What could be wrong with this simple radio kit?

March 20, 2009 8:45 AM

ElectronicsFilter: Help me figure out what might be wrong with my son's "safe, solderless, exciting, educational and fun!" AM/FM radio kit.

My son got one of these solderless electronic radio kits at a recent cub scout meeting - everything mounts on a carboard-box frame via little springs and jumper wires. The instructions say "you'll be amazed at how well it performs!"

Indeed.

It runs off a 9V battery, two transistors, a single diode and half-dozen or so capacitors and resistors. The tuning board is pre-assembled and simply attached the board at the right spot(s). I posted the schematic for the FM variation here - the user-assembled part is on the right. For AM, a couple of extra hook-up wires are added, and the tuner board is attached so that the variable capacitor is working with the ferrite rod.

Our first iteration through the kit got us nothing but a burst of static in the earpiece when power was applied.

I went through it again last night and I think I'm making some progress. I can now hear a varying, AC-type of hum in the earpiece that goes away if I ground the circuit (by touching the spring at point 11 on the schematic). This tells me that I'm receiving...something. But when I turn the knob to tune, I get nothing. No static, no local stations, zilch. This goes for AM and FM alike.

Where might I check next? I've checked and double checked the transistors, germanium diode and single electrolytic cap to make sure that they're connected properly and they are. The battery is also brand new. The back of the variable cap on the tuner board has two tiny trim pots that I've twiddled a little (and restored) to no effect.

This thing is driving me batty because there just isn't that much to this kit and it Ought To Just Work. But it's not, as far as I can tell.

posted by jquinby to technology (6 comments total)

Really difficult to debug this kind of thing without a multimeter... I know buying more stuff probably isn't what you want to hear, but it's really useful for debugging this kind of thing. Unfortunately, I'm not familiar enough with AM/FM radio to help you out further.

Once/if you have a multimeter, check continuity between each component in the diagram that shows as connected and make sure everything has connectivity.
posted by jangie at 9:03 AM on March 20

I once made these (well, similar) AM radio kits with a class of 7th grade students - the most common problem was the inefficiency of the antenna. We connected to a conveniently located TV antenna cable and were able to get a surprising number of stations, though none were crystal clear even then. Any electrically conductive material should work, but copper wire for cables should be handy and work very well.

I know it's not what you were looking for, but this debugging or trouble shooting is a great way to learn - it forces the user to understand each component rather than simply put pieces together like a lego kit (which are also fun, but different). I suggest you involve your son in finding the problem/issue and you'll learn a lot about how he problem solves and you might learn a new way to approach problems yourself ... either of which would be more meaningful and useful in the long run than the know-how to build an AM radio. Though I understand that's the goal and hope you get the answers you need from mefi.

Best luck!
posted by unclezeb at 9:19 AM on March 20

Could be a bad component, could be a lot of things. Id call the manufacturer and ask what they suggest. They might just RMA the whole kit and save you several hours of playing with a bad resistor.


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posted by damn dirty ape at 9:25 AM on March 20

jangie - thanks for the suggestion. I pulled the multimeter out just now and went through the circuit. Continuity looks good. I probably should have done that after the first go-round - I might have caught whatever was wrong at first.

unclezeb - a better antenna will be the next step. We live near a few local transmitters, so I'd expect to hear something, but you never know. Agreed on the troubleshooting together. My electronics knowlege is sparse at best, so short of things that ought to be obvious (like continuity testing), I'm not sure what to look at next.

damn dirty ape - yeah. I was on the mfr. website last night looking for tips. One of the other dads bought the kits for all the boys. I'll be curious to see how everyone else fared at our next meeting.
posted by jquinby at 9:30 AM on March 20

I'll second the recommendation to get a btter antenna. For tiny little circuits like this a good antenna will net you more gain than almost anything else.
posted by GuyZero at 9:56 AM on March 20

You don't say how it's assembled, but if you are going exclusively from the schematic I can imagine a simple mistake, which is to connect the "crossed" wires between circles 6,9 and circles 7,8. These should not be connected because there is no dot on the intersection there. Connecting them will cause no signal in transistor 9/10/11.

Perhaps you can debug the circuit a bit by using a divide-and-conquer approach. You can use a signal detector (ear) and signal injection tool (wide-band antenna made of meat: you and your finger). Unhook the choke between 10,13 and touch circle 13. You should hear a buzz/hum in the earphone. If not, the problem is in the last amplifier stage around transistor 14/15/16.

If that works, put the choke back, and unhook the resistor between 5,6 and touch circle 6. Listen for the buzz/hum again (probably louder this time). If none, the problem is in the detector (?) stage around transistor 9/10/11. If this works, maybe your tuner module is bad.

Are you hooking circle 11 to a real ground in your home, like a cold water pipe? You may need to do that for it to receive much.
posted by fritley at 11:34 AM on March 20 [1 favorite]