39. The Tuner IC: MK484

Project AF Amp Sound
Ineke's Retro BC547B PLAY
Matchbox none PLAY
Gerard L'AMiGo ECC40, EL41 PLAY
Paeansonic 210SP TDA2822 PLAY
Indin BC-R22 TDA2822
Loop Antenna Monitor none
A few years ago, I bought myself some MK484 linear ICs. Priced at just a few quarters, this IC claims to be an almost complete AM tuner. The component is similar (or equal) to ZN414 and TA7642. In the eighties, the ZN414 was priced at 8 guilders, but in 2016 I bought 50 TA7642 for just €1.80.

This article describes the IC and some projects I built with it, and comments on the receiving quality. Some very useful additional information is in this note by Dan McGilles.

Data sheets of the MK484 are easy to find, but only have two pages with basically little more than the construction of the match box radio. Fortunately, it is as good as equal to the older ZN414 and its sheets can be used. Only be aware of the reversed pin layout! The IC has become very cheap; Spring 2017, I bought 50 TA7642's for 88ct!

Brief description

The radio needs, besides the IC, just an LC circuit of which the coil is also the antenna.

The quality factor Q of such a tank is reasonnably high, and so is the input impedance of the IC, giving a fair selectivity. The IC amplifies and detects the AM signal and the current through its output wire has four components. First, a constant DC current of approximately 0.3mA to feed its transistors. The exact value of the DC current depends a little bit on the signal strength, and this was done on purpose; this second, signal strength dependent DC component makes the voltage drop over R1 (800R) signal dependent. Third current component is amplified RF current, which we don't want anywhere on the right side of the chip, so we short it through C1 (100nF). The fourth component is Audio Frequent, which can be taken as current through a transistor amp, or as voltage over R1 for a tube amp.

R2 and C2 form the AGC circuit. R2 (120k) connects one side of the tuned circuit with the DC voltage of the output, so the DC level of the input now depends on the signal strength. Amplification in the IC depends on the DC level, which closes the control loop. We don't want the AGC to react too quickly (it would counteract the modulation), so C2 serves as a buffer.

My Tuner IC Projects

The Retro Construction Kit

In 2014 I gave a few Conrad construction Kits as presents and decided to built one with my wife. Many aspects of the radio hobby turn up in this little project, so my wife now understands much better what I do in the attic. Running from a 1.5V battery, the circuitry is barely able to drive a speaker, and the sound is quite weak.

The Matchbox Radio

Inspired by a discussion on the Transistor Forum, I built this little gem which is about the most simple thing that can be constructed with the IC. It has no AF amplification at all and produces just barely audible sound in a headpiece. But the sound is clear and undistorted, and you'll hear only one station at a time! The sound becomes a bit stronger still when a capacitor (220uF to 470uF) is connected over R1.

Gerard L'Amigo

While the previous projects are like fun projects with little practical value, this radio is for real use. It receives LW and MW, has four preset stations, and a powerful AF amp with ECC40 and EL41 tubes. The amplifier has an external input jack, so you can use this device also to play songs from an MP3 player.

Paeansonic 210SP

The tuner IC appears to be used mostly in construction kits and home designed product, while real radio's with it are hard to find. This construction kit is a little bit more pretentious than the Conrad one. It plays reasonnably well on FM (though tuning is hard due to hand effects), but has mediocre AM reception. On the lower AM frequencies, it tends to oscillate (both units I built), despite a total capacity of 300nF over the IC output.

Indin BC-R22

The Indin is the only factory made radio that I know of with the tuner IC. With the CD9088 for FM and the TDA2822 audio amplifier, it can be considered as a pre-constructed version of the Paeansonic. So, if you don't like the hassle of building the radio yourself, or you find the Paeansonic's price of 4 euros too high, or you insist on having a nice cardboard box for your radio, you can order this product for about 3,5 euros (including delivery!).

Loop Antenna Monitor

How is that, being able to check the operation of your antenna without a real receiver! This little machine is a bit like the Matchbox, but without the load resistor and the tuning tank. Use it with any of my loop antenna's.

Receiving Experiences

The construction projects and radio differ a lot in design and appearance, but have the IC tuner in common. How is the reception?

Not too much can be expected from the AGC operation. To avoid overloading on strong stations, you should use a large AGC resistor (R2 is 1k) and the supply voltage should be low (1.3V). To avoid noise on weak stations, use a smaller R2 (400R) and/or higher supply voltage (1.6V). To receive strong and weak stations, some manual gain control will be necessary.

Because there is just one tuned circuit, selectivity is limited. If your region has a strong MW station, you may hear it all over the MW band, and it may even mask distant stations. So, your TA7642 project will be a typical local broadcaster receiver.

The antenna coil is hardly loaded by the IC (its input impedance is 4M), and may show a strong directionality. This is nice for handheld or small sets, but a table radio with fixed ferrite rod is not so easy to rotate to optimum reception!

I experience a quite good quality of sound. With my own transmitter, the quality is not limited by the usual 10kHz bandwidth. Also, when a broadcast station comes in with the right signal strength and uninterfered, the sound is good.

Gerard Tel.