Modifying CM108 For GMRS nodes

The cm108 is a sound card used to build a node or repeater on gmrs it allows you to connect to a actual radio. You don’t use this if your using a USB radio.
You need to make sure you buy one without a case so you get the chip you see here. The ones in the case will likely have a potted chip that yo can not connect to.

The chip was designed for another use and has unused io pins that can be accessed by USB so its perfect for a node to control a radio.

In the above drawing.
This chip has 4 unused GPIO pins. And unused button inputs VOLUP, VOLDOWN and SELECT.
It has 2 LED outputs LED0 is used and it starts out ON then flashes in operation.
LEDR is a unused mute indicator.

COS is pin48 which is a unused vol down button.
PPT is pin 13 a unused GPIO3.

The above is my schematic for modifying this card. Some use torids on the cables and I do to but there is so much noise on these wires that it needs a coil choke.

The unused audio line could be used to drive a local speaker.. And this card doesn’t have a shield around the USB so it needs a extension cable since it will move around if plugged direct into the PI.

Most people have removed the jacks in the past but you don’t need to remove them. You could actually plug into them and use them. Or just solder on top of them. They are very hard to remove and the pads under will likely break. I have found good luck inserting a small screwdriver into them to pull them off while heating. But now I really see no point in removing them. Just solder on top of them.

The easiest way to connect to COS and PPT is to use enamel wire and run it from the bottom under all the pins till it comes out at the top. Having it under the other pins holds it in place to prevent it moving while soldering and after.

For me I find its easer to strap it to a project board and place all my parts on that board so I can use normal sized parts and not the chip parts seen online.
If you need some of these and cant do it yourself I will do it for you let me know.
Im going to make up some more and look for a final photo soon.

I Use glasses to do all the work not a microscope, These are all over amazon.