TDA7000 receiver project

[audioguru]

Hi Zach,
My General Electric cheap clock radio is pretty good. It uses a National Semi "almost an entire radio in an IC" that is a real super-heterodyne that had its own power amp. I added a new power amp, replaced the speaker and added EQ to make it sound fabulous. I have it with tonnes of bass boost at low volume and the bass level stays the same as I increase the volume. At max volume the bass response doesn't have any boost so the amp never clips. :lol:

I had a very hard time finding the datasheet for National's IC because they sell it only in quantities of 10,000.

 

[zachtheterrible]

Hi audio, I just completed your mod4 tx and the sound quality is very good :lol:

Weird thing though: I accidently had the 9v connected to the 5v region of the circuit and it was working great. Then I realized my error and connected it correctly and all I can get now is a little static :cry: . Maybe I damaged the 7805 when I did that.

Can't wait to test range.

 

[audioguru]

Hi Zach,
You had 9V on the output of the regulator which fried it. :cry:
A 7805 will regulate well until its input voltage falls to about 7.5V or 8V. My brand new 9V alkaline battery dropped to 8V in about 20 minutes.
I always use an LM2931-5.0 low-dropout regulator in my 9V battery powered circuits because it still regulates well when its input is only 5.3V. :lol:
The range of my FM transmitter is so far that you'll need a map and ruler to measure it. I used www.earth.google.com satellite pics which is even better than www.maps.google.com . My son spotted my new car in the driveway of my daughter's house! But it showed my old car that I traded-in in my own driveway.

 

[zachtheterrible]

Hey audio, I finally got it working, I think. Not sure if I'm getting harmonics over my radio though. Take a look at the inductors that I have, are they too small or too big?? My capacitors are something like 5-60pf. Does that sound about right?

BTW, the sound quality is GRRRREAT as tony the tiger would say. I wish I had a portable radio to test range. all ive got is the stupid alarm clock radio that plugs into the wall. I'm not too worried 'bout batteries. I'm just gonna get a 9v wallwart for it.

How much current does yours consume? Mine is only consuming about 40mA, which leads me to believe that the amplifier might not be working and that I'm just getting a signal from the oscillator part of the circuit. That reading is coming from my PSU, and I don't put too much confidence in what that meter says :lol: . I guess I'll just have to get a radio and test range to be sure.

 

[Nigel Goodwin]

Hey audio, I finally got it working, I think. Not sure if I'm getting harmonics over my radio though. Take a look at the inductors that I have, are they too small or too big?? My capacitors are something like 5-60pf. Does that sound about right?

No disrespect, but have you compared your board to Audioguru's?. His looks really smart, in comparison your's looks like a heap of xxxx :lol:

In particular compare the coils, do your's look anything like his?, did you follow his coil winding instructions (obviously not).

 

[audioguru]

Hi Zach,
My FM transmitter draws 34mA from a brand new 9.39V alkaline battery.
Yours must be working because its 40mA is a good sign. I think it would draw much more current if it wasn't oscillating. Your whopping big regulator has an operating current a few mA more than mine.

Your layout is nice and tight which reduces stray capacitance, but your coils are small with few turns.
Your radio is probably overloaded by your transmitter so get some distance between them for tuning. My FM Walkman radio has a local-distant switch which is handy to reduce its overloading.

Here is a comparison pic of mine vs one made by a guy who complained that his didn't work:

 

[zachtheterrible]

Nigel, the main reason that perfboard exists is to experiment with, not to make a final product. I always like to make an experiment board first, whether its on breadboard or perfboard before I go and make a PCB that does not work (I hate troubleshooting PCB circuits). Perfboard circuits are not meant to be pretty. When I make a PCB, I'll take a picture and you'll have to eat your words :lol:

As for my coils, mine are quite a bit smaller, never saw the directions on how to make them. The strange thing is, my radio is not overloading. It is probably about 50 feet away, and the signal has to go through the whole house to reach the radio, yet the signal is clear as a bell and very strong too. I would think if I made coils that big, the carrier frequency would be way below the FM band and all I would get is a load of harmonics up in the FM band. So what exactly are the specifications for making those coils?

 

[Nigel Goodwin]

Nigel, the main reason that perfboard exists is to experiment with, not to make a final product. I always like to make an experiment board first, whether its on breadboard or perfboard before I go and make a PCB that does not work (I hate troubleshooting PCB circuits). Perfboard circuits are not meant to be pretty. When I make a PCB, I'll take a picture and you'll have to eat your words :lol:

Audioguru has posted his layout a great many times now (including one more just above :lol: ), why not simply copy that? - there seems no reason to make a PCB when you've already got a picture of an excellent stripboard layout? - particularly when layout is quite critical at VHF. As Audioguru's layout works well (according to him, and various others who have built it), changing the layout can only potentially make it worse!.

Perhaps Audioguru would like to post the complete project in the "Electronics Projects" forum?, it would make a nice change to see a post there I don't delete :lol:

 

[audioguru]

Hi Nigel,
Sure you'll delete my project if I posted it with phrases like, "Make the f***ing coils this way".
If I ever find a way to transmit a pic (using IR?) of my Veroboard layout from my cell phone to my pc then I'll post it. I think my son's laptop pc has an IR port, but then I would just use his digital camera.

Here is a sketch of my FM coil that I stole from Rod Elliot's project:

 

[Nigel Goodwin]

Hi Nigel,
Sure you'll delete my project if I posted it with phrases like, "Make the ******* coils this way".

If it was a good project I'd probably just edit it 8)

 

[zachtheterrible]

I don't really want to use stripboard though, I would much rather have it on PCB. Thanks for those dimensions by the way :lol:

 

[Nigel Goodwin]

a PCB layout?? Ive never seen it. Yeah, I'd rather use that than mine, save me some work :lol:

No, it's a stripboard layout - but more to the point it's a GOOD WORKING stripboard layout - you making a PCB 'may' equal it's performance, but (presumably?) aren't likely to better it, and it may well be worse?.

Why not just use a tried and tested stripboard layout?, rather than spend time and effort making a PCB that won't improve anything?.

You 'could' simply etch a PCB of the stripboard layout, but again it's work for no purpose!.

 

[zachtheterrible]

Ive made plenty of PCBs for these bugs and transmitters and all of them work fine. I can model it after the stripboard (place the components in the same place) and I'm sure it'll work just fine. Plus it'll look very good.

 

[zachtheterrible]

I made those new inductors and was able to go almost to the other end of my entire neighborhood (probably about 400 meters at least) and that is on the other side of about 5 or 6 houses and got a very strong and clear signal until a more powerful rap station came on :evil: . I'm very excited about this :lol:

I'm having one problem though: I get the 50hz hum over the radio. I put a 100uf cap across the amplifier part of the circuit and still get the hum. I also tried taking out the audio cable from my CD player to the TX, yet it still persists. There is a 220uf cap and a .1uf cap across the +9v part of the circuit as well. Can't quite figure out what's going on. Any ideas?

 

[Nigel Goodwin]

I'm having one problem though: I get the 50hz hum over the radio. I put a 100uf cap across the amplifier part of the circuit and still get the hum. I also tried taking out the audio cable from my CD player to the TX, yet it still persists. There is a 220uf cap and a .1uf cap across the +9v part of the circuit as well. Can't quite figure out what's going on. Any ideas?

What are you powering it from?, a cheap crappy wall wart supply? - these are generally worse than useless!.

 

[zachtheterrible]

No. This: **broken link removed**

 

[Nigel Goodwin]

Try it with a 9V battery!.

 

[zachtheterrible]

Hi Nigel, I did just that and the hum disappeared for the most part. Now the question is, how do I get this to work with a wallwart supply? as this is the way that I am aiming to power the transmitter.

I have a 100uf cap across the audio section and a 220uf cap across the amplifier section. I also tried two 220uh inductors in series with gnd and +9v to no avail. Any suggestions?

 

[audioguru]

Hi Zach,
I think your transmitter is being modulated by the power supply's very high frequency harmonics of its rectifiers' switching.
Doesn't the transmitter have about 1000pF of a ceramic disc cap connected across the supply input? Together with the series chokes you added they should stop the interference.
A 9V/300mA wall-wart will have an output voltage of about 12V with the circuit's 40mA which might destroy the RF amplifier transistor. Try a 9V/500mA wall-wart which will have an output voltage of about 10.5V.

 

[zachtheterrible]

Thanks for the info on the wallwart audio, I probably would have ruined my tx since I would've gone with the smallest possible size (I only need 40mA :lol: )

I just realized that the buzzing isn't all that bad. It was so pronounced because I had my CD player volume turned down so low and the radio turned waay up loud. I was fearing that i might get distortion but I turned up the CD Player and I don't, so the buzzing is in actuality very small. Gonna order that wallwart right now, don't have one laying around :cry: