The 2400VH-1, Videolynx Z70A and VM-70X are designed for a single line level audio input of 200-250 mV RMS and do not have an internal 25 - 50 kHz deviation limiter. Adding the P. C. Electronics FMA5-G sound subcarrier boards mic amp, line mixer and volume compressor type of deviation limiter will let you adjust proper sound levels and use a low impedance dynamic mic and/or line audio as desired. The transmit LED also doubles as an over deviation indicator.

Before mounting the FMA5-G board, the 4.5 MHz VCO must be disabled by applying a solder short across the base to emitter solder pads of the 2N2222 surface mount oscillator transistor on the bottom of the board - see photo.

A wire is run from the audio test point solder pad, as shown in the photo, to the line audio input of the transmitter. The DC resistance at the transmitter line audio input must be greater than 5K.

The sound deviation limit needs to be set for between 25 and 40 kHz. This can be done with a tone applied to the line input, line level pot raised to the point where the transmit LED is off, then setting the deviation pot in the middle of the FMA5-G board for about 35 kHz with a communications monitor. What? You don't have one of those? OK, preset the pot to about half way and you will be about 25 kHz. Note the sound level on a TV of a commercial broadcast station, then switch to the cable channel you are transmitting ATV on, and adjust the deviation pot for about the same volume - that's close enough.