Radio Kit Issue

[Muhasaresa]

Hello

I made an FM Radio Kit by Velleman. When I turn it on, the LED light up but there is no sound coming through the speaker. Here is a link with schematics:

http://www.vellemanusa.com/downloads/0/minikits/manuals/manual_mk118.pdf

When I was trying to fix it, I noticed something strange. I used my multimeter to measure the frequency coming out of pin 12 (AF OUT) and it was around 100-300 KHz. Then I tried measuring the frequency after R9 (68K) and it was only 50Hz. How can this be possible? Might this be a reason why there is no sound?

Muhasaresa ---||---
Capacitors Unite!

 

[Harald Kapp]

On AF out you should see the audio siognal (AF = Audio Frequency), not 100 kHz.
After R9 you should read the same frequenyc. Possibly the multimeter is not suitable for measuring these signals. With RF (Radio Frequency) circuits there is always a chance that some RF "spills" into parts of the circuit where it is unwanted.

First check the audio amplifier (IC2). Remove R9 from the connection to pin 12 of the TBA120. Connect an audio source (MP3 player or similar to the open pin of R9. Check the output of IC2. You should hear a signal from the speaker. If not, the audio amplifier doesn't work and you will have to check this section for correctnes (parts, soldering etc.).

If the audio amplifier works, you have an issue with the receiver part. Unfortunately I cannot help you there.

 

[CocaCola]

Always a good idea to post of pictures (clear and in focus) of your actual built circuit so that a second set of eyes can take a look at construction...

 

[Muhasaresa]

Thanks for the advice. I have connected an external speaker to pin 12 and ground and I can hear a quiet but clear sound. Then i connected the external speaker to pin 3 of IC2 and again the sound was extremely quiet. Then I tried connecting it to the output (pin 5) and no sound was produced.

Is the amplifier faulty? I checked my soldering multiple times and everything was ok.

Muhasaresa ---||---

 

[Harald Kapp]

Check the supply voltage of IC2 (pins 4 and 6) is it 8 V as it should be?
Is the LED LD1 on?
Did you mount the regulator VR1 in the correct orientation?

 

[john monks]

You may try connecting a speaker through a 100uF capacitor to IC1 pin 12 and the other end to GND. Pin 12 is meant to be AC coupled that is it goes into a capacitor first.

I am wondering if you really meant that you were getting 100-300kHz with your multimeter. You are sure you didn't mean volts or something else. If this is the case C11 is not connected or R2 is not connected. These parts are to prevent parasitic oscillations. You might check R2 with and ohmmeter to make sure it is about 22 ohms.

 

[CocaCola]

I'm not even going to hazard a guess without pictures...

 

[Raven Luni]

The most common problem you'll run into with kits is that you accidentally either bridge 2 points on the board with a blob of solder that arent supposed to be connected, or you manage to cut or burn the copper traces. I remember the first kit radio I built as a child - I accedentally scored through one of the traces and it took days to find it.

The only thing I can suggest is looking over it very carefully. If you dont see any broken lines or suspicious big blobs of solder, then go through every component 1 at a time and make sure they are in the correct positions. You'll have to be able to read resistor and capacitor values since they wont be in their nicely labeled packets anymore. Here is an info page in case you need it: http://www.mds975.co.uk/Content/components01.html

 

[Muhasaresa]

Solved!

The problem that the LM386 was not working. So I got a new one and now it works perfectly!

Thanks for all your help!

Muhasaresa ---||---