K40 speech processor

[Lil'Yeshua]

I had a chance the other day to buy a NIB/NOS K40 radio but i didn't get it. I was kinda hoping to get my hands on a Grant 2.

 

[bob85]

You made mention of an Astatic cartridge in these mic's. I'm looking for a replacement cartridge for my K40 speech processor mic.

several older astatic mobile mics have the same looking cartridge as what id call mk2 k40's,
i tried one from a none amplified noise cancelling astatic but it still sounded poor, the diaphragm has a gap around the edge unlike old D104M /555 trucker style mics,

you may get away with using a kobetone cartridge from mouser for a few dollars.

 

[Lil'Yeshua]

Thanks. However,RB Micro's website is presently broken because of an update and the host is rewriting it.

Em:"Webhost has upgraded the version of PHP on the server and ended up breaking the webshop. It will be a while before I have time to rewrite the system to accommodate the new version"

 

[Lil'Yeshua]

When I get home I can get the number off the eight pin IC that the K40 mic element input's to and get the specs on that to see if I can modify the circuit in order the run a different mic element.

 

[bob85]

the mk1 used a custom marked ic, mk2 and 3 have conventional markings you should be able to find a datasheet for.

 

[Lil'Yeshua]

I'm looking for the data sheet but so far no cigar
Here's a pic of the K40's IC

 

[Lil'Yeshua]

Okay, after coming up empty on a long extinct IC,I just happened to look at the green colored K40 speech processor mic schematic again from CB Tricks. A sub note next to the IC says LM4250cn.
http://www.ti.com/lit/ds/snosc17a/snosc17a.pdf

Pin 2 is the inverting imput. That's what the mic element and a series 100K resistor goes to.

 

[Lil'Yeshua]

IC

 

[Lil'Yeshua]

Ident pin#1

 

[Lil'Yeshua]

Schematic

All I need to do is figure out how to modify R6 on the pcb board to work a regular dynamic mic element.

 

[Lil'Yeshua]

I hooked back up the Galaxy stock mic element to the K40's mic plug and made sure the voltage was 4 volts. As I held the key down I tapped on the Galaxy mic element and could hear a faint tapping noise over my 959's talkback. My next step would be to use a variable VR pot of 100 k ohms and jumper that unto the 100k ohm resistor and adjust that until I get the right volume response. Upon the success of that I'll just solder another resistor of a predetermined value in parallel and hopefully I've have my K40 mic functioning again.

 

[bob85]

check you have no dc voltage on the cartridge wires without the cartridge,
ceramic cartridges usually output higher voltage than a dynamic cb mic.

 

[Lil'Yeshua]

This looks nice
Samson MTR201 | Sweetwater.com

 

[Lil'Yeshua]

RB Micro website is back up but I had RB Micro send me a PayPal invoice. Just ordered two
Kobitone 25LM022 Hi-Z Crystal Elements.

25LM022 Crystal Element - Mic Elements

 

[bob85]

good luck with the kobetone, not tried one in a k so im interested in how it turns out,

whats up with the original cartridge, is the diaphragm damaged or has it just stopped outputting a signal,
i have resurected k40 and astatic cartridges by giving them a good tap on the back then hooking them to the speaker output of a radio/cd player and playing music through them to loosen them.