Elenco Radio Build: Part I

October 18, 2024 · by Budi Isdiyanto

Elenco have this great AM/FM “Superhet” Radio Kit that I came across on a YouTube channel called The Offset Voltage that has a series of videos on building the kit, going carefully and thoroughly through all the steps detailed in the excellent manual, as well as explaining the operation and theory of each section. Highly recommend watching them if, like me, you have ever wondered how AM/FM works.

Of course had to build this kit myself as it looked super interesting and a good introduction to RF, a topic that was already on my radar.

You can find lots of reviews and unboxings online so I’m not going to go into any details on the kit itself. I only plan to document my experience with building and testing the radio. If you want to know more about the kit I recommend watching Voltage Offset’s introductory video as it is quite detailed and he gives some good suggestions and insights.

The kit includes everything needed, all nicely labeled and bagged. However you will also need:

1. Soldering iron
2. Lead trimmer
3. Multimeter
4. Philip’s screw driver and pliers
5. 9V Battery or equivalent power supply

Having an oscilloscope and/or signal generator will make testing and tuning the radio much easier and be more educational too, though not a requirement.

The only other thing I would point out, and it should be obvious from the PCB, is that this kit is meant for edification and not function. Don’t expect great performance out of it, though I am curious how well it will or won’t work.

Here is a shot of of what comes out of the box:

Everything is nicely packaged and labeled.

I rarely check parts lists for kits but strongly recommend doing so for this one:

I even tested all the resistors, diodes, large caps and transistors with a multimeter to minimize simple bugs later on. There are a lot of parts to go through but it also helps create a memory map that makes following the instructions easier.

The first step is to build the amplifier section using the LM386, which is what this post is really about:

Along with the battery holder and the on/off/volume potentiometer:

As well as the speaker and earphone jack:

In the last picture I included the whole backside of the PCB to show the huge sizes of the traces and pads. Just like the PCB for Elenco’s power supply kit it makes working with the supplied lead-free solder very challenging indeed. You’ll need a lot of heat from your iron. Ran mine at 750F and worked hard on getting the pads and leads up to temp so that the solder would flow well. It was a struggle and in places got partial delamination of the traces. Would recommend practicing on simpler kits first if your soldering skills are minimal.

Construction of this first section took just under two hours. Took it slow and focused on getting good solder joints.

The next night I did all the tests and measurements specified in the manual:

It’s these test and measure portions of the instructions, along with the explanation of the circuits, that makes this kit stand out, particularly for relative newbies like myself. You get to know how the circuit should work and how to check that it is working as it should. Really nice.

So how did mine turn out? Really well. It aced all its tests!

I did have one annoying issue and a funny one at that. When turning on the device you can hear very mild noise from the speaker as you’d expect. However, when connecting an oscilloscope lead to the J3 jumper (which is the input to the audio amplifier) I could hear a station playing! “Wow!”, I thought. “The kit’s working already!”. As amusing as that was it made it hard to accurately measure the mV signal levels going into the amplifier due to that interference. [I remember in Offset Voltage’s videos him mentioning how to minimize interference so need to look into that more.]

Was able to get around this somewhat by using the cursor feature of the oscilloscope:

Which was my first time using this feature in anger. You can see from the screen capture that the oscilloscope clocks the peak-to-peak voltage (of the 1 KHz signal) at around 7 mV (it was jumping about a lot). Using the cursors I could see that the level was more like 6mV.

Used the cursor feature again when doing the bandwidth test, in this case to show where -3dB was visually:

Doing all the testing and measuring took about another two hours. A lot of that was due to configuring and setting up equipment. It was also my first time using the signal generator I recently bought specifically for this project.

Have to say I’m really enjoying the kit so far. Am very curious how the AM and FM receivers will ultimately perform.

If you are following along, here are all my test results for the LM386 version of the amplifier:

Metric	Expected	Measured
Idle Current	<10 mA	4.4 mA
Gain	100-180	166
AC Bandwidth	>30 Khz	52 Khz
Clip Voltage	~	6.7 V
Max Power Out	> 200 mW	714 mW
Max Source Current	~	143 mA
Source Voltage	9 V	9 V
Max Source Power	~	1.29 W
Efficiency	~ 50 %	55 %
Fun Factor	100 %	100 %

The next step is to build a discrete version of the amplifier and test its performance.