trying to get an electret mic to work with a radio looking for a carbon style

[MikeMl]

I think I may officially be quiting this job! With a direct connection to the radio mic audio, there is still no transmit. In fact now that the radio is connected to the head unit, voltage across a 480 ohm resistor from mic audio is 14 volts ppt floating and then rises to 22 volts with ppt to ground. With the NAT audio panel connected, you get a whole different set of results, also depending upon whether pilot isolate is selected or not. I think the radio is doing something very weird. I'm going to try to contact Marconi. Wish me luck with that!

Ok, sounds like the Marconi Mic input is not directly compatible with either aviation carbon mics or modern headsets.

Have you been able to find the mysterious resistors/capacitor circuit under the panel? Sounds like you will have to find those components, determine their values, and how they are wired. Then you either use the existing circuit between the Nat and the Marconi, or duplicate it.

 

[jfarrow]

mic

This is the mic that currently works with the Gemelli:

**broken link removed**

and check out what this site says about the mic:

David Clark Microphone M87 Aic [09168P-18] - AviationLogs.com

 

[MikeMl]

Well, that confirms that the Marconi's input circuit is not going to work with the Lightspeed directly:

Re our phone conversation about how to cobble a test circuit to see if you can get audio from the Lightspeed mic into the Marconi: Look at the attached circuit. Note that the Lightspeed's mic is between Ring and Sleeve. Tip would be the PTT contact (to Sleeve) if the headset had a PTT button, which AFIK your's does not.

See if you get any audio out of the Marconi with this temporary test circuit. The level may be way too high, but I have a fix for that. Goal is to get any audio at all. If you get audio, let me know and I will design an interface...

 

[jfarrow]

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.

 

[MikeMl]

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:

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...

 

[santiago36]

Hi Am locking for the diagram of a sony tv it is a tv proyeccion model KP-53s55 or Kp-46s55 Thanks for every help.

 

[jfarrow]

santiago36, I suggest u start your own thread - this thread is certainly not about tv's.

The Intercom is working! Thanks to you Mike. Sound quality is good and that is without the shielded wire to the mic audio. The ground control were easily able to hear my transmission with the aircraft in the hanger with the doors closed. I'll finish it up this week.

I've learned a lot about radios! I've also learned that almost 100% of avionics shops know nothing whatsoever about electronic components. I must have visited five in my area for advice and they all stare blankly when you ask them about a voltage bias circuit for the mic. All they do is make wiring harnesses now. Never heard of ohms law. Too bad the basic knowledge of how this stuff works is being lost.

 

[MikeMl]

... I've also learned that almost 100% of avionics shops know nothing whatsoever about electronic components. I must have visited five in my area for advice and they all stare blankly when you ask them about a voltage bias circuit for the mic. All they do is make wiring harnesses now. Never heard of ohms law. Too bad the basic knowledge of how this stuff works is being lost.

Here is some more evidence that some avionics installers don't know what they are doing.

Also attached is a simulation of the whine filter.

 

[jfarrow]

solution to alternator whine

here is a wildly simple solution to the alternator noise I hear in the headsets. You can only hear it when NOT transmitting. That's because in non-xmit mode you have two pieces of equipment connected with grounds that are physically separated but in xmit mode the headsets are connected directly to the radio so only one ground in that mode. At least that's my diagnosis.

The relay is a 24 volt. Do you agree with the 2 micro-farad capacitor across the relay (will that reduce bounce currents?) and the diode? Just a gut feeling.

For more reliability, I may add a second relay in parallel?

 

[MikeMl]

here is a wildly simple solution to the alternator noise I hear in the headsets. You can only hear it when NOT transmitting. ...

OK. That is a useful observation. However, I have lost the bubble regarding what is the current state of the Mic connection between the Marconi COM and the Nat Audio Panel? Have you bypassed the mysterious resistor/capacitor network and replaced it with a direct connection?

If so, do you have a resistor between the Nat "Com1 Mic Hi" and the mic wire to the Marconi to reduce the DC voltage across the Lightspeed Mic during PTT? If so, it is desirable to keep the Lightspeed Mic voltage at 8V or less while transmitting.

I recall that you "got it working", but don't remember how. Connecting the Mic connection caused you to hear whine in the headsets during times when the PTT is Not keyed. Breaking the Mic wire to the Marconi stops the whine during non-PTT. You probably hear whine during PTT (with mic path connected); but you are willing to live with that. Is that right?

The relay is a 24 volt. Do you agree with the 2 micro-farad capacitor across the relay (will that reduce bounce currents?) and the diode? Just a gut feeling.

No. The capacitor there will harm the PTT switch. It is not needed. A 1N4001 through 1N4007 diode across the relay coil (cathode to +28V, anode to the switched end) will suppress the voltage spike produced by the relay coil as the PTT switch opens.

I would add a second diode as shown in my diagram below. This is to prevent the relay coil sourcing current into the Marconi's PTT line.

For more reliability, I may add a second relay in parallel?

I wouldn't. Putting two of anything doubles the probability of a failure. The relay below is cheap, so buy two and keep a spare

Suggested relay has Double Pole, so just parallel the two normally-open contacts. That relay may have an internal suppression diode across the coil built-in inside the relay. If so it matters which end of the coil goes to +28V and which end goes to the PTT line. If the relay does have the internal diode, leave off D1.

 

[jfarrow]

To bring you up to speed:

Bypassed the mystery capacitor/resisitor network with a new shielded line to the NAT.

Used a resistor (1.4K ohms) with capacitor in parallel to drop bias voltage to about 4v when xmiting.

I can hear several noises in the Mic audio line, 90% is the alternator.

If you pull the mic line in to the NAT, noise ceases.

If you shut of the alternator in flight, noise ceases.

If you press PPT, noise ceases (this, I imagine, is because by pressing PPT you connect the mic directly to the radio, bypassing the NAT and so eliminating for that moment the ground loop).

EVERYTHING is grounded! NAT to NAT case, all shields, radio to radio case, shields at radio end plus I think the headset shields also. This relay idea seems to be a reliable, simple solution.

 

[jfarrow]

The circuit with the relay is a success. No more noise.

Mike - how do you that idea - thinking outside the box!

One last question re the music in - how do you wire a stereo socket for a mono input? Connect left and right together? Do a need to put a 1k resistor in series in each before connecting?

 

[qdjs]

dyanmic mic to aircraft radio input

I'm to do something very similar to this thread. I provide sound for a local air show and I have asked if i can provide conversation between the announcer and the pilot.

I know I can do this by used of a scanner connected to the pa mixer but the announcer would have to hold two mics to his mouth in order to do this (one for the radio and one for the pa). I would like to do this by feeding the dyanmic mic signal into a handheld radio using a ptt switch for the annoucer to key the xmitter. --Any suggestions ???