Cobra HG M75 Power Mike Mod Questions

[GMC guy]

Hey everyone,

I was looking around and found a Mod for the Mike on CB tricks website. It says that it''s a 2 resistor mod, I can see where he replaced the 1.8kohms resistor, but the other one he says Notice 1Kohms resistor in place of a cap..remove resistor and short pads. So when i go to Radio Shack what exactly am i buying? I know a 200 ohm resistor, but for the one in question.

[Nagant]

Quite a bit of information on the modifications for them in this post.

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[Night Crawler]

Notice 1Kohms resistor in place of a cap..remove resistor and short pads. So when i go to Radio Shack what exactly am i buying?

What that's telling you to do is remove that 1k resistor and replace it with a jumper so you won't need to buy anything for that.

The purpose of that 1k resistor (R6) is to keep RF from getting into the preamp for the microphone it has negligible attenuation so by jumping it there won't be much of an improvement in gain and could cause rf feedback and cause the microphone to squeal.

Try jumping it and see if that happens if there is rf feedback replace the 1k resistor with a RF Choke which will act like a jumper but attenuate the rf.
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The other resistor 1.8k (R7) sets the amount of gain for the transistor the lower the value the more gain that is obtained that's why in the mod it was decressed to 220 ohms you could use a 100 ohm resistor there if you like it's not that critical.

[GMC guy]

Notice 1Kohms resistor in place of a cap..remove resistor and short pads. So when i go to Radio Shack what exactly am i buying?

What that's telling you to do is remove that 1k resistor and replace it with a jumper so you won't need to buy anything for that.

The purpose of that 1k resistor (R6) is to keep RF from getting into the preamp for the microphone it has negligible attenuation so by jumping it there won't be much of an improvement in gain and could cause rf feedback and cause the microphone to squeal.

Try jumping it and see if that happens if there is rf feedback replace the 1k resistor with a RF Choke which will act like a jumper but attenuate the rf.


The other resistor 1.8k (R7) sets the amount of gain for the transistor the lower the value the more gain that is obtained that's why in the mod it was decressed to 220 ohms you could use a 100 ohm resistor there if you like it's not that critical.

Thanks guys,

I went to The Shack today, asked the guy for a 220 Ohm resistor, i need to know if it's a 1/4 watt, 1/2 watt or full watt resistor? As for the other 1Kohm resistor, should i just leave it blank and solder ends up? what Gauge wire is best to use? Or just use another resistor cut down?

[Night Crawler]

I went to The Shack today, asked the guy for a 220 Ohm resistor, i need to know if it's a 1/4 watt, 1/2 watt or full watt resistor? As for the other 1Kohm resistor, should i just leave it blank and solder ends up? what Gauge wire is best to use? Or just use another resistor cut down?

For R7 the 220 ohm resistor use a 1/4 watt for the jumper cut one of the ends off another resistor remove R6 the 1k resistor and replace it with the jumper.

[GMC guy]

I went to The Shack today, asked the guy for a 220 Ohm resistor, i need to know if it's a 1/4 watt, 1/2 watt or full watt resistor? As for the other 1Kohm resistor, should i just leave it blank and solder ends up? what Gauge wire is best to use? Or just use another resistor cut down?

For R7 the 220 ohm resistor use a 1/4 watt for the jumper cut one of the ends off another resistor remove R6 the 1k resistor and replace it with the jumper.

Awesome Night Crawler, i can honestly say to everyone that has answered me since i came here, Thank you!! You guys have all been a great help.

[GMC guy]

Just wanted to give an update. Went and purchased the 1/4 watt 220 Ohm resistor from Radio Shack today ( 5 for .99 ), took the Mike apart and desoldered and resoldered the new resistor and jumper in, and the Mike is Crystal clear. Very easy job with the help of others that did it before me.

Thanks again everyone

[Night Crawler]

For better low frequency response from the microphone C1 a .47uf capacitor can be replaced with a 2.2uf Radio Shack #272-997

[GMC guy]

For better low frequency response from the microphone C1 a .47uf capacitor can be replaced with a 2.2uf Radio Shack #272-997

Does it make that much of a difference Night Crawler? I mean it's a cheap Mike and all, can't expect miracles.

[Night Crawler]

Probably not too much it something to experiment with if you want to.

[Radio 75]

Actually, for the price it is a very nice mic. I ran one on my 142GTL that I had done a similar cap mod on,
and some of the locals said I sounded like an FM broadcast station on SSB.
Love getting reports like that!

[blue goose]

Notice 1Kohms resistor in place of a cap..remove resistor and short pads. So when i go to Radio Shack what exactly am i buying?

What that's telling you to do is remove that 1k resistor and replace it with a jumper so you won't need to buy anything for that.

The purpose of that 1k resistor (R6) is to keep RF from getting into the preamp for the microphone it has negligible attenuation so by jumping it there won't be much of an improvement in gain and could cause rf feedback and cause the microphone to squeal.

Try jumping it and see if that happens if there is rf feedback replace the 1k resistor with a RF Choke which will act like a jumper but attenuate the rf.
[Please login or register to view this link]

The other resistor 1.8k (R7) sets the amount of gain for the transistor the lower the value the more gain that is obtained that's why in the mod it was decressed to 220 ohms you could use a 100 ohm resistor there if you like it's not that critical.

Hi Night Crawler,
I'm having the feedback problem after doing this mod....you suggested the rf choke from Radio Shack. I bought one and it is quite large....to install it would require soldering longer leads from the PC board o the choke's leads to tuck it in somewhere out of the way, probably up in the big hollow area the mic element sits in. I'm wondering if rf might still be a problem getting in perhaps via the extra wire leads. Aren't components recommended to be placed as close to the pc board as possible to eliminate noise and rf? Guess I just need to try it and see , but curious minds wonder. I am happy with the two resistor and 1uf cap mod so far. Just too much feedback. Get excellent reports except for the feedback. Has anyone else here tried this choke instead of just jumpering where the resistor has been removed.? Thanks guys,
Blue Goose

[war wagon]

I did this same mod to my cobra m75 power mic and a friends and the results are fantastic i use this mic with my 148gtl and i haven't had any feedback issues and it sounds great. I don't know why cobra doesn't make them like that from the factory? Enjoy! War Wagon.

[Spook]

New to the site, and all of the info on here is extremely helpful. I've been out of radios for many years, and recently decided to get back in and am setting up a new rig. I want to thank everybody for their help. What happens if you don't jump the resistor at R6? I am asking because I am wanting to do this mod, but I am only wanting to do it once. I don't want to have to worry about feedback issues if I don't have to.

[MDYoungblood]

Hello Spook,
Welcome to the forum. Here is the site this information came from, [Please login or register to view this link] , if you read page 5 it states the reason to remove a jump R6.
As a new member could you post an introduction in the "Welcome to the Forum" topic, [Please login or register to view this link] , so others can greet you to our forum.

3's

Greg