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The hum you are likely hearing on the air is a product of the power supply and the audio input circuit.
First, use a car battery or regulated/filtered 12 volt power supply. The “wall-wart” is meant to be used only for testing. It has a poor filtering circuit and poor voltage regulation. Make sure you follow the set up instructions to the letter.
Next the wire antenna needs to be clear of all metal surfaces and objects, period. the antenna needs to clear of nearly everything else and oriented vertically. You should consider building the loaded antenna mentioned in the instructions. Most of the signal is remaining on the chassis of the transmitter. And that leads to the last possible problem.
The RF signal on the ground side of the chassis is causing the audio circuit to “motor boat”; a slow oscillation of the audio amplifier circuits. The RF energy is de-sensing the audio section causing the audio to go in and out. Am transmitters operate by significant audio level driving the modulator. Make sure your audio source has enough audio output to drive the transmitter to full modulation.
Until you get a better, more stable power supply, better DC filtering, a somewhat resonant antenna, better isolation/decoupling for the audio circuit you are going to continue with a fairly common problem with Ramsey AM transmitters. I have an AM 25 and in the beginning it did the same thing. I changed all the previously mentioned items and it improved quite a bit.
The best range I got with my Ramsey AM transmitter was about a half city block. Much less than the 1/2 to 3/4 mile or more with the Rangemaster AM1000 I use now. Plus Ramsey transmitters work very hard to make 90 percent modulation. The transmitter kits can however produce some pretty respectable quality audio. Mine unit does. I use it as a back up transmitter.
Good luck. Sounds like you can easily fix this unit to do what it was designed to do. Check the audio final transistor and the RF output final transistor. They should be “hot” under normal operation. It is normal for these devices to run hot according to Ramsey. They run hot without failing. I hope you find this helpful.
Marshall Johnson, Sr.
Senior Pastor, President
Rhema Christian Fellowship, Inc.
Rhema Radio – The Word In Worship
AM 1660 – FM 93.5