Need Help -Thought I knew it all but I was wrong

Radio Joe · 2013-04-27T17:50:21Z

I need help, I thought I knew it all but I found out I didn’t. I have a "few" transmitters that use the BH 1415 Chip. These are "factory" made transmitters -here is an ebay link to the transmitter so you know what you are looking at:I have six of these transmitters, of various power levels, basically the lower power level transmitters are the same circuit board as the higher power models but they don’t have various RF amplifier components installed. I have found these transmitters over all are very good, the sound is excellent and I haven’t had any problem with spurs or harmonics although I do use the 1 mw version. Here is the problem I am having. Three of the transmitters I have work very good in general, but they will not transmit a stereo signal. Three work perfectly. The transmitters do transmit the pilot tone, as the stereo light lights up on my receiver. My input source is true stereo and I have also tried a audio signal generator feeding each channel separately. The audio always ends up with both channels coming out equally on the receiver. This is not to be confused with limited stereo seperation, the audio output is total mono. The stereo mono switch is not in mono position on the receiver it is set up to receive stereo. I have tried the transmitter(s) using different stereo radios and it still will only transmit in mono. I went further in the trouble shooting of the problem and traced all the audio circuits right to the audio input pins on the BH 1415, the audio is not combined anywhere preceding the inputs directly on the chip. Then I removed the BH 1415 and replaced it with a good BH 1415 chip and I got exactly the same results, then I changed the crystal and got the same results again, the audio does not come out in stereo at the receiver. I tried several receivers and its all the same, the stereo light on the receiver comes on, so I know the 19 kHz pilot is being sent but the audio is not stereo, it’s very clear audio, with no distortion or whatever but it just doesn’t come out stereo. I use a good 5/8 wave antenna and LMR 400 cable. The problem is not with any of the audio going in, or my antenna or power supply. I have six of these transmitters, all the same models, three works perfectly and three are duds. I opened all the cases and checked to see if there were any obvious production circuit changes and there were not, all components are the same. in all the transmitters. I checked across capacitors to see if any were shorted by solder or whatever and connections looked good I didit find any shorts. I am not sure about any hidden traces being broken but I ohmed out what I could and could not find anything wrong but obviously there is something wrong. I know the composite signal output with full stereo encoding is suppose to be at pin 5. The problem is I don’t know what to look for at pin 5 to see what kind of signal is suppose to be there. I haven’t tried looking at a wave form at pin 5 on a good transmitter and comparing to a transmitter with the no stereo problem, I just thought of that,, I do have a good oscilloscope but not sure of what I should see on pin 5, but I could compare to see if there isn’t any difference. Pin 5 has a 10uf capacitor - perhaps there is a problem there I’ll have to look further. Anyway, for those who have more experience in this than I do, please let me know what your thoughts are on further trouble shooting this problem. Again here are the symptoms: The transmitter works perfectly in all respects but the audio will not come out in stereo at the stereo receiver, the stereo indicator light comes on, and the signal is not over driven at the source, the stereo light does not flicker, the audio is pristine and not distorted, the audio levels are correct, all is well but there is no stereo separation. I have three transmitters with this same problem, and three more that work perfectly. The transmitters were like this when I got them. This is not a component failure after some use. I hate to spend the money sending back to China! It really doesn’t pay off to do this; I did it once before for something more expensive but in this case I would just like to fix myself or if I have to use the case for somthing else and junk the rest. I believe this is an external (to the chip) component issue as I have tried changing the chip and crystal and got the exact same results as the original problem. It would seem to me that the 19 kHz pilot tone is being transmitted but none of the other parts of the stereo signal are being transmitted. Since I have changed the chip it doesn’t appear to be a bad chip. Any help you can give me on further troubleshooting will be apriciated! Working with surface mount parts make the job hard for an old do-jigger like Radio Joe to do! Thanks guys, Radio Joe

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Anonymous

(@Anonymous)

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Mr. Radio Joe

You are a true Part 15 Man and I respect your sticking to the problem and fighting for a solution.

You've got many minds out here thinking about the problem.

Posted : 29/04/2013 6:19 pm

Anonymous

(@Anonymous)

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Always a Tech.. Just can't let silly stuff like that kick your butt! I totally understand. I've chased my tail for days just to find a bad resistor or cap, or a bad trace. You go, Joe!

Posted : 30/04/2013 2:22 am

Anonymous

(@Anonymous)

Posts: 0

I ordered a few BH 1415 chips from another supplier they claim the chips are True ROHM mfr, I still think the problem is in the chip and it is faulty, all else checks. Ill post after I go through all of that again chaning the chip. Hard to do with my gass torch used to clean off old paint on the house but ill manage...
Radio Joe

Posted : 08/05/2013 8:25 am

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