Total posts : 45366
This sounds really wierd. You are on the right track with your tests and believe me it will work out. Keep trying.
I wonder if you are getting the proper AC at your transmitter. If the voltage is low, the 7815 regulator will drop out and allow 120 Hz. hum on the SSTRAN 15 volt bus.
Did I understand correctly that you have hum even when the audio is disconnected. This really sounds like the AC is too low in voltage at the transmitter. You might try running an extension cord out to the transmitter and plugging the transformer there and connecting it to the transmitter. This will eliminate the drop in the Cat 5 wire. I really doubt that Cat 5 would drop enough voltage to cause this problem, but this could be a rule out.
Another thought, did the transmitter work OK on your bench? If not, maybe you have a diode in wrong. Check D1, D2, D3, and D4 and make sure C4 is not in backwards.
Another old trick to test DC supplies is to connect your meter and set it on the AC scale. If you have DC only the meter will read millivolts. If there is hum on the supply, it will read volts. Connect the black lead to T2 and the red lead to the “hot” side of L8 ( the right side lead as you face the front of the PC board.
Keep trying. It WILL work.