Newbie's First Questions RCA TS 721 Restoration

[Electronic M]

Isn't there 2 outputs on that knight (one audio one radio frequency)? There should be a fixed frequency audio output (something like 400Hz) that you can use to inject audio.

If there isn't an AF output on the Knight, do you have a junky (something won't be sad if you accidentally break it) radio or audio player (battery powered is preferable) with a working headphone or line output? You can make a cable that plugs into the output of your audio test source and has alligator clips on the other end. Put a roughly .1uF cap in series with the audio hot lead and feed that to the grid, clip audio ground to the chassis.

Alternately if you want even simpler just use a roughly .1uF capacitor to connect the heater hot lead to the grid...All you'll get is 60Hz hum, but it's a simple method to use when you don't have test equipment or sacrificial audio gear on hand.