[arm-allstar] YET, YET, Another Cheap USB FOB Mod

Doug Crompton doug at crompton.com
Tue Nov 18 14:20:36 EST 2014

First of all this is (at least at the moment) a Beagle bone black site and not a repeater builders site and we do not recommend nor support usbradio on the black at this time. That may change in the future but for now the use of discriminator or non pre/de-emphazised audio is not really well supported in simpleusb so it is suggested that for the best audio, fixed speaker and mic audio should be used. This is very often available at data connections on many amateur radios like on the Alinco DR series using the 1200 baud connections. Motorola radios can be programmed to supply it also. When setup like this audio is excellent. We have many users doing it this way with great results.

As far as audio in and out of the FOBS one could simply use mini phone plugs (like is done on IRLP with the Raspberry PI) to bring the mic and speaker connections from the FOB and only solder two internal connections to the COS and PTT takeoff points. Of course if you could solder those wires on you could solder a couple of extras to the un-removed audio connectors also and eliminate using the plugs.  

The addition (if necessary) of an electrolytic blocking cap is a lot easier than first finding and then removing a surface mount resistor on the board. The resistor is not always in the same place or the same reference number on the myriad of boards out there so finding it and removing it by a less experienced person could be a problem. There is plenty of audio gain in the FOB so a fairly high value of coupling resistance or voltage divider should eliminate any loading and as you point out this would only be a potential problem when using discriminator audio which in general we are not using. 

My goal was to make the mod as easy as possible. It is just NOT necessary to remove parts from the FOB and try and shove all the mod parts inside. Those who operate repeaters or who want to explore more advanced things would already know some of the potential problems with interfacing to their devices. 

One of the goals of the Beagle Bone project is geared towards helping the less experienced person get on the air with Allstar in the easiest possible and least expensive fashion. I try to write documents and explain things in the simplest possible way and the latest code makes it very easy for a newcomer to enter our midst. I try to do the same with any hardware modifications.

There are many ways to do things and I simply offer another one that works well. The user can decide what is best for them.  

73 Doug

Date: Tue, 18 Nov 2014 09:17:57 -0500
From: kuggie at kuggie.com
To: arm-allstar at hamvoip.org
Subject: Re: [arm-allstar] YET, YET, Another Cheap USB FOB Mod

    In the grand scheme of things, if a
      person has the ability to add wires to the CM108/CM119, removing
      the audio connectors is a piece of cake.


      Besides, all of the better modifications tell you to cut the jacks
      off the board, and then do the clean-up.  Some modifications
      require the jacks to be removed, so in those instances it is a


      Bottom line - cut the bands or pins with a good pair of cutters
      first so the jack falls off, then minimal heat is required to
      remove the remaining stuff.  Same goes with the USB plug in those
      mods which require it.


      A bigger problem is those modifications that do NOT instruct the
      builder to remove the MIC bias resistor.  Many radios with DATA or
      discriminator audio outputs have a fairly high output impedance. 
      If you don't remove the bias resistor (commonly around 1k) you can
      severely load the discriminator and the radio won't work
      correctly.  Removing the resistor accomplishes two things.  First,
      it removes the voltage on the MIC input lead and eliminates the
      need for a coupling capacitor.  Second, it raises the input
      impedance so loading of the discriminator doesn't happen.  Sure,
      on those installs that use speaker audio (yuck) for driving the
      FOB, there are no loading effects, but why buy a coupling
      capacitor when one component can be de-soldered or otherwise
      removed?  You aren't saving this component, so you could care less
      if it gets damaged or lost in removal.







      On 11/17/2014 11:14 PM, Doug Crompton wrote:

       My only concern is that for the
          average person absolute minimum modification of the FOB would
          be the best avenue. It is so easy to damage and taking out the
          audio connectors is just not necessary although so many of the
          mods seem to think it is.




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