![]() So, we have only two levels of volume full and something a bit less. This is obviously done to avoid a space consuming potentiometer. The power switch is a minature slide switch on the side, which has an extra position for volume. In any case I would prefer AAA cells rather than the LR44's. I don't know what current it's drawing so I can't say whether it's consuming much more battery current than a class B amp would. The audio amp appears to be one transistor ie. The enclosure is all clipped together, and once I'd opened it, sure enough, a TDA7088T was visible. So, I use the normal kind of headphones instead. Apart from the appalling sound quality, they are insensitive, unhygenic and dirty, fragile, and do not block out external sounds. Supplied with this receiver were a pair of those awful "in-the-ear" type of miniature type earphones. I'll be on the lookout for LR44's at the markets and $2 shops now that I've got this radio! As with all these sorts of radios, the headphone lead functions as the aerial. It is powered off two LR44 button cells, which are expensive and I assume wouldn't last terribly long. Some people might want to build the set from the start with a trimmer potentiometer in place (e.g., Mouser 569-72PM-25K).This tiny receiver is not much bigger than an AA cell. Once the circuit is working, you can remove the potentiometer, measure its value, and replace it with a fixed resistor. Adjust R4 until you can reliably tune in stations. Replace R4 with a 20K or larger potentiometer (up to 50K). If you don’t make any progress, then you need to change R4. If you hear a squeal or hear nothing, then the circuit is oscillating too little or too much. Try the tuning control and see what you get. If you hear a rushing noise, you will probably be able to tune in a station. You can change the tuning range of C3 by squeezing L1 or change C1. Use another FM radio to see where you are on the FM band. If you got a radio station, you are in good shape. If the radio is wired correctly, there are three possible things you can hear when you turn it on: 1) a radio station, 2) a rushing noise, 3) a squeal, and 4) nothing. ![]() The resulting arrangement works quite well. Tighten the screw just enough so it stays put as you tune the capacitor. ![]() The #4 screw is the wrong thread pitch and will jam (bind) in the threads. ![]() Twist the nylon screw into the threads of the C3 tuning handle. A knob is important to keep your hand away from the capacitor and coil when you tune in stations. Mount the ends of the wire on your circuit board keeping some clearance between the coil and the circuit board.Ĭ3 does not come with a knob and I have not found a source. Find the midpoint and solder a small wire for C2 there. Spread the windings apart from each other so the whole coil is just under an inch (2.5 cm) long. Wind 6 turns tightly, side-by-side, on the cylinder, then slip the wire off. #20 bare solid wire works the best, but any wire that holds its shape will do. I used a thick pencil from my son’s grade school class, but a magic marker or large drill bit work just fine. Get any cylindrical object that is just under 1/2 inch (13 mm) in diameter. Although it has many important jobs, it is easy to construct. L1 sets the frequency of the radio, acts as the antenna, and is the primary adjustment for super-regeneration. If your hand is too close to L1 while you tune the radio, it will make tuning very difficult. Arrange L1 fairly close to C3, but keep it away from where your hand will be. Only the outer two leads are used the middle lead of C3 is not connected. The tuning capacitor, C3, has three leads. ![]() Because this is a superregenerative design, component layout can be very important. ![]()
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