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TDA7000 FM receiver

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mdanh2002

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Hi,

I am building the TDA7000 FM radio using the schematics from this website TDA7000 FM Radio Receiver Circuit

**broken link removed**

I added a LM386 amplifier to power a 8 ohm speaker. I use a trimmer capacitor 2-35pF.

I build it on a perfboard and have tried to keep the connections as short as possible. All capacitors are ceramic. I have most of the capacitor values, except for C1 where I used 0.1uF, C10 - 160pF, and C12 - 56pF.

Upon first power on (and until now), all I can hear is white noise on the speaker. The white noise is stable with MUTE switch ON, e.g. artificially generated by the TDA7000 chip. With the MUTE switch OFF, the white noise remains but seems to vary slightly when I touch various parts of the board. However, I am unable to tune into any FM stations despite tuning the entire range of VC1 and trying various inductor sizes.

Any ideas how I can debug the circuit, other than keeping on checking connections and hope to find something wrongly connected? I have a 100Mhz oscilloscope, though I know it's probably useless at FM frequency. Any expected waveforms/voltages at various pins of the TDA7000?

Is there a way to know which FM frequency the circuit is trying to receive?

Also, regarding the antenna, what is the purpose of the seemingly complicated capacitor/resistor connections at pin 13, 14? The Philips datasheet simply suggests a capacitor to ground at pin 14, and another capacitor to the antenna at pin 13.

The photos of my circuit are attached. Any help is appreciated. Thanks.
 

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FM is 88MHz to 108MHz here. I believe it's similar in Singapore so your 100MHz scope should be able to see something. (Make sure you use a 10:1 probe).

The antenna circuit is to tune the antenna to the RF frequency. What antenna do you have connected?

Did you carefully wind the coil L1 to the specified dimensions? That value affects the tuning range of you circuit.

The IF frequency is 70kHz, so the local oscillator frequency at pins 5 and 6 will be 70kHz below the RF frequency it is trying to receive.

The data sheet and the example on your referenced website both use a PCB with a ground plane. Without a ground plane the performance of the circuit is problematic. 100MHz circuits really need a ground plane for proper operation. A PCB is best but you probably could get by with building it on a perfboard that has a ground plane.
 
Before AG comes in and tells us what an awfull circuit the TDA7000 is/was, just good for toys, not serious receivers.

I build it on a perfboard
In the words of the guy on Mythbusters... "There's your problem!"

Perfboard is not a good idea for VHF circuits, there will be stray capacitance and inductance all over the place.

What would I do?
Rip it apart and re-build it using "Ugly" or "Manhatten" construction. (Google them).

Is there a way to know which FM frequency the circuit is trying to receive?
If you have a frequency counter or calibrated receiver, try to find the oscillator frequency at L1.
Be aware that coupling to the coil will change the frequency.
The oscillator frequency will be 70khz away from the received frequency.

Also, regarding the antenna, what is the purpose of the seemingly complicated capacitor/resistor connections at pin 13, 14?
The idea of the capacitors is to connect the antenna which will have a low impedance, to a low impedance point in the tuned circuit.
If the antenna was connected to the top of the coil, the low impedance antenna would load the tuned circuit and there would be no selectivity. (Not that there was much there to start with).

JimB

PS, AG is probably quite correct!
 
The horrible TDA7000 "radio" is obsolete and has not been made for many years. It was replaced by the TDA7088 that "scans" for tuning and I think it is also obsolete and is not made by Philips anymore but Chinese copies are around.

I can buy a radio and flashlight that uses the TDA7088 radio IC for only $1.00 at The Dollar Store. I got some free for buying a newspaper. Their performance is terrible.
 
So another typical Audio McDuck response:

"What you're using is crap. Your circuit is no good. I will offer absolutely no help whatsoever, nor answer any of your questions."

Yawn.
 
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mdanh2002 said:
I build it on a perfboard
In the words of the guy on Mythbusters... "There's your problem!"

Perfboard is not a good idea for VHF circuits, there will be stray capacitance and inductance all over the place.

Yes, true that.

But--and mind you, I'm just asking here--is there any way in hell that one could make such a project work on perfboard using more careful construction? Like creating something of a ground plane?

I ASS-U-ME by "ugly" construction you mean what's sometimes called "dead bug"? I've seen that used all over the place for RF circuits.
 
I think your perfboard construction is fine for an FM radio circuit that has a real LC tuning circuit at the input (unlike this simple cheap circuit), so that the RF amp transistor is not overloaded by all the strong local stations. But this one's RF amp transistor doesn't even have automatic-gain-control nor input tuning so of course it gets overloaded.
Perfboard is fine for a real super-heterodyne FM radio that uses crystal filters for an excellent 10.7MHz IF, not a lousy 70kHz IF that uses opamp RC filters.

Why are you making an obsolete cheap lousy FM radio?
 
School kids can build whatever they want.
If they make an obsolete circuit then they will not get much help on the internet. Many of us have never seen an old TDA7000 IC.
Many school kids complained that this cheap lousy old circuit doesn't work or that it has horrible performance.

Teachers should teach about a real super heterodyne FM radio that works well, not simply let the kids find out for themselves that this cheap lousy old circuit does not work or works poorly.
 
Hi all,

Thanks for your replies. I am well aware of the limitations of the TDA7000 FM receiver chip. This is just meant to be an experimentation for fun for myself, and not part of any project/assignment or things like that. As long as I can get some audible sounds from a true FM radio station coming from the speaker, I am happy that I have built something that works. The intention was never to build a sophisticated FM radio.

@crutschow: thanks for your answers. I am using a 1m antenna as indicated in the reference website. I also tried various antenna sizes.

I will use my oscilloscope and try to find the oscillator frequency. Is there a way for me to measure the inductor value using an oscilloscope?
 
I add one more turn to the inductor and got the circuit to to play many local FM stations satisfactorily without much change in the circuit. I am using a 1m antenna. The MUTE switch needs to be disabled during tuning and a plastic screwdriver needs to be used, otherwise the frequency will change once the screw driver is removed.

I knew the circuit started to work when I heard a distorted speaking sound from the speaker. I squeezed/stretched the inductor and change the trimmer until the sound becomes clearer. By experimenting eventually I could tune the entire FM band using the trimmer cap.

Without a proper pcb or a perfboard with a ground plane, the circuit is very sensitive. Touching the board or moving my hands close to it could change the frequency significantly.

Hope this helps others who are trying to build the same circuit.
 
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I would say comparatively nice construction..:)

But there's some excessive solder applied I think, that you should take care off in the next build..

Without a volume control may add noticeable distortion at full punch..

Built-in white noise of LM386 will add some horror along with that, result is terrible audio performance from the little speaker..

Better amps like TDA7052, TA7368P, CXA1522, volume control, a bigger full-range speaker would be nice improvements and may add noticeable difference in performance with the same radio-chip.

As a beginner radio maker, I think you've done it well.
 
Why do you care?

Why can't you just answer their questions, or STFU?

Who died and appointed you Chief Arbiter of What Thou Shall and Shall Not Build?
Carbonzit, this is a techinical forum and we don't appreciate personal and abrasive invective. If you don't like the answers that someone gives then give some alternate answers. It serves no useful purpose to attack the poster. We're not in high school anymore (at least most of us).
 
Hi transistor495, thanks for the encouraging words. :) Although I have built several FM transmitters, this is my first FM receiver, and I am happy just to see it work. The excessive soldering is due to the repeated replacements of various components to see if it makes a different (especially near the oscillator section). The speaker is from a junk computer (you can see the connector socket) and you are right that the audio is distorted when the volume is very loud. When my time allows, I will redo the circuit on a proper home made PCB with another amplifier chip (I do have a CXA1522 in my junk box) and see if any improvements can be made.
 
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you are right that the audio is distorted when the volume is very loud.
All amplifiers distort when the output tries to exceed the clipping level.
You show the "radio" with a 5V supply so we assume that your LM386 amplifier IC also has a 5V supply. Its datasheet shows typical distortion of only 0.2% before clipping which is fairly low. But with a supply that is only 5V it clips when its power is only 0.12W into 8 ohms. Your poor quality tiny speaker is not sensitive so it needs more power. It also needs an enclosure.

A Sony CXA1522 stereo amplifier IC has so much distortion that it is difficult to see clipping on the power vs distortion chart on the datasheet. At an output of only 0.1W its typical distortion is 1% into 8 ohms with a 5V supply. At 0.05W the typical distortion is 0.6% (its max distortion is listed at 2.5% with an output of only 0.05W) and at 0.2W the typical distortion is 1.5%. At 0.3W the distortion is typically 10% because it is clipping like mad.
 
I do have some pieces of CXA1619BS( Sony AM/FM with integrated power audio :). This chip is a lot stable in operation than TDA7000 and offers a very nice audio output. With lesser peripheral components you get a nice mono FM receiver. Also it's supply voltage range is 2-8V. This chip is quite difficult to procure nowadays.
 
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