I am installing the 527B push-to-talk (PTT) microphone into a paging system where the mic cable run will be several hundred feet. The mic input on the amplifier is balanced and will only have a single 527B connected to it. To reduce unwanted noise when the mic is not in use, I want the 527B mic output to be shorted (not open) when the PTT switch button is released. How do I rewire the mic?
A modification to the 527B microphone allows shunting the microphone audio line instead of opening the audio circuit. This will prevent any hum and buzz pick-up due to the routing of the microphone wiring to the amplifier.
To open the microphone, remove three Phillips screws from the back side of the case. Separate the case halves.
The red and black wires coming from the microphone curly cord should be left intact on the PTT switch terminals. These wires provide access to a set of spare contacts for operating a precedence circuit, keying a radio transmitter, etc. These wires are left in place and not changed.
To modify the audio circuit for "shunt" operation as opposed to the factory default "open circuit" configuration, perform the following steps:
- Locate the Black wire that connects between the microphone element and the push-to-talk (PTT) switch terminals. Unsolder and remove this Black wire from the PTT switch terminal.
- Locate the Green wire coming from the microphone curly cord that connects to the PTT switch. Unsolder and remove the Green wire from the PTT switch terminal.
- Reconnect the Black wire to the PTT terminal that originally terminated the Green wire.
- Reconnect the Green wire to the PTT terminal that originally terminated the Black wire.
- Locate the White wire that connects to the "+" microphone element terminal. Add a jumper wire from the "+" white wire terminal to the unused PTT switch element.
- When the PTT switch is up, the green and white wires are connected together - shorting the mic connection to the amplifier.
- When the switch is down, the microphone element black wire connects to the green wire. This connects the microphone element audio through to the amplifier.