FM transmitter stereo encoder help

mbates14

Member
2002-06-02 2:14 am
non
Im not sure if this is a proper forum or not, but here goes:

I dont have a schematic, since I built it myself using different concepts from other schematics, but Ill describe it the best I can.


The problem: NO stereo seperation. none. nill. If you listen very hard, there is very very very little seperation. but mainly the channels are mixed together. It was working perfect one day, and the next day, i just have no seperation. makes no since. All parts test good.

The design: Here is the layout. I have a 555 timer tuned to 76khz, with the output feading to a CD4013 IC. This divides it by two and gives me both a normal and inverted 38khz signal which is fed into the gates of a CD4066. then the /Q is fed into the other side which developes the 19khz pilot. Its then fed into a 741 op-amp with a slick-filtering design I found on anohter schematic to filter the pilot. Its then fed into a transistor along with the 38khz modulated signal from the CD4066, both mixed together with another filter. It sounded very clean when it worked. Then one day, I lost seperation. I even went as far as replacing all the ICs and rebuilding the simple transmitter.

I get a strong pilot. the stereo decoding switches in on all my tuners except for one, which it used to at one point and stopped.

If i switch my tuner from mono to stereo to back, I hear no difference except the low level white noise generated from the decoders. No seperation.

The transmitter is simply a colpits oscillator fed into a buffer stage centered around 106.5mhz, delivering about 5W output. But im not certain of this, since my freq counter cant count that high

Any Ideas? the 4066 is switching as it should, and I hooked a relitivly low freq counter up to the osc stages and outputs, they are all within spec. Hmmmm.

Everything is prototyped onto breadboards with wires everywhere. The encoder runs off of 12V and the transmitter 26V. the only thing on circuit is the osc, since they cant be prototyped easily.
 
From your description and the things you tried, it seems unlikely that there is a problem with a part. What I would do in your place would be to recheck and doublecheck again all the interconnect wiring especially to make sure that you are looking/listening at the right points. It being a proto sort of, there is the possibility of an unseen short/open that happened one time after some mech movement.

Jan Didden
 

moamps

diyAudio Member
2002-10-11 8:15 am
Croatia
www.moxtone.com
mbates14 said:
...The problem: NO stereo seperation. ...
... then the /Q is fed into the other side which developes the 19khz pilot. Its then fed into a 741 op-amp with a slick-filtering design I found on anohter schematic to filter the pilot. Its then fed into a transistor along with the 38khz modulated signal from the CD4066, both mixed together with another filter.......

Hi,

It is essential that the pilot (19kHz) is in phase with the subcarier (38kHz) after all filtering and before final mixing. Use the scope to do this. Also, the pilot level should be max. 10% of max. deviation.
Btw, channel separation is also limited with the transmitter modulator's phase characteristic.

Regards,
Milan
 

mbates14

Member
2002-06-02 2:14 am
non
no, this is an oooold 70s dial tuner. IT used to switch on, but not anymore. It will on other strong stations.


I have another dial tuner the stereo comes on, and my sony digital pll tuner, the stereo light comes on.

if I send the filtered pilot output to a 386 op-amp and boost the output signal, the Stereo will switch on, on that tuner, but i get more noise if I amplify the signal.
 

mbates14

Member
2002-06-02 2:14 am
non
I switched the connection from q to /q on the pilot, didnt make a difference. AARRGGH.

one thing I did notice, is when I switched it, the forground voices sound like it went in the background, so I switched it back


The only thing I can think of, is the modulator bandwidth is screwed. because the highs sound like AM. never did before.
 
MBates, you state that WinAmp does your pre-emp. That tells me you are probably feeding your audio to your transmitter from a PC. Have you checked to make sure that the audio cable coming out of the soundcard hasn't wiggled out just a bit? If the tip and ring were in just the right (wrong place) place, it would short the left and right signals together and make mono to your transmitter.
Maybe I'm way off-base here, but check the simple thing too ;)
Scott
 

mbates14

Member
2002-06-02 2:14 am
non
I figured out the problem, but it ended in disaster.

The problem was the 0.1uf cap off the base of the colpitts. It was forming an R-C filter, which was attenuating anything above the audio band, So, i decreased the capacitance, and what do you know, the seperation came back.

But then, the stereo encoder started to screw up, it went psyco, spurs all over the spectrum, so I got mad and ripped it all off the board. (pieces went everywhere), so thats the disaster.

Im thinking about going the MJN2035 route.