If it has a "click" inside the radio when you key the mike, or jumper across the transmit pin and ground, it is a "relay-keyed" radio... many older 23 channel radios were. Sometimes the relay contacts tarnish and don't make good contact. I have taken the covers off the relays in the radios and used a "burnishing tool" to clean them with success usually, though sometimes temporarily. I ran several different Midlands back in the day, they were fairly durable radios. Make sure the PA switch is not set to PA also, as this setting will bypass the transmit audio the PA output jack on the back of the radio. If I remember right, there were several pinout versions for Midlands, and most had 4 prong plugs.