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[SOLVED] Overmodulating CyberMaxFM+ SE V2 STEREO DSP+RDS

 
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[SOLVED] Overmodulating CyberMaxFM+ SE V2 STEREO DSP+RDS
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ragnar.jensen
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Joined: 02 Jun 2019
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Location: Stockholm, Sweden
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Post [SOLVED] Overmodulating CyberMaxFM+ SE V2 STEREO DSP+RDS Reply with quote
New user, first post. So... Hello world!

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.


I would be grateful for any and all help.

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... d'oh!

--
Ragnar


Last edited by ragnar.jensen on Sat Jun 08, 2019 10:38 am; edited 3 times in total
Mon Jun 03, 2019 10:08 pm View user's profile Send private message Visit poster's website
pcs
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Joined: 18 Jan 2002
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Location: Radio Land

Post Reply with quote
Solution is quite simple.

On the MAXPRO8015 board you will find a trimmer labeled Audio. Reduce the level at that trimmer and over modulation will disappear.

I think the original owner probably opened that trimmer way too much.

If audio level cannot be reduced check the soldering on that trimmer, the tab connected to the ground may have a poor contact making adjustment ineffective.

Should be a 5-minute job to fix.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Tue Jun 04, 2019 1:05 am View user's profile Send private message Visit poster's website
ragnar.jensen
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Joined: 02 Jun 2019
Posts: 2
Location: Stockholm, Sweden
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Post MPX out adjustment = success! Reply with quote
Thank you for the quick response!

I found the Audio trimmer...


audio_trimmer by Ragnar Jensen, on Flickr

... but discovered that it was already adjusted to almost zero. I could barely touch it without losing the MPX audio completely.

The key to success was the blue trimmer that is reachable through the small hole in the rear:


mpx_out_trimmer by Ragnar Jensen, on Flickr

This time, I had better lighting and could actually read the text on the board.
The MPX out was cranked way too high, I had to reduce it a lot to get the pilot to 6.8 kHz. A quick readjust of RDS and audio levels and everything is now pretty much perfect.
Even the audio level meters are bouncing around happily.

I must say, now that it is properly adjusted, this transmitter is one sweet sounding machine. Applause

--
Ragnar
Thu Jun 06, 2019 12:59 pm View user's profile Send private message Visit poster's website
pcs
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Joined: 18 Jan 2002
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Post Reply with quote
Yes, last year I added this MPX trimmer also for adjustment of level without having to open the box.

And I forgot to mention that

You have discovered it yourself, congratulations.

Maybe the previous owner opened that trimmer too high.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Fri Jun 07, 2019 12:05 am View user's profile Send private message Visit poster's website
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