed.

I then installed the 5K potentiometer with built in power switch. This is labeled RP in the schematic and it will serve as the volume control. There was no way to test the potentiometer with my component tester. I just checked with my VOM (Volt-Ohm-Meter) that it measured 5K Ohms between the two outer leads of the potentiometer. You should see the resistance change from 0 to 5K Ohms or 5K Ohms to 0 if you place the meter leads between one of the outside leads and the center lead.

Next, I installed the audio transformer. I did some resistance checks with my VOM meter to determine if there were three windings, two should have a lower resistance while the third has more windings and should be slightly higher in resistance. The audio transformer must be installed into into the printed circuit board in the right orientation. If you look closely, there is a little raised bump on the bobbin of the transformer, this should line up with the white dot on the component side of the printed circuit board. See areas circled in the picture below:

The audio transformer is installed in the corner of the printed circuit board. In addition, I installed the oscillator coil and the first and second IF transformers. The oscillator, first, and second IF transformers slugs are color coded. You must install them in the proper locations. 
T2: Red Slug (Oscillator Coil)
T3: White Slug (First IF Transformer)
T4: Black Slug (Second IF Transformer)

Time to install the audio jack, circled in the upper left-hand corner.

The next step is to install the variable capacitor, used for tuning, to the printed circuit board. The mounting screws used for the variable capacitor are also used to hold the nylon loopstick antenna mount in place.   

The two screws provided secures both the loopstick antenna mount and the variable capacitor to the printed circuit board. You will need to solder the three variable capacitor connections to the printed circuit board. 

The HX-6B has a Red LED (Light Emitting Diode) power indicator, circled in the picture below. It must be routed though the hole in the printed circuit board from the component to the foil side. 

The LED is polarized, the flat side of the component corresponds to the side the arrow points to on the printed circuit board.

The loop stick antenna consists of two windings on a ferrite rod. The windings act like a step-up RF transformer. It is important to connect the wires to the right connections on the printed circuit board. As you can see from the picture below taken from the instructions, connections a and b connect to 100 wire turns while connections c and d are only 10 wire turns. It is easy to identify a and b connections using a VOM (Volt-Ohm-Meter) as the resistance between these connections are higher than between connections c and d.

The easiest way to make the proper connections from the printed circuit board to the windings is to lay it out as shown below. 

Once all of the connections to the windings are made, it is time to slide the cardboard tube that secures the windings over the ferrite rod. Next, attach the ferrite rod to the printed circuit board using the nylon loopstick antenna mount. I also attached the dial to both the volume control and the turning capacitor, so that the radio is ready for alignment.

There are several places on the printed circuit board foil side that have broken traces. This is intentional so as to allow current checks of different stages of the radio. My recommendations is to just put a solder bridge across them as I found the current ratings listed in the schematic to be inaccurate. If you don't bridge these connections with solder, the radio will not work!

Time to attach the two provided blacks wires between the printed circuit board and the speaker. The yellow wire to the +3V point and the blue wire to the -3V point. I circled the points in red, on the sparsely populated component side of the printed circuit board, to show you where to make the connections.

On the ends of the yellow and blue wires, you must solder the battery contacts. 

Soldering leaves sticky flux residue on the foil side of the printed circuit board. I spray flux remover on the foil side then use an old tooth brush to loosen the flux. I then use compressed air to blow away the flux residue and to dry the board.

Assembly

I found it easier to assemble the HX-6B radio in its case before performing alignment activities. Double-check the tightness of the screw that holds the dial to the tuning capacitor.

Place the tuning indicator sticker over the tuning dial. I recommend temporality placing the printed circuit board into the radio enclosure then rotating the tuning dial back and forth so that you can determine the proper dial indicator sticker placement. 

Next assembly steps:

1. Lay the front half of the radio shell on a soft cloth.
2. Place speaker in as shown in picture.
3. Install printed circuit board as shown.
4. Secure printed circuit board with single provided screw.
5. Install battery clips, they simply press in the mounts.
6. Check tightness of screw that holds the volume control dial in place.
7. Apply several dabs of adhesive, such as E6000, to keep the speaker in place. 

Install the batteries, the HX-6B radio is now ready for alignment!

Alignment

You must align a Superheterodyne receiver, in order to get maximum sensitivity and selectivity, and so that the tuning indicator indicates the correct station when being tuned. 

Note: Do not use a regular screwdriver with a metal shaft to adjust transformer slugs and trimmer. It will skew your adjustment. It is best to use a non-metallic radio/TV alignment tool. 

Note: If an RF Generator is not available, satisfactory results can be obtained by tuning into stations on the low and high end of the radio dial. 

Here are the steps to align the HX-6B radio:

1. Turn on your RF Generator, set for a 530kHz carrier with AM (Amplitude Modulated) audio signal.
2. Place the lead of the RF Generator close the HX-6B's loop stick antenna.
3. Turn on the radio using the switch attached to volume control, adjust volume control midpoint. 
4. Turn the HX-6B tuning capacitor fully counter-clockwise. 
5. Adjust the red slug on the local oscillator transformer for maximum volume of the audio signal.
6. Adjust the white slug on the first IF transformer for maximum volume of the audio signal.
7. Adjust the black slug on the second IF transformer for maximum volume of the audio signal.
8. Set RF Generator for 1600kHz.
9. Turn the HX-6B tuning capacitor fully clockwise. 
10. Adjust the trimmer on the tuning capacitor for maximum volume.
11. Repeat steps 1 through 10 so as to double check adjustments.
12. Turn off RF Generator.

Antenna Alignment

In order to further maximize the sensitivity of the HX-6B, you must adjust the antenna trimmer and loop stick antenna. 

1. Turn HX-6B radio on using the volume control, adjust volume control midpoint.
2. Find a station midpoint on the tuning dial. 
3. Adjust the antenna trimmer for maximum station volume.
4. Using an insulated tool, move the antenna coil back and forth along the ferrite core until maximum station volume is achieved. 
5. Repeat steps 1 through 4 so as to double check adjustments.
6. Apply a dab of adhesive, such as E6000, to keep the antenna coil in place.

Conclusion

Below is a picture of the finished HX-6B. Simply install the back cover after the alignment process. At a price point of under $5, the HX-6B is a great little kit for a hobbyist or a STEM program. It can be used to teach basic radio principals, soldering, and assembly skills. However, with its only single IF stage, it lacks the sensitivity of most standard AM radios. The single IF stage makes the HX-6B only suitable for serious radio listening in metropolitan areas were strong radio signals are present.

The HX-6B in action