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CDM VHF CDM1250 transmit audio

The_B_Chief

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Sep 4, 2007
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Side Alpha With The Command
I have a CDM1250 that I use as a base station. I have noticed that the transmit audio is slightly muffled or hollow sounding on 2 meters. Above 2 meters, around 150 mhz, the transmit audio is crystal clear.

The radio is not operating out of the manufacturers recommendations nor has it been modified.

The radio is getting sufficient power and the antenna system appears to be operating just fine. There is also no reflected power and the swr never reaches higher than 1.3.

I feel like this is a turning issue and welcome any suggestions or comments.
 

RFI-EMI-GUY

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Dec 22, 2013
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3,193
modulation compensation?
What band spilt is the radio? In other words, is it operation out of band?
 

mmckenna

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OK, so let's try from this angle.

On the 150MHz and up frequencies, are you running 12.5kHz or 25kHz? Does it sound OK if you try a 25KHz channel? Does it sound OK if you use a 12.5KHz channel?

And you are using 25KHz, not 20KHz, right? (just checking).
 

The_B_Chief

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Sep 4, 2007
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Side Alpha With The Command
OK, so let's try from this angle.

On the 150MHz and up frequencies, are you running 12.5kHz or 25kHz? Does it sound OK if you try a 25KHz channel? Does it sound OK if you use a 12.5KHz channel?

And you are using 25KHz, not 20KHz, right? (just checking).
Right on! So I set everything to wideband, 25khz, when testing into a dummy load and I still had the same issue. That's kind of why I was leaning towards it being a tuning issue. I have the tuning software but have never messed with it. I also have no hardware to assist in finding where I need to be. I would have to enter values and then re-test every time.
 

mmckenna

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I would have to enter values and then re-test every time.
That would be dangerous. Without the right equipment you can easily get things way out of whack and cause issues with adjacent channels. Recording all the settings would help, but some of the settings are interdependant, so you may have to adjust multiple settings at the same time.
A huge task to do without the correct gear.

You'll save your sanity by having a shop do it, or since these radios are really cheap, just buy a new one.
 

KK6ZTE

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Mar 27, 2016
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307
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California
Are you using a CDM specific mic? I've seen CM mics cause weird audio issues when used on a CDM in police use.
 

Tech792

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Mar 25, 2003
Messages
1,598
Location
Central NJ
I had this problem once on only a few channels. In the CPS, Per channel, check to see if you have pre emphasis and de emphasis enabled.
 
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