Richard,
 
I was having the same problem as you, since I own a first generation ThumbDV which runs at 230400 bauds.
When I removed the –n argument in the command: python AMBEtest3.py -n -s /dev/ttyUSB0, it then worked for me.
 
So for people with first generation ThumbDV’s, the command python AMBEtest3.py -s /dev/ttyUSB0 must be used.
 
Also, on page 3, Steve says at point 8: DO NOT edit DMRGateway.ini UNLESS your DV is not at /dev/ttyUSB0
If you run a first gen ThumbDV, the baud rate must be changed in it also to 230400.
 
And for Steve:  thank you very much for all the efforts in the software and the documents.  I went thru it all, and except for the items mentionned up here, everything went well and I’m ready for the next steps.
 
Thanks and 73s from Guy VE2VAG