Expert: Marc Ebelini - 11/25/2008

Question
QUESTION: Hello,
Like a previous question reference wiring to comm radio to an intercom - no audio selector panel. We had to change one of our comms. The new #2 comm was replaced on the field, expeditiously bypassing the intercom system. The original panel was set up is an old but functioning Telex tc 200 intercom, independent jacks pilot/copilot on each side wired to intercom. This intercom has only one "output (speaker)", one input (mic)and one PTT with their respective grounds. Each control wheel is equipped with a build in PTT switches. The 2 radios can receive simultaneously on the headsets (no speaker installed) but only one radio at a time can be selected to transmit using a switch (up for transmitter 1, down for transmitter 2). Our situation is that to install the new radio (transceiver #2) we have to deal with an overwhelming quantity of wires connected to a bus that is constituted of 2 matrix of 3X12 connectors (each line of 3 is connected). To our dismay the transmitter switch is composed of 12 (yes 12) connections. Right now the pilot can transmit/receive on #2 (switch on #2), pilot can transmit/receive on #1 (switch on #1) BUT ALSO simultaneously transmit on #2 despite switch position on #1. Copilot cannot transmit on #2 but receive on #2 (switch on #2). Copilot can transmit/receive on #1 (switch on #1). Any insight would be appreciated. We are down to rewiring the whole avionics. Hardly any wires are identified and and (of course) we have no plan of this "bus". The starting point would appear to be the connectors of the transmit switch: why 12 connectors?
Cheers

ANSWER: Let me begin by saying that what you have sounds like a mess. First of all trying to modify or add to an already poorly layout system is a real challenge that would be better off starting from scratch. To begin with you first have to decide how the system should work. Most light aircraft do not normally use a selector for the copilot to transmit. But this can be done especially in multi engine multi pilot operations. Such as when the copilot will be using number two to talk to ground control but the pilot was using number one for the control tower. This type of control panel is used on some military aircraft.
Unless there are two Com antennas the system must have an inhibit circuit to prevent number two transmitting into number ones receiver and vise versa.
The simplest thing would be to have a pilot selector switch if only one antenna is used to prevent the receiver damage. That switch would decide which PTT will work. Using an inhibit system works but would be hard to make it fail safe. My original audio control panels contained auto switching a feature, that then became popular on commercial panels made by Kings, Narco and other companies. What it does is automatically switches the radio selected to transmit on to the speaker or phone which ever was selected.
Now tell me is this a twin engine plane and normally flown with two pilots? If so a dual control may be the answer. They don't allow me to insert a picture but if you include your email address maybe I could dig up a picture of one of my old designs. Have been retired for many decades but I may have one in the archives. Be aware that many things are now preformed my microprocessors and solid state switches rather than relays with use back them. Relay logic is easier to trouble shoot especially in my old designs because I used gold plated plug in relays to make it easier beside those devices were not available then. I say KISS, bet you know what that means. I used and still have 27 different colors of wires with a list to identify what each controls. Unfortunately PVC wire is now outlawed in aircraft and Teflon or teflex coated wires are required. Thought I would mention that incase you were not aware of the regulation that came into effect in the early eighties. Let me know what you plan?

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QUESTION: Thank you for your reply. The antenna radio setup is on top and one on the bottom of the fuselage. The aircraft is a single engine Rallye (like a Grumman Tiger) used in Europe extensively. The aircraft is used partly to instruct tow pilots (glider operation) and the transmit capability is required from the right seat. I suppose you are correct to mention a mess. I will probably have to meticulously identify every wire. Why do you suppose the transmit switch contains 12 connections I fifigured out perhaps Tx 1 PTT incoming, Tx 1 mic incoming, then tx 1 PTT outgoing, tx 1 mic outgoing, same thing for tx 2 This would make 8 connections. Are the other connectors grounds of some sort? In the end we imagine the previous owner went for a cheap 2 radio setup. However it worked well until radio number 2 packed it in. That is the reason why we had to play around this well defined mess. Our life would indeed be more easy with color coded wires (they are all white of course) and we have no plans of this artistic work!
As far as planning I suppose I am down to identifying each wire and at same time map this once and for all. Failing I would every thing start from scratch and only install the new VHF com and use it's internal intercom. The number one VHF would then have to be scrapped. Not a good idea since I always prefered flying around with 2 comms. This would be a last resort.
Best regards

ANSWER: okay I got the picture and my recommendations that obviously says rewire the system and end up saving time and aggravation. Now to have two comms you need two comm antennas or a diplexer. Not being up to date I went to the Internet and found a diplexer. Fortunately I was sitting down. They cost about $1,000.00 USD. Did not see any diplexer antennas that were used in my day that I would not recommend. My designs always used two antennas. On your aircraft I suggest both on the top as bottom mounted antennas work poorly on the ground as in the case of ground control. Mount them as far apart as possible. With your short fuselage (If I remember correctly)One could be mounted on top of the tail. (separate VHF antennas by at least one half wave length or about 1.6 meters.) The tail top will not work as well because of the poor ground plane area. Use one switch to connect the PTT between intercom and transmitters. Don't recommend being able to transmit on both at the same time so there will be a switch to select between one and two with a center off. Why? Should a short develope in the key line you can still use the radios. Separate switches to select speaker or phone for each set. But the radios need to be connected to intercom should someone call. Also any warning devices need be on that bus. The speaker line needs to got to a relay so that the speaker can switched over to a load (I used 33 ohms 3watt resistor) to always terminate the speaker output. The relay is keyed by the PTT so that the transmitter does not feed back into the speaker system.
The nav sections of the radios can use a splitter to the nav antenna.
This is my recommended logic. To start the job you will needed the wiring diagrams of the intercom and radios and different colored wires. To accomplish the 27 colors I used there were three colors on some wires. Did my logic flow help you to picture what must be done to start this project? If not let me know using a format of spaces where you want my reply to see if I can make it clearer. If I may tell a story. I was once accused by an FAA inspector of doing poor work. When asked why, he said I used all scrape wire for my installations. They had to explain to him it was for color coding. He was from the military where the white wires are stamped for identification. To expensive to do on a custom level, besides the ID usually ended up being on the back side and out of sight.
Flew gliders in Germany in 53 but I was winched up.

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QUESTION: Well, just to keep you posted. I finally had some time to work it out. Not being a mechanic nore an avionic as you have guessed, it took me 3 days to track and identify every wire in the system. I map the whole thing and then created the actual schematic. This in turn guided me towards my end. It was a patience trick. There was plenty of abandoned wires. I cleaned them up. The transmitter switch was figured out using a small relay to cut of the audio from the "standby" radio. Then antenna issue was resolved by ensuring one on top and one at the bottom of the fuselage.
Thank you very much. Best regards
Marc Arsenault

Answer
You could have some time if you acquired the wiring diagrams for the radios. Most companies will supply this. These are not service manuals only wiring information that usually comes with the radios to identify the pin connections of the radio. You will have less of a chance of doing any harm. Remember to use a center off switch for the transmitters so that if the event of a shorted jack or stuck mic key you can continue using the radio's without interference. Also highly recommend a selector switch to isolate the PTT function from intercom to transmitters. If you send me your email address I could make some simple drawings as a suggestion as to how to wire it up. Do you plan to have a audio control panel? Running wires to a central control can help later in troubleshooting the system. It's works great when you need not play with the volume controls to mute listening to the radio not in use and or switching it to speaker or phone. Remember the speaker output should always have a load on it when not in use or disconnected during transmit. I found 33 ohms at 3 watts give adequate protection without needing a resistor large enough to handle the full radio output power. Just remember to connect any warning devices to the headset and speaker. Like stall warning if they are used. Contact me if you have any questions Marc