Introductory Note: I purchased this kit for this article and I do not represent the Elenco Company. They have not paid me or provided any materiel for writing this article.
This article is a review on the Elenco Model AM-780K AM radio kit. It was designed for students in an electronics lab class, and when I was working on my associate’s degree in Electronics Engineering the college I was attending issued us both the AM radio kit from this company as well as the AM/FM version of it.
The knowledge I want to pass on here is that should you decide to purchase a kit for yourself or possibly for your children as a STEM project, it would be a great choice for learning basic electronics, electronics parts identification, soldering, reading assembly instructions, and general knowledge of radio theory. In fact, should you build this radio to operating completion you will have learned enough that it would suffice as a good portion of knowledge needed to pass the FCC ham radio test for both Technician and the General license classes. The rest of those exams would be FCC regulations and such.
There have been many changes in the kit, the manual is much more detailed in its effort to educate the builder and has 10 pages complete with theory, parts identification, detailed building instructions step by step, and a complete inventory list. The components used are modern design types that are far more stable than what was offered way back when I first built the kits.
The kit I previously built used several transistors and had only one Integrated Circuit (IC) that was used to amplify the sound to the speaker.
The current iteration of the kit uses two ICs with one of them that looks like a normal three-leg transistor but it s really the first stage of detection and amplification of the detected AM radio signal.
PARTS IDENTIFICATION Page 1&2
Let’s look at IC-1 and then IC-2 and compare the difference between the version used in today’s radio as was used in the kit 24 years ago.
IC 1 (MK484 radio IC)
This in all outward appearances looks like a traditional three-leg transistor you’re most likely familiar with seeing. But inside is another device altogether. This integrated circuit provides a complete RF section, Detection and AGC (Automatic Gain Control) and amplifier circuit, which requires only a few external components to make it a high quality AM Tuner. Good audio quality can be achieved, and current consumption is extremely low. No setting up or alignment is required and the circuit is completely stable in use. The MK484 (IC1 in the kit) has an input impedance of 4MΩ and operates over a frequency range of 150KHz to 3MHz. The IC will run off a DC supply of between 1.1V and 1.8V, a current drain of 0.3mA (thousands of an amp) makes it suitable for battery operation. The IC provides an output of 40 to 60 mV. Here is a calculator for these types of units of measure https://www.unitconverters.net/current/milliamps-to-amps.htm as well as there is a unit converter listed that is listed on the bottom of page 2 of the assembly manual.
The original AM radio kit had twelve transistors, all the external circuitry needed to create the system and use substantially more power to operate. Consider a TEOTWAWKI situation where ” is none, two is one.” A person could invest enough money to buy several kits, and for a modest price several spare IC units and store these things in an EMP-proof case: just in case. The completed radio weighs only 2 pounds with battery … easy to store, easy to hide, easy to use. The PCB (Printed Circuit Board) is only 5.5” long and 3.5” wide. One could make a protective case for it, if you wanted to.
IC 2 LM386 Amplifier:
The LM386 is an audio power amplifier designed for use in low voltage consumer applications. The (Signal) gain is internally set to 20 to keep external part count low, but the addition of an external resistor and capacitor between pins 1 and 8 (C8 and R6) increases the gain to any value from 20 to 200. The inputs are ground referenced while the output automatically biases to one-half the supply voltage. The quiescent (at rest) power drain is only 24 mW when operating from a 6-V supply, making the LM386 preferred for battery operation.
Today’s kit is truly a smaller footprint design and far more stable and reliable in operation than the older version that I first built.
Construction Pages 4 to 8
Page four starts the construction part of the project. It has detailed data on soldering, parts identification, location, and installation. The Printed Circuit Board (PCB) has the parts identification printed on the surface on the top of the board. It also has the schematic diagram printed on it as well. The schematic diagram for any electronic circuit is a diagram that shows the electrical connections in the circuit, and not always the physical location of the part in the circuit. In the case of this circuit the physical location and the electrical location have been made close to each other for ease of building as well as for teaching the circuit theory to the application. Page three of the manual has a detailed description of how the radio works.
What you will need is a soldering iron, preferably one with a stand and a tip cleaning bin that has either a sponge-type pad that can be soaked to clean your tip, or better yet one with a “wooly pad” type cleaning element. The water can corrode the tip with continued use whereas the wooly element type (think Brillo Pad) that will clean the tip but not contaminate it with minerals usually found in most tap water.
Page 5 under the construction steps note that diodes D1 & D2 have a particular way to mount them to the PCB, pay attention to the orientation of all teh components.
Figure 2 at left shows the parts as you build the kit. Also, in photo Figure 2 you can see the diodes with the cathode end (Black Band to the left) and resistors R4 & R2 mounted. Resistors do not have a particular mounting orientation, some may have to be mounted with one end flush to the bard and the opposite lead bent over and soldered in, but they are not going to go bad unless you install the wrong resistor in. Notice though that I have installed them such that the primary color band (See page 2) facing to the left. It could as well have been to the right; the point I want to make here is that by installing the components so that the values such as color bands or markings on the part can be read from the position of the top of the board facing upwards. Not all parts will let you do this as they must be mounted in such a manner as to let them work properly. But if you can mount them as I have so far then if I have to troubleshoot the circuit later it will be easier for me to see the correct installation like with the diodes cathode end, or the resistor color bands in one orientation for most of the PCB.
At the end of Part 2 of this article is a link to one source of an inexpensive tool kit that a first-time or inexperienced builder can buy and would work well for this project. If you do not want to buy from Amazon look at the listing in the tool kit and find a source to purchase those items. This particular kit is a complete kit for the beginner builder and has everything you will need to start this project. The radio kit comes with lead-free solder, and I recommend that you replace it with 60/40 rosin core solder (See link at the end of Part 2) since the lead-free solder requires much higher heat to melt and doesn’t make for good solder joints that are reliable. DO NOT use acid-core solder as that will destroy the connections.
This is merely a link for reference to what will suffice for many electronic / electrical and home building projects. There will also be other tools you will want to have to have on hand for this and many other projects. I also want to discuss Electrostatic Discharge (ESD), which is the enemy of any electronic circuit of modern design. This kit is no exception. If you are planning to build this kit you will need to consider a way to ground yourself before handing the IC components in the kit. I put a link to a pad you can buy for your bench that can become a grounding pad for keeping your parts in order and safe from ESD (See link at the bottom of Part 2, tomorrow.)
I have an ESD pad at my home bench, and my bench at work is a professional ESD system for the whole room. Consider this, if you walk across the room, you will generate an electrostatic charge in your body. If you do not dissipate this charge before touching something that is ESD sensitive then you stand a good chance of either damaging the device or destroying it. If you see a spark and feel the spark as you touch something like a doorknob, you have discharged as much as several thousand volts or more — in theohry enough to stop a human heart, if it was in direct contact. It may or may not destroy something electronic, but it will create internal damage and that device may well work, but its working life will definitely be shortened. Or you just might get to see some of the magic smoke roll out of it.
Troubleshooting Page 8&9
These pages will give you some steps to troubleshoot if something doesn’t quite work out well. But before you move on from one step to another as you build the circuit have a second or third look. Have someone else look it over, look for solder bridges, parts you may have missed soldering into the board, parts in the wrong location or maybe installed backwards as in the case of a diode, a capacitor or such. I use a head mount magnifying hood that has 5X lens built in and this really helps me to see things much better. At 76 years young, my eyesight is better since I had cataract surgery, but I still need good lighting and the second set of 5X eyeballs to really make sure I have the right part and place it in the right location.
(To be concluded tomorrow, in Part 2.)
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