#462 HX108-2 AM Receiver

Build and analyse the HX108-2 superheterodyne AM MW radio receiver kit.

Notes

AM/MW radios are of very little use these days in Singapore. There is no local AM broadcasting, just a few stations from Indonesia and Malaysia that can be picked up if you’re lucky. However, there’s still a great deal that can be learned from studying simple AM superheterodyne circuits. I have built a similar kit before - the HX-6B, see LEAP#123 - but I probably know more than I did then, so I’m looking forward to this little opportunity to revisit similar ground.

Although it’s possible to build the entire kit in under an hour, I decided to carefully follow a sub-system by sub-system build from audio out to RF in. Co-incidentally, K7QO is posting an excellent series covering the build of an HX108-2 at the same time … so I am heavily cribbing from there;-)

Tutorials and Resources

The HX108-2 is a pretty popular kit, and a number of very good tutorials can be found on YouTube describing construction and tuning. Here’s a selection…

K7QO HX108-2 AM Receiver: Theory and Construction

K7QO had a series covering the build that appears to have been subsequently re-cut into a series on RF Receivers. The main videos covering this particular radio are part 6 and 7:

RF RECEIVERS – 6 – Audio Amp, Driver and Demodulation Circuits

RF RECEIVERS – 7 – First and Second IF Amps

Other Tutorials

shango066: HX 108-2 am kit radio assembly alignment and test

The Radio Mechanic: an excellent desktop freeform build of the HX108-2 circuit:

The Kit

I purchased my HX108-2 kit from seller on aliexpress. It was generally OK, with no missing parts. The only negative was that the speaker was somewhat corroded.

The basic specification for the kit:

• Frequency Range: 525-1605kHz
• Intermediate Frequency: 465kHz
• Output Power: 100mW
• Power Supply: 3V, 2xAA
• Speaker: 57mm diameter 8Ω

Parts

Electronic components:

Transistors

The transistors are classified by ß(hFE), and it seems many kits substitute different parts of similar capability:

• 9018G ß = 80-100
• 9018H ß = 97-146
• 9013H ß = 144-202

I stuck all the parts in a component testers, and found that one of the 9013H had a much lower ß(hFE) than the others, so I replaced it from spare parts. Might not have been necessary, but avoided a potential issue.

Audio Transformers

There are two audio transformers used in the final stage:

• B6 input audio transformer (蓝 blue or 绿 green)
• B7 output audio transformer (黄 yellow or 红 red)

I received a 绿 green and 黄 yellow in my kit. For reference, I measured the resistance of each coil:

• 绿 green: 220Ω : 104Ω + 104Ω
• 黄 yellow: 2.5Ω : 6Ω + 6Ω

IF Transformers

There are four transformer cans used in the design:

• B2 红 red: local oscillator & mixer
• B3 黄 yellow: first IF
• B4 白 white: second IF
• B5 黑 black: demodulator

While not essential, I decided to tune the IF and demodulator cans to 465kHz prior to installation so they should not require much fine tuning later. Note: 455kHz is the conventional intermediate frequency, but the circuit states it is designed for 465kHz so I went with that.

I tuned the transformers with a 2V peak-peak 465kHz signal from a function generator directly connected to the 2-pin/single coil side of the transformer, and monitored the output with an oscilloscope with the following results:

For B2 红 red, I simply measured its tuning to make sure it was broadly in the ballpark. I’ll fine tune it in-circuit:

PCB

The PCB has a reasonably good silk-screen to assist with construction:

Circuit Design

The schematic provided with the kit contains a number of errors. This is a copy that I have annotated with the necessary corrections:

The HX6B is a superheterodyne receiver design with two IF stages. The circuit basically maps to the following stages:

• RF Filter: B1, C1a antenna tuner
• RF amp: V1
• Local oscillator and Mixer: B2, C1b
• First IF Filter and Amp: B3, V2
• Second IF Filter and Amp: B4, V3
• Demodulator: B5, V4
• Audio amp:
  • V5 audio driver/preamplifier
  • B6, B7, V6, V7 push-pull class B power amplifier

A quick sketch from my notes on the functional design:

Test Points

The PCB includes a number of test points with a specified current range:

• V1 collector: 0.13-0.22mA
• V2 collector: 0.4-0.8mA
• V3 collector: 1-2mA
• V5 collector: 3-5mA
• B7 high-side: 4-10mA

I believe the way these test points should be used is as follows:

• build the complete circuit without bridging the test points
• power the circuit
• measure the current at each test point - it should be within the limits above
• after testing, bridge the test point

Since I’m going to build the circuit by stages, I don’t think I’ll be able to make these measurements as I go. I could go back after the kit is complete and un-bridge and test each stage.

Construction

Step 1: Power

Can’t achieve much without power! There are actually two “power rails” in the circuit:

• the full power of the battery is used to drive the audio amplifier
• and a ~1.4V rail is established using 2x diode drops to power the RF stages

Verification:

• with 3.15V applied (2xAA) the voltage at the anode of D1 measures 1.43V - this is as expected, about two diode drops of ~0.7V √
• the current drawn is about 7mA √. This is about right: (3.15V-1.43V)/220Ω = 7.82mA

Step 2: Speaker and Battery

Mounting the speaker and battery clip in the housing. Speaker is connected to the PCB, however I’ve left power disconnected for now as I’ll use a bench power supply for testing so I can read off total current and voltage.

Step 3: Audio

This step adds the audio pre-amp and power amplification stages, starting from C8/R9 - which is where I’ll inject a test signal to verify things are working. Note:

• C11 is mislabeled as C6 on the schematic (so I ended up installing both by accident)
• one of the 9013H tested with much lower ß(hFE) than the others, so I replaced it from spare parts. Might not have been necessary, but avoided a potential issue.

Verification:

• powered with bench supply at 3V
• injecting a 0.4V peak-peak 1kHz sine wave at C8/R9 junction (CH1-Yellow in the scope traces below)

First, with the volume set to roughly 20%, total current drawn is about 40mA, and the output (CH2-Blue) is amplified to about 1.55V peak-peak (gain ~ 3.9)

Next, with the volume set to roughly 60%, total current drawn is about 86mA, and the output (CH2-Blue) is already clipping, running at about 4V peak-peak (gain ~ 10)

As I increase the volume, current peaks at about 120mA. Interestingly, the current draw and volume does start to fall off at over say 80-90% on the volume control pot. This probably indicates something in the push-pull configuration is starting to break down (but in a “safe” way).

Step 4: Demodulator

The V4 transistor is being used as a detector in this stage.

Verification:

• with circuit powered, probe the B5 test point - noise on the speakers √
• with circuit powered, inject 465kHz carrier with 1kHz AM via 22nF capacitor to the B5 test point - peaked 1kHz tone on the speakers √

Step 5: Second IF Stage

Verification:

• with circuit powered, probe the B4 test point - noise on the speakers √
• with circuit powered, inject 465kHz carrier with 1kHz AM via 22nF capacitor to the B4 test point - peaked 1kHz tone on the speakers √

Step 6: First IF Stage

Verification:

• with circuit powered, probe the B4 test point - noise on the speakers √
• with circuit powered, inject 465kHz carrier with 1kHz AM via 22nF capacitor to the B4 test point - peaked 1kHz tone on the speakers √

Step 7: RF Front-end

All the remaining parts:

Verification:

• turn it on, and it works!

Step 8: Final Tuning and Testing

shango066 has a great video on tuning the HX108-2.

Since we don’t have any AM stations in range, I did a rough manual alignment against some digital signals in the local MW band. I read the frequency with a commercial radio, then adjusted the HX108-2 so that the signal appears at roughly the same frequency according to the front dial.

Since I’m tuning with an ugly digital signal, I can’t really tell if the IF and demodulator stages are perfectly aligned, although they seem fine and couldn’t be improved much with fiddling. The main thing I needed to do was adjust the oscillator so that the frequency range roughly lines up with the decal/front dial. I got “close enough for now” by just adjusting B2 红 red:

• set the variable capacitor to position the front dial at the expected frequency of the signal I was aiming for
• adjust B2 红 red until the signal is received at best strength/quality

Step 9: Putting it all together

Conclusion

My local AM band is completely dead (has been for years) so now I have a perfectly functioning but absolutely useless doorstop;-) But this has been great revision on simple superheterodyne receiver design.

Next steps?

• next time I travel to a place with AM, I might take it along for a test and some tuning in the field
• build a low-power AM transmitter for a little local area transmit/receive test

Credits and References

• HX108-2 from seller on aliexpress
• 中夏科技 - zxradio - manufacturer
• 108-2七管半导体收音机 - the product on the manufacturer’s site
• LEAP#123 HX6B - a similar circuit
• Superheterodyne receiver - wikipedia
• Introduction to the Superheterodyne Receiver - by Lloyd Butler VK5BR
• Block Diagram of Superheterodyne Receiver Radio - Ian Pool. A good description of the modules.
• Radio Receivers Book - Chapter 4 Superheterodyne Radio Receivers - Miomir Filipovic
• Superheterodyne: BJT AM receiver - fascinating project notes by He Yong Li
• IF AMPLIFIER TRANSFORMERS