Science Buddies: "Ask an Expert"

I've built the AM radio project - but no sound comes out of the radio. The only part that I could not *exactly* spec was the transformer, I ended up with an 8 - 1200 ohm as opposed to 1000 ohm. Would this be the reason for the radio not broadcasting? I'm also questioning my source, I tried just connecting the three pins of the crystal to the source and antenna (found in another experiment) and I got no audio from that either.

I ended up with an 8 - 1200 ohm as opposed to 1000 ohm. Would this be the reason for the radio not broadcasting?

Probably not. If the audio source is an 8 ohm speaker/headphone jack and you are connecting it to the 8 ohm winding, then either a 1000 ohm or 1200 ohm winding should work to amplitude modulate the crystal oscillator. If you mix up the windings or wires so that you are using one of each winding, then it won't work.

With the audio source disconnected, can you tune your AM radio slowly through the oscillator frequency and hear a squeal? If not, is there a real radio station at that frequency? What city/state/province are you located in and what frequency oscillator do you have? With this information, we can look up the location of commercial stations broadcasting at that frequency or an adjacent frequency and determine if you need to use a different frequency oscillator for your experiment.

If you can find the squeal, as you tune through the oscillator frequency, the pitch of the squeal should decrease and then increase as you go past the carrier frequency. You want to tune your AM radio in this center null. You should then be able to plug in the audio source and increase the speaker/headphone volume until you hear the audio on your pre-tuned AM radio.

Probably not. If the audio source is an 8 ohm speaker/headphone jack and you are connecting it to the 8 ohm winding, then either a 1000 ohm or 1200 ohm winding should work to amplitude modulate the crystal oscillator. If you mix up the windings or wires so that you are using one of each winding, then it won't work.

According to the project the 8 ohm side has two connectors, and the 1200 ohm side has three. I hooked it to the 8 ohm side.

With the audio source disconnected, can you tune your AM radio slowly through the oscillator frequency and hear a squeal? If not, is there a real radio station at that frequency? What city/state/province are you located in and what frequency oscillator do you have? With this information, we can look up the location of commercial stations broadcasting at that frequency or an adjacent frequency and determine if you need to use a different frequency oscillator for your experiment.

I tried two different radios, one with a digital tuner I was able to set up to 1000 kHz exactly. I am in Halifax, NS, and there appear to be no radio stations past 920. The second crystal produced the same results. Just white noise, with no difference in quality.

If you can find the squeal, as you tune through the oscillator frequency, the pitch of the squeal should decrease and then increase as you go past the carrier frequency. You want to tune your AM radio in this center null. You should then be able to plug in the audio source and increase the speaker/headphone volume until you hear the audio on your pre-tuned AM radio.

I'll try a stereo headphone jack connected to the 8 ohm side and see if I get anything that way today, something's not quite right. As I mentioned, I saw another experiment where they just hooked the crystal oscillator up to an antenna and the source to produce an even lower quality signal, and that failed to work as well - leading me to believe it's the source connection that's wrong. (two wire source jack)

According to the project the 8 ohm side has two connectors, and the 1200 ohm side has three. I hooked it to the 8 ohm side.

The one shown in the project has a single 8 ohm winding and a center tapped 1000 ohm winding. The center tap lead is left unused. If your transformer has an 8 ohm winding and a 1200 ohm center tapped winding, you still need to identify and use the correct leads to connect to the same locations in the circuit.

I tried two different radios, one with a digital tuner I was able to set up to 1000 kHz exactly.

With this information, I'll assume that you are using a 1 MHz oscillator, otherwise, your radio and oscillator are at different frequencies so it isn't going to work. A digital AM tuner isn't very helpful in diagnosing a problem as it will only tune to the exact carrier frequency so you can't determine if you are producing that carrier frequency by slowly tuning through that frequency. Assuming your 1000 KHz oscillator is working, with an analog tuner tuned to 997 kHz, you would get a 3 kHz squeal (difference between the carrier and the local oscillator in the radio). This squeal would reduce to 100 Hz as you slowly tune to 999.9 kHz and then disappear and then reappear as it is tuned to 1000.1 kHz and increase in pitch to 3 kHz as it is tuned to 1003 kHz.

I am in Halifax, NS, and there appear to be no radio stations past 920.

That is surprising. A quick check of stations in Nova Scotia between 900 and 1350 kHz shows: CJCH 920 Halifax, CHER 950 Sydney, CHNS 960 Halifax, CKBW 1000 Bridgewater, CBI 1140 Sydney, CKEC 1320 New Glasgow, CJLS 1340 Yarmouth, CKAD 1350 Middleton.

If your radio isn't picking up a 10 KW CHNS 960 in Halifax during the day, your AM receiver has some problems. At night, CHNS is directional and if you are south west of its towers, then it is understandable that you won't pick it up.

The 1.2288 MHz oscillator might be a better choice for your area. CKBW 1000 Bridgewater is a 10 KW station about 60 miles away. During the night, it uses a directional pattern to send its signal toward the east toward Halifax.
Note: 1.2288 MHz can't be properly tuned by a commercial digital tuner as they only tune to every 10 kHz.