I think our postings had crossed, I was editing an earlier posting I made.

Yes, I realise now. The Marconi set's DC bias (from the radio) doesn't seem to be reaching the LS mic. What I didn't realise was that during PTT, the raw mic circuit is transferred away from the intercom, over to the radio.

The missing sidetone of course is a symptom, not a cause.

Red herring I'd inadvertantly created was me assuming that an electret mic (few millivolts) is interfaced into a carbon mic circuit (few hundred mV) without an amplifier.

 

What a great response! Mike, I'm in Los Angeles - my cell is 323-428-0559.

So how is it that the headsets with the carbon mics and the old Gemelli audio panel/intercom manage to work with that capacitor and resistors in the way?

Although the Marconi radio is operating on civilian frequencies, it is worth noting that the aircraft (it's an SF260, a 2 seat trainer) was in service with the Italian military back in the 90's. In addition to the Marconi VHF radio it also had installed a military UHF radio. Both radios fed through the Gemelli audio panel to the headsets. Perhaps this is why the headsets required carbon mics?

If I put a 500 ohm resistor across the mic in of the radio, I should see 4 volts, right? If that's the case then I think I could do no damage to radio if I jump the radio's "mic in" directly to pin 12 of the NAT.

 

I'm guessing that for whatever reason, in the old set-up, the resistor network supplied the mic bias current from a regulated voltage source which may have been inside the old audio panel. When that was removed, the resistor network stopped working.

I'm 90% sure that the Marconi (Collins) will source the bias current. Possibly, the military UHF radio didn't, so someone cobbled that network to make it work with both radios. If one did, and the other didn't, then the capacitor would block the dc from the the one that did so as not to change the operation of the carbon mic when that transmitter was selected.

Yes, use a 500Ω resistor as a mic substitute. Key the radio with the PTT, and you should see 2 to 6V. Unkeying will likely make the voltage be zero.
If for some reason, you do not get the mic bias when the Marconi is keyed, it may be an option. Otherwise, write back and we will cobble our own bias network, or trick the NAT into doing it.

btw: I got a 1 hour ride in a Fouga here in the SLC area with Jon Bowman about 12 years ago (his son Tim worked for me), during which I got to try a loop (~4 Gs), aileron rolls, a hammer-head and some other cool maneuvers.

 

Mike - I pulled the transmitter from the plane today and fired it up (with no head unit attached so no freq selected). Across a 480 ohm resistor between mic audio in and ground I see:

14 volts with ptt floating
6-7 volts with ptt ground

Looks like we are on the right track.

I couldn't trace the wire or find the capacitor so I stopped for the day. Will borrow a tone generator tomorrow.

 

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.

 

This is the mic that currently works with the Gemelli:

Item # M87/AIC, Microphone Element on CJ Component Products, LLC

and check out what this site says about the mic:

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

 

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...

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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:
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...

 

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.

 

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.

 

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.

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Attached Files

spg.txt (7.7 KB, 8 views)

__________________
Mike ML.

 

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?

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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?

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.

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.

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__________________
Mike ML.