The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

I've just got a K40 compression microphone & was thinking of wiring it up to the Cobra 148 GTL-DX

Will the voltage of the switching line (approx. 3.6v on my Cobra) be enough to charge the capacitor inside the mic?

I seem to remember some sort of 'power conversion' kit being supplied with the original mic (diode & resistor?) does anyone know the values of these components?

Cheers

Steve

Remove the TX wire from pin 3 on your radio of the mike socket.
Put a diode IN4001 to the wire just removed with it's unbanded end to the wire and a 1k resistor to the other banded end of the diode.
Place the other end of the 1k resistor to the 13.8v at the on/off switch and the banded end of the diode and resistor back onto pin 3 in place of the wire just removed.

Cheers Dave

I tried it last night without the mod & the charge from the capacitor died after about five seconds!

CQ DX CQ DX CQ DX CQ DX .....................

Here is couple of pics from original K40 user manual which may help some people to visualise the mod:
A common fault on this microphone is bad solders on the main capacitor which can give same symtoms as a radio that has not had the mod done to it.

EDIT: user manual and Schematic now on my next post down...

Hi, the upload limit for a post is set to 5MB, so you may be able to post it in another message, would be helpful for people to see it here.

Cool Scott, I just didn't wanna clog up the forum with big files, but with your ok, here are the files:

Thanks for the additional info Norm

It's working very well but I can hear a very slight buzz (which stops when I touch the mic plug) on another receiver

The other thing I noticed is a trimmer inside the mic (not the gain control which can be adjusted via a hole in the back of the mic)

Anyone shed any light this?

Cheers

Steve

Hi Steve, that other pot is the voltage adjuster. Connect volt meter between ground and the center terminal of that pot and adjust it to obtain 4.0volts DC.

The buzzing is possibly caused by a bad ground connection, Check for bad solders on the board