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
Here is a basic look at them:
http://www.freeimagehosting.net/8kox7
One thing to check is the phase of the audio waveforms applied to the stereo generator -- they should be wired to be in phase with each other. If they are opposite, then the output of the L-R subchannel modulator can be zero instead of the normal value, and the output signal will be a form of mono, with pilot.
This could happen if a pair of wires was attached to oppposite sides of the audio path in one channel compared to the other. You could try reversing the termination points of the audio cable wires in/at either the L or the R connector plugged into the "bad" transmitter, to see what happens.
.. of the pilot tone. It could be off enough that the rcvr(s) don't decode it properly. Might be a series capacitor in-line with one of the leads of the pilot crystal. If you know what it is, try replacing one. Seems you ruled out the crystal..
Is there a peaking coil for the reference osc.? Try tweeking it a little. Might be a harmonic creepin' through, riding along with the pilot..
Can you vary the supply voltage and see what happens?
Just a thought..
Havent checked eitehr suggestin yet but ill try both... its a real mystry because there really isnt anyting I can find wrong, but somting is because it isnt working right. Like I said Ihave three other exact transmtters and they all work normally, and I did try a new chip and new crystal ! it has me stumped - I dont think ill return to china, I can buy new for few dollors more than shipping but wuld like to salvage the duds - wish it wasnt surface mount caps and resistors, so hard to work with on a deal like this.- Anyone else that has suggestins please let me know, ill post any findings I have..
THank you so much
Radio Joe
The received signal on the FM radio, does it sound like a mono mix? Are you hearing both left and right mixed rather than sounding like the center channel is gone and only the difference between them is heard?
It would seem as you said, if the stereo indicator is lit, the pilot must be decoding.
If you are hearing the mono mix of both left and right from both speakers, it could be the audio inputs to the transmitter are tied together somewhere.
What happens if you disconnect first the left input and then the right input one at a time? Does the received audio do the same thing; first the left channel is silent, then the right channel?
If not, I would think the inputs are shorted together somewhere. If yes, then I would think the stereo encoder is faulty.
Do you have access to a spectrum analyzer such that you could see the 38 kHz stereo subcarrier? Perhaps its not present and only the L+R mono is being received.
Try putting two different tones at the inputs so you can distinguish which channel is being sent where. It may shed some light on what's happening.
I checked the audio circuits right to the audio input pins on the BH chip, the audio is not mixed from either input prior to going into the chip, in other words the audio is not tied together some place prior to going into the chip.. I dont have a spectrum analyser wish I did ! Putting in a seperate tone into either channel alone still makes the one signal from that channel come out equally from both channels on the receiver.. and this happens no matter which input left or right on the transmitter. Something is happening to the stereo encoding not sure what... I lost my check book some place so now that takes priorty but soon as that problem is solved then ill look into the stereo problem more.
Seems strange that three of these exact transmitters have the same problem so my guess at this point some external compontet isnt the correct value or somee production problem in the board... some times life is too short if you understand what im saying, I orderd two more transmitters so ill see what those do when I get them.. If I end up with five duds then ill be depressed. I gotta find the check book fist.....
Radio Joe
Connect to a different antenna. Ya never know.. 😉
MRAM mentioned checking the stereo subcarrier, and I want to explore that idea further.
The FM subcarrier, centered on 38kHz, is the carrier for the L-minus-R signal, which gets decoded at the receiver where, mixed with the L+R Mono signal, gives a proper left and right set of channels.
The 19kHz signal tells the receiver to turn on the stereo light, but this "pilot tone" is not a stereo carrier.
Here is a useful description of how it all works
http://en.wikipedia.org/wiki/FM_stereo#Stereo_FM
The pilot tone serves the function in the receiver of restoring the carrier to the double sideband suppressed carrier L-R signal so it can be demodulated. The 19 kHz is doubled to 38 kHz and the phase is important as well as accurate frequency.
Joe, this is a wild speculation but it is well known that some Chinese components are "knock offs" which have been reverse engineered and are of substandard quality. Is the chip you substituted a genuine Rohm part? Maybe a bad run of chips was used by the manufacturer.
Neil
The 19kHz signal tells the receiver to turn on the stereo light, but this "pilot tone" is not a stereo carrier.
True. But to elaborate ...
The real purpose of the 19 kHz pilot signal transmitted with an analog stereo FM signal is to provide the frequency and phase reference needed by a receiver to accurately demodulate the L-R, DSBSC* subchannel centered at 38 kHz in the baseband spectrum -- which for improved signal/noise ratio at the receiver, does not include its 38 kHz subcarrier frequency.
In the receiver, the 19 kHz pilot is doubled in frequency to 38 kHz, and used by the detector to demodulate the L-R subchannel. The output waveform of that L-R demodulator is matrixed with the L+R (mono) waveform that was transmitted on the main channel, to produce the discrete L&R audio of the original program source.
Some FM receivers have used the presence of the 19 kHz pilot also to turn on a front panel stereo light or text display, but that is not the reason that the 19 kHz pilot is present in the transmitted waveform.
* Double Sideband Suppressed Carrier
Thank you gentlemen for the chalk board classroom on how it works.
It's even more intricate than my simple view ever imagined.
I hope I at least get a D in the course.
By way of experience allow me to share a related story...
Back awhile I was involved sending programming to a monophonic FM public station, and one night we mixed a 19kHz tone into the broadcast at 10% modulation.
The stereo light on our FM receiver lit up.
Is that exciting, or did we just think it was.
Back awhile I was involved sending programming to a monophonic FM public station, and one night we mixed a 19kHz tone into the broadcast at 10% modulation.
The stereo light on our FM receiver lit up. Is that exciting, or did we just think it was.
The stereo light on that FM receiver might have lit, but that 19 kHz signal should not have been transmitted by that monophonic FM station.
The FCC does not permit a monophonic-only, analog FM broadcast station to transmit a baseband signal at/near 19 kHz.
The reason for this is to prevent an analog FM stereo receiver from enabling its L-R subchannel demodulator under those conditions -- which would degrade the signal-to-noise ratio at the output of the receiver.
Where were you when we needed you, Rich?
We had piles of FCC manuals and the station manager was wide awake and tried to keep abreast, but we never knew about the ban on fake 19kHz pilots.
Have I forfeited my career retrospectively?
OK. So from everything you've done and said, I'd go with a problem with the stereo subcarrier. It's either not present or does not contain the L-R info.
Well, Good News/bad news the good news is I found my check book, but on the other hand still dont have stereo, By the way, the chip I used was a ROHM branded chip so hope that one was good. Question, On pin 9,does anyone know what I should see on an oscilliscope with audio input?
I am not at the workbench today but I may try to scope a good transmitter on pin 9 and then scope a dud to see if there is any difference in the signal, Pin 9 is suppose to have the composite audio signal which is then sent to the oscilliator - The circuit which is in the transmitter is pretty much the standard circuit for the BH 1415 - I am thinking something is shorting out the stereo (signal) or somthing is not letting them get to the oscilliator or the oscillator isnt modulating them.
I may put a good transmitter along side a dud and take the composite out from the good transmitter and inject it into the oscilliator of the dud after disconnecting the composite out from the oscilliator on the dud transmitter to see if I get a stereo signal using the dud oscilliator with a stereo generator from the good transmitter- here I thought being retired was gonig to be simple !
Thanks for the good suggestions so far, I havent been at the work bench last couple of days or I would be tearing into the problem, sometimes its best to think on it for a while anyway.
The answer has to be pretty simple I would think.. As far as the replacement chip, I have a few more I suppose I could try another one, but this reminds me of the time I had a 89 dodge caravan, the alternator wont charge,I replaced the alternator, but it still didnt charge, so figured it was the voltage regulator, problem is the voltage regulator on this model caravan is in the ECU. Ok so went to the junk yard picked up different ECU, no luck -wont charge, figured the junk yard ECU was bad, ok went though 3 ECUs finally said it has to be in the wiring as three ECUs and still no charge ?( alreday replaced the alternator, well I finally gave up and took it to a shop I trusted guy said its the ECU.......... turned out all three ECUs were bad ... so its possible it still could be the chip even though I replaced it with a Rhom chip and can tell you I dint burn it up replacing it either.I used proper solder tools etc,,, Ill figure it out probably isnt worth the effort but I dont give up that easy..