I need schematics for cobra 148 gtl - Cobra 148 GTL 120-Channels Base CB Radio

There appears to be help here and that forum is the best place to ask. It is a Post Factory modification according to this.

WTB cobra 148 gtl side mic

WorldwideDX Radio Forum https://www.worldwidedx.com > threads > wtb-cobra-...
Feb 27, 2019 - 148 GTL side-mic uses 5 pin mic receptacle. If the radio in question is a side mic and has a 4-pin receptacle, then it was put in post-factory ...

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Cobra technical support here: Support Cobra Electronics Corporation

This sounds like a problem with the MB3756 Regulator IC. It controls voltage regulation and TX/RX Switching. It is a common failure in these radios. Check voltages with schematic that can be found at
CB Tricks Home Page

http://www.cbtricks.com/radios/cobra/148gtl_dx/index.htm

Nice choice of radio i have the Grant LX Uniden which is roughly the older version.
This site may be of interest also if you choose the channel mod make sure you have the right PLL. http://www.radioaficion.com/HamNews/mods/mods-cb/2428-uniden-grant-xl-mods-pll-agc-power-clarifier.html

I believe you will find everything you need here http://www.cbtricks.com/radios/cobra/148gtl_dx/index.htm
If it helps please rate my service as, it appears, i have no rating.Thanks.

• Locate the MB8719 chip that is right behind the channel selector. Wire a SPST switch from pin 10 to ground and another from pin 11 to pin 12.
  

• Schematic http://minus.com/my8tfHQMD#1

• Channel Chart
  

• Position 1: sw1 down, sw2 down
• NORMAL CHANNELS
  

• Position 2: sw1 up, sw2 down
  
• 15=26.815 20=26.885 25=26.925 30=26.985(ch3)
  
• 16=26.835 21=26.895 26=26.945 31=26.995(ch3A)
  
• 17=26.845 22=26.905 27=26.955 32=27.005(ch4)
  
• 18=26.855 23=26.935 28=26.965(ch1)
  
• 19=26.865 24=26.915 29=26.975(ch2)
  

• Position 3: sw1 up, sw2 up
  

• 1=27.605 6=27.505 10=27.555 14=27.605
  
• 2=27.455 7=27.515 11=27.565 16=27.475
  
• 3=27.465 8=27.535 12=27.585 20=27.525
  
• 4=27.485 9=27.545 13=27.595 23=27.575
  
• 5=27.495
  

• Position 4: sw1 down, sw2 up
  

• 1=27.605 11=27.725 21=27.855 31=27.955
  
• 2=27.615 12=27.745 22=27.865 32=27.965
  
• 3=27.625 13=27.755 23=27.895 33=27.975
  
• 4=27.645 14=27.765 24=27.875 34=27.985
  

you would probably want at least a 4watt continuous power supply or better. do not use as switching power supply.

Most units do NOT allow modulation adjustment as any more than 90-100% modulation causes adjacent channel signal "Splatter" therefore is subject to restrictions, hence generally that is locked. Also there really is no reason to have more than 100% modulation..as well ... why? I would suggest a voice/speech "Compressor" would be more suited.

if the transmitter produces power output with modulation on SSB but NOT ON AM then the AM Pass Regulator is defective, more times than not.

This is the Manual for the C148GTL