Welcome
to part 3 of my Allstar Link node build.
In this installment I will start work on the CM108 USB sound modules and show you how to modify one for use with an Allstar Link node. The module forms a link between the modified Baofeng radio transceiver and the Raspberry PI single board computer.
The CMedia CM108 is a highly popular single chip USB audio solution. All essential modules are embedded into an LQFP 48 pin package. The chip includes a dual DAC, earphone driver, ADC, microphone booster, PLL, voltage regulator and USB 2.0 compliant transceiver.
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| Shown enlarged, actual IC is 7mm by 7mm | |
Acknowledgements
I acknowledge the work of Hamlib and Asterisk Allstar for creating the API & software distributions that support and allow the use of the CM108 device.
WB6NIL & W9SH for designing the CM108 modifications. Jim Dixon WB6NIL is now sadly silent key but his work and vision for Allstar live on.
The following documentation is a guide on how I convert the popular USB CM108 based sound dongle as found on Amazon, eBay and AliExpress.
Overview of modifications
- Remove manufacturers components no longer needed from PCB.
- Make several cuts/breaks to PCB tracks on the module.
- Create wire links from the CM108 IC to COS & PTT points.
- Add a Diode & Capacitor for the COS input
- Add a Transistor & bias Resistor for PTT
- Add a Resistor to limit the microphone audio input level
- Fit Molex connector and sub PCB.
I take no credit for the creation of the modifications outlined above, they are the work and ideas of others documented on various websites. The idea of adding a Molex connector and sub PCB is my own idea with a view to improving the connectivity, assembly and repair of my Allstar Link node.
Lets get started
The
first step is to remove a few components no longer required from the CM108 PCB. I used my soldering stations hot air gun with a fine nozzle to remove the components, they all came away easy without causing damage to the tiny PCB pads or tracks. The image below shows the PCB with the two jack sockets removed, capacitor C2 and resistors R6 & R7.
PCB track cuts & wiring
Fit two short wires to pins 13 & 48 of the CM108 IC and connect them to the points illustrated in the image below, I had some thin yellow PTFE sleeved wire that was ideal for this. Create four breaks to PCB tracks as indicated, I used a sharp craft knife to make the cuts and the hot tip of a soldering iron to heat and lift the unwanted track bits away. A spot face cutter or small drill bit could also be used to make the breaks
Remove solder mask
Next remove a small amount of the solder-mask coating to expose the copper PCB at the points shown in the next image. This is best done with a small/fine craft knife or blade. The exposed points will be used for adding components in the next step.
Install some SMD components
For this step you will need the following SMD components
- 10K SMD Resistor (1206 Package)
- 1K5 SMD Resistor (1206 Package)
- 100nF SMD Capacitor (1206 Package)
- 2N3904 SMD NPN Transistor (SOT23 Package)
- BAT54 SMD Diode (SOD-323 Package)
Mount the components as per the image below taking care with the orientation of the 2N3904 Transistor and BAT54 Diode.
Making the connection
This stage of the CM108 modification requires a 6 way Molex header and a small piece of veroboard or prototype board 6 holes by 7 holes both with 2.54mm pitch.
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| Two proto-boards & Molex headers for making CM108 sub PCB's |
