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Found that modulation was only possible to 60-70% before distortion occurs.
Measuring set up used – Retro 75 transceiver built by W1UJR, Elecraft DL1 Wideband Dummy Load,
Elecraft CP1 Directional Coupler, Elecraft 2T-gen 2-Tone Test Oscillator, and Elecraft W1
140W Computing Wattmeter and SWR Bridge for initial set up. All test equipment built
by W1UJR from Elecraft kits.

Found power output about 2.5 watts with an unmodulated carrier, does rises to about 4 watts at 60-70% modulation.
Waveforms were recorded on Tektronix THS720A Digital Oscilloscope.

AM Modulation Measured on W1UJR Retro 75 Transceiver

Ideal measured waveform – 100% modulation

Elecraft DL1 Dummy Load
Elecraft CP1 Directional Coupler
Elecraft 2T-gen 2-Tone Test Oscillator
Elecraft W1 SWR and Power Meter