Up until a couple of years ago, I was involved in a local community radio station, as the guy that handled all things technical. I don't do any regular work there anymore, but now they have asked me to help with their new transmitter. The magic smoke escaped from the old one, and they have bought another one second-hand from another station.
The problem is that it over-modulates quite a lot. It was actully I that discovered the problem, I noticed one day on my receiver that the station was coming in much louder than it used to do.
I dusted off my FM analyser and discovered pilot tone deviation at 17.6 kHz, RDS at 11 kHz and the audio peaked at almost 300 kHz deviation
I have managed to tame the audio somewhat by installing an audio processor between the PC and the transmitter and lowering the audio level going into the transmitter. I have to set the audio levels very low to keep the deviation inside 75 kHz. The audio level meters on the transmitter just occasionally flickers the first bar when I'm at 75 kHz.
I got RDS working by adjusting the Level and Phase trimmers on the RDS encoder.
The pilot is still at 17.6 kHz. I found a "Pilot Level" trimmer on the stereo encoder board, but it does nothing or only very little when I turn it.
I'm not sure which stereo encoder I see, though. I think it's an SB8000, but maybe someone can confirm that?
cybermax_modules by Ragnar Jensen, on Flickr
stereo_rds_encoders by Ragnar Jensen, on Flickr
As you can hopefully see in the pictures, there's a MAXPRO8015+ and an RDS4000+ MICRO, which sits piggy-backed on what I think is an SB8000.
Now to my questions:
- Are there any instruction manuals for the MAXPRO8015+ and the SB8000? I have searched a lot on here, but haven't been able to find any.
There's a trimmer on the stereo encoder board, that is accessible through a small hole in the back of the chassis. What is that?
Can the MPX audio output level from the stereo encoder and/or the audio input sensitivity on the MAXPRO8015+ be adjusted? I can see a couple of trimmers on the 8015+ circuit board, but I'm unsure about what they do. Seeing as everything - pilot, RDS and audio - was over-modulated, I'm thinking that the MPX audio going into the exciter is too hot. If I can bring the MPX audio level into the the exciter down, that would lower both pilot and RDS and then I would be able to increase the audio to normal levels.
P.S. I have talked with the previous owner of the transmitter. He admits that he has "adjusted" everything he could find, in order to solve a bad stereo reception problem he had. He also admitted that he really doesn't know anything about FM transmitters. He didn't manage to solve his problem and that's why he sold the transmitter...
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Ragnar