Cobra 21 LTD Classic Mic Conversion

From one of my eBay escapades, I picked up an older Cobra 21 LTD Classic. It was a really clean radio and I really like it actually! A step up from the Cobra 19 but with Cobra's Dynamike control. I received it without a microphone, however it didn't matter what microphone I tried. I would always get around 20 to 30% peak modulation. What's going on?

Understanding CB microphones

CB mics come in two different flavors - dynamic and speaker type. Dynamic microphones are usually higher in impedance, around 1000ish ohms, and contain an active voltage-driven electret element. This element needs voltage to drive it - fluctuations in voltage produce changes in sound.

In a speaker type mic, fluctuations in air movements produce sound. These mics usually have anywhere from 400 to 600 ohms of impedance. These are your standard 4-pin coffin microphones that you can get from truck stops for less than $20.

From the factory, the Cobra 21 came with a dynamic microphone and the design was similar to the stock mic that came with the Cobra 19XS (also from another eBay haul). Since this 21 came without a microphone, I learned the hard way that speaker mics aren't compatible with radios that use dynamic mics. Using these mics will result in subpar audio and severely attenuated modulation.

So how can we fix this?

Dr. Owen to the rescue

It's actually rather easy to do. I found this mod on an old Angelfire webpage, and it involves removing and adding components. Here is a basic schematic of the Cobra's microphone circuit:

We need to make the following changes:

• Remove D9

• Bridge R50 (sends the other side of the Dynamike pot directly to ground. As of right now, it has a range of ~1 ohm to 11K of resistance.)

• Change VR6 (Dynamike pot) to 5k

• Change R130 to 50k (47k works fine also, might be a hair hotter on the audio. Adjust modulation accordingly)

After this, realign the radio so that when the Dynamike pot is fully clockwise, the modulation should be 100% peak using your standard 1KHz tone at 30% modulation, 1000uV. In this way, you actually have a fully functional Dynamike setting.

Congratulations, you can now use a standard truck stop mic with this radio! This works with all radios but components and swaps will vary greatly based on the radio's model. Also, if you use a power mic, you'll have an actual adjustable Dynamike control too! Now in order to do this to other radios that use dynamic mics, you'll need to remove the voltage bias on the audio line and increase the audio heading to the modulator. That's the most important part.

It's very unlikely that you'll need to do this mod to modern-day radios such as the Cobra 25 or 29. But if you ever find yourself getting poor modulation even after you've adjusted the AMC circuit on an older 80s or 90s radio, consider doing further diagnostics and seeing if this is a possibility.

That's all for now!