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Could a oscilloscope software which utilizes your computers soundcard be of use in modulating the 130% positive and 90% negative peak capabilities of the Rangemaster transmitter?
In order to do that the frequency response of the soundcard would need to include the r-f frequency band having the AM carrier and its upper and lower sidebands. AFAIK no soundcards are available with that performance.
Below is a screen clip showing the audio frequency response of a typical soundcard for sample rates of 32, 48 and 96 kHz (left to right). Its upper frequency limit even at a 96 kHz sample rate is less than 1/10 of that needed to display an AM waveform in the medium wave broadcast band.
A simple diode demodulator circuit (Google) would provide an audio output from an r-f sample of an AM transmitter, which could then be displayed on a PC using its soundcard and the needed software. The display would show gross distortions in that waveform such as flattened peaks when the tx is over-modulated by a sine wave, but it would not permit accurately setting positive and negative modulation peaks. Also diode detectors add distortion to their output signal, for some conditions.
Probably the least expensive way for a Part 15 operator to set +/- AM modulation peaks ~accurately is using an oscilloscope. I looked at EBay, and saw some for under $100. Just look for one with an analog input bandwidth extending to at least 5 MHz.
Here is a clip showing the oscilloscope waveforms for +125/-100% AM, and +100/-100% AM using sine wave modulation.