Cobra 138XLR mod limiter

[Deleted]

Someone gave me this radio about a year ago, and I am finally going to take a look at it. Supposed to be working and great audio.

Of course, the first thing I do is look for a clipped mod limiter and reattach it (I never clip). This radio has that compression circuit that makes it sound great. So I want it stock. If anyone know which this is, please let me know. I am thinking of rigging a stock mic with an electret condenser element.

[Sandbagger]

Feb 3, 2018 18:06:39 GMT -5 @vinnies said:

Someone gave me this radio about a year ago, and I am finally going to take a look at it. Supposed to be working and great audio.

Of course, the first thing I do is look for a clipped mod limiter and reattach it (I never clip). This radio has that compression circuit that makes it sound great. So I want it stock. If anyone know which this is, please let me know. I am thinking of rigging a stock mic with an electret condenser element.

Not sure I understand the question. Are you asking where the limiters are? Yes that radio has the mic AGC "compression" circuit. I usually leave this alone as it does a great job at limiting, and without the distortion and bandwidth limiting that occurs from the diode clipper circuit (and low pass filter) that follows it. I usually jump out the diode clipper and low pass filter circuit, and adjust the mic gain to about 12:00. That gives me 100% modulation and the waveform is much cleaner.

[Deleted]

I wanted to know which component is was to make sure it is intact.

[Sandbagger]

Feb 4, 2018 10:27:52 GMT -5 @vinnies said:

I wanted to know which component is was to make sure it is intact.

Well, there are several "components" in the modulation control circuitry. The only way to really know if it's working is to test it with a modulation meter or preferably a scope to see if it overmodulates. You can adjust VR7 to see if the clipper circuit is working. TR18 and 19 make up the active components of the mic AGC circuit. You can test if that circuit is working by setting VR7 wide open (defeats the clipper), backing down the mic gain to around 11 O'clock and then varying the audio input signal to see that it is not allowed to overmodulate.

There are many ways to defeat these circuits. You could remove TR18 and the mic AGC is disabled. But you could also remove C64 to achieve the same result. The clipper circuit can be easily defeated by removing TR23 or clipping D23, or by jumping out D21 and D22.

[Deleted]

I'll post a pic on the scope.

[2600]

Most every CB made since the early 80s has a NPN transistor wired in parallel with the mike audio. A control current into this transistor will control (decrease) the mike-audio level. The more current feeding into the base terminal of this transistor, the more it cuts down the mike audio. Typically a separate trimmer pot will control this transistor for limiting AM modulation level and sideband power.

But this radio is different. It controls the mike-audio level two different ways. First, in a chip that combines the mike preamp with a compressor circuit. The uPC592 chip has no adjustments, but a wattmeter worshipper might clip on leg of D20, or TR18 to disable this "compression" circuit.

Bad idea. Sounds great the way it is. Sounds like dirt if clipped.

The second way in this radio is for AM limiting only, and uses two diodes D21 and D22 to hold back the AM audio. You can't just "clip" them because your audio passes through them. A wire jumper around them is one way to defeat it.

There is a trimmer pot that sets the AM modulation limit. It's VR7.

And if turning VR7 does nothing, someone has disabled it already.

73