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Because you are using a known good cable, used before with a signalink, I would open the both the signalink and the 1234 and verify the jumpers are the same. While you are there, grab a jumper and briefly short out the ptt line at the jumper block and see if the radio keys up. That will test the jumper block to the radio.
To test the 1234, I would bring up a command shell, either ssh in it, or using VNC and opening one up there, and at the prompt type 'gpio toggle 0'
That should make the 1234 key the PTT line.
Same command again to unkey.
That should indicate the basic hardware of the 1234 is working.
If all of that seems to work, I would start to check the power available to the pi. Are you running a local monitor on the 1234, and if so, does the little lighting bolt appear? If you do not have a monitor connected to the pi, you many not know you are having brown outs. I believe these messages are being logged in /var/log/messages
use the command 'tail /var/log/messages' to see that tail end of the log file.