Tried the circuit (I used a 470 ohm resistor) - nothing - even tried shouting. For troubleshooting purposes:
Using Lightspeeds - 9V battery connected or disconnected - can hear yourself in headsets when xmt NOT pressed (ie intercom mode)
Using Lightspeeds - 9V battery disconnected - can't hear yourself in headset when press xmt.
Using Lightspeeds - 9V battery connected - can hear yourself in headsets when press xmt.
Bias voltage across mic is 4v when using intercom function, and 4-3 volts when xmit pressed.
I modified the circuit to run from the 28v aircraft buss by dropping the voltage using a resistor/potentiometer combo of between 1000 to 1400 ohms. Still nothing but now you get a steady 4v across the mic.
As an aside, I tested the voltage on the radio side of the capacitor. 20 volts in non xmit, 16 volts in xmit.
To keep the continuity going, I'm adding this which came by PM:
jfarrow said:
Mike - cuircuit didn't work as per my message.
Today, I tried connecting the new mic directly to the radio. Upon grounding ptt it transmits. So we know it can work. I saw 15v across the mic.
What next - connect directly to the NAT no capacitor?
Ok,
It sounds to me that like a modern aircraft COMM radio, the Marconi supplies a bias current to the dynamic mic to power its built-in amplifier that you showed several posts ago. The difference is that the open circuit bias voltage out of the Marconi is much higher than what a modern COMM radio puts out. We still are not sure exactly what level the Marconi expects, but since both the older dynamic mic and the newer electret mic are amplified, but let's assume that they are close.
As I posted earlier, I looked up the mic input of a King KX170B aircraft COM transmitter, and it uses a regulated 8V internal power supply to power the mic circuit. The open circuit voltage at the mic jack with the radio keyed is obviously 8V. When the transmitter is keyed, and it is connected to a Lightspeed Headset mic, I have measured the voltage across the Lightspeed mic at about 3-4V.
The 15V that the Marconi puts across the Lightspeed mic sounds too high to me. Here is my proposed fix:
First, add a resistor R in the path between the NAT COM1 MicHi (pin 12) and the Marconi Mic Pin. Try 1200Ω. The goal is to get about 4V across the Lightspeed Mic with the PTT pressed by dropping some of the Marconi's voltage across the resistor, leaving the rest for the Lightspeed mic. If it is still too high, double the resistor, say 2.2K. If it is too low, try 470Ω.
After finding the resistor value that drops the voltage across the mic to about 4V, have someone listen to the Marconi's transmitted signal off the air. Three possibilities: If the audio level sounds right, you are done. If the transmitted audio is too low, add the polarized capacitor C (+end toward the Marconi in parallel with the series resistor, effectively letting it drop the DC level, but shorting it out at audio frequencies.). Try 4.7uF @15V or higher. If that brings the level up, you are done. If not, see if there is a mic level adjustment pot inside the Marconi; try turning that up.
If the transmitted level is too high even without the capacitor, you have a two choices. Turn down the mic level inside the Marconi (assuming you can find an adjustment pot), or write back and I will come up with a resistive network which simultaneously reduces the bias voltage to the Lightspeed mic, and attenuates the audio level between mic and the Marconi...