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Aviation Products Frequently Asked Questions
Sigtronics Aviation Products Frequently Asked Questions page. On this page you will find answers to questions on the use or installation of Sigtronics headsets and intercoms.

For technical information on the use or installation of Sigtronics products not covered here, E-mail Technical Support: or call Monday through Friday 8:00 a.m. to 4:30 p.m. Pacific Time. --909 305-9399

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General Questions

Q. How do I get technical assistance for my Sigtronics product?

A. There are several ways to get assistance with any Sigtronics Products:

1) This FAQ document has help on the most common asked questions.

2) All Sigtronics products installation and operating instructions are available for downloading on our site, press here to go there.

3) By Telephone: 909-305-9399 for assistance 8:00 a.m. to 4:30 p.m. Pacific time Monday through Friday except holidays.

4) By E-mail:

5) By FAX: 909-305-9499.

6) Or you can write us at: Customer Service

Sigtronics Corporation
178 East Arrow Highway
San Dimas, CA 91773

If you do contact Sigtronics for technical assistance along with the problem you are having, you will need to provide specific information to your application. Information like the Sigtronics product model number, type of aircraft, make and models of the aircraft radio(s) / audio panel, and make and model of other headsets you are using if applicable. If the problem is with a Sigtronics intercom, also include the serial number.

Q. How do I send in a Sigtronics product in for repair?

A. If possible contact Sigtronics first via phone, E-mail, fax, letter as above to determine where problem may be. Most intercom / headset system problems turn out to be installation or usage related and not caused by a faulty product.

If it is determined that a Sigtronics product is in need of repair box it up in a suitable shipping container and send shipping prepaid to:

Customer Service
Sigtronics Corporation
178 East Arrow Highway
San Dimas, CA 91773

No return authorization number is required. We do however, want you to include a note with your daytime phone number and description of the problem you are having. Include additional information like type of aircraft, make and models of the aircraft radio(s) / audio panel, and make and model of other headsets you are using if applicable. If unit is out of warranty and requires payment for repairs we normally send it back UPS COD. If you prefer you can include credit card (VISA or Master Card only) information in your letter. We will need the card number, the expiration date, and the card holders name as it appears on the card. Normal repair turn around time is 1 1/2 weeks in house or less plus shipping time to and from Sigtronics. If necessary, faster turn around times can be arranged by request.

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Adobe Reader or PDF

Q. Why are all Sigtronics instructions in Adobe Reader (PDF) format?

A. The PDF format allows you - our customers - to get Sigtronics product instructions that look and print out exactly as the originals. This is true regardless of the computer system you are using for example Mac or PC. The PDF format printouts are much cleaner and therefore much more readable than FAXed documents. As long as you have Adobe Reader version 5.0 or greater you can view and print any of these instructions.

Q. How do I get the Adobe Reader for my computer.

A. Just click on the Adobe Reader button on the Sigtronics technical pages. It will take you to Adobe's site and allow you to download and install the reader for your particular system. If you are in a hurry press here now!

Q. I have Adobe Reader and I can download the tech pages but can't view them. Why?

A. You do not have Reader Version 5.0 or greater. Sigtronics technical files will not open correctly in Readers versions 3.X and lower. Just download the latest Reader now and install it and you will be able to use the technical pages.

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Portable Intercoms

Q. Can we use the aircraft's built in PTT with a Sigtronics portable intercom?

A. Yes, all Sigtronics Portable intercoms will work. There is one caveat though. When you transmit this way all microphones on the unit will go out to ATC. On two place intercoms this is usually not a problem. On four place and above it can be more problematic. You will have to tell every one to be quiet while you are talking to ATC.

Q. I do not hear music or I have very weak music when I plug into the AUX input on my SPO-22 / SPO-42 / SPO-62 portable intercom. What can I do?

A. There are three possible causes for this problem. The first is to make sure you are using the headphone output jack on your music source and not the line output jack - if it has one. The second is to make sure the cable that you are using between the walkman and the SPO is correct. It needs to be stereo on the music source side and combine both channels into one (monaural) on the SPO side. Also make sure it is a straight through cable without any attenuation in it like a "dubbing" cord. Third and finally if the other two check out fine and you are still having low audio, you could have an older SPO that does not amplify the AUX input audio. We made a change a few years ago so we don't have this sort of problem with the newer lower powered walkmans. We can update your unit to the new circuit, if necessary. There is a small charge for this. Contact Sigtronics for the details.

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Built In / Panel Mount Intercoms

Q. Can we use portable hook-and-loop type Push-To-Talk (PTT) switches with Sigtronics built in or panel mount intercoms?

A. Yes they can be used but they have to be hooked in correctly. For complete instructions download to learn how.

Q. Co-pilot cannot transmit through the intercom to ATC. The radio is activated but the tower hears no voice. The Pilot can transmit fine. What is wrong? Intercom models SPA-400, SPA-600, ST-400, and ST-600.

A. The white wire is tied to the white/red wire because of preexisting PTT wiring. To test for this disconnect the white red wire as close to the intercom as possible and then try to transmit on the co-pilots side. If you now can transmit you do have this wiring error. Rewire the pilots PTT switch to only go to the white/red wire and of course to point "A" ground. If necessary you can download your intercom's installation instructions by going to the Aviation Intercom technical page and selecting your intercom model.

Q. Can we add pilot isolate to a SPA-400 or SPA-600 intercom?

A. Yes it is possible by adding an additional switch to the installation. Download www.sigtronics/pdf/air_pdf/pilotiso.pdf for the details.

Q. Can I use Sigtronics intercom with a portable or hand held aircraft radio?

A. Yes, but it depends on the intercom model, the radio model, and a few other factors as to how this can be done. Please contact Sigtronics technical staff with the details of your installation and we can provide instructions.

Q. Can I install Sigtronics Stereo Intercoms for use with regular monaural aviation headsets?

A. Yes, you can install any Sigtronics stereo intercom system to use regular monaural aviation headsets. It requires a small wiring change to the installation. See the installation instructions for your particular Sigtronics intercom model for details.

Q. I have an older ST-400 / ST-600 with the RES-400 / RES-600 music switcher. How can I use low level music input like a portable CD or tape player?

A. If your portable player has at least 30 milliwatts of total headphone output power (check your players specifications) a modification to the Sigtronics music switcher (models RES-400 or RES-600) will allow it to be used. Download www.sigtronics/pdf/air_pdf/resms1.pdf for the details on this modification. If your portable player has less output power than 30 milliwatts you have two options. Option 1: Upgrade to the Sigtronics RES-401 or RES-601 music switcher. These new switchers just plug in place of your current switcher and allow even very low output players to be used. Option 2: provide an audio amplifier between your player and the Sigtronics system. You will probably have to have a qualified technician do this for you.

Q. I have an older ST-400 / ST-600 with the RES-400 / RES-600 music switcher. How do I hook it up to a stereo music source with four speaker output wires labeled left +, left -, right +, and right - (floating speaker outputs)?

A. The older ST-400 / ST-600 with the RES-400 / RES-600 music switchers have a three wire music input and cannot directly hook to a four wire stereo. To use a four wire output stereo you have two options. Option 1 - Upgrade to the Sigtronics RES-401 or RES-601 music switcher. These new switchers just plug in place of your current switcher and allow a four speaker wire output stereo to be used directly without any adapters. Option 2 - Connect a three to four wire or floating ground adapter between the stereo and the Sigtronics music switcher. These adapters can be purchased from some stereo supply houses. Alternately, an adapter can be built using basic electronic components. Download www.sigtronics/pdf/air_pdf/fltngrnd.pdf for complete instructions details.

Q. The intercom squelch and volume controls work fine until I select or turn on the aircraft radio(s). I still get talk between the headsets but the intercom squelch and volume controls have no effect. What can I do?

A. A small number of aircraft radios when they are turned on will bypass the intercom of some Sigtronics models. This is because of the design of these particular radios in combination with part of the intercom fail safe design. The most common aircraft radios that do this are the King KY-92, and most Terra panel mount radios. The specific Sigtronics intercom models affected are the SPA-400, SPA-600, ST-400, ST-600, SDB-800, SPO-22, SPO-42, and SPO-62. There are two recommended ways of remedying this situation. The first is to send in the Sigtronics intercom for a factory modification known as the "mic interrupt mod". There is a small fee for this modification. The second way to fix this is to install a small relay in the aircraft that effectively does the same thing as our mic interrupt mod. For complete instructions on how to do this download the "SPA-400/600 External Mic Interrupt Modification" at

Q. Why can't my passengers and I hear my voice when I transmit on the aircraft radio(s)?

A. Most aircraft radios, when transmitting, provide what is called "sidetone". Without sidetone, you will not hear yourself or your co-pilot transmit to ATC. This can be distracting, especially during instruction, because no one on the intercom will hear the out going transmission side of the radio conversation. Some aircraft radios do not provide this function and other radios are designed to drive only one headset. Therefore, if you have such a radio, the sidetone may be extremely weak or non-existent. If this is the case and the sidetone cannot be turned up in the radio(s), a simple modification can be performed within the Sigtronics intercom to simulate sidetone. The Sigtronics intercom models affected are the SPA-400, SPA-600, ST-400, ST-600, SAS-440, SAS-640, ST-440, and ST-640. For a small fee Sigtronics can modify your unit for simulated sidetone for you but you will have to send it to us. If you prefer to do it yourself or have someone do it for you locally, instructions on how to do the modification are in the installation instructions that came with your particular model. If you don't have the instructions you can contact Sigtronics and we will get be glad to get them to you. Alternately, they are all available on our web site. Just go to and select the appropriate instructions and download.

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Q. Can I buy less expensive headsets for the other seats? After all it is just my wife and other insignificant non-pilots!?

A. The answer is a qualified yes. You can buy less expensive headsets but be careful about buying poor performing or poor quality headsets. There are two main reasons for this. The first is your friends and loved one's hearing are just as important as yours. Many low priced headsets do not attenuate the back ground noise very well putting the wearer's hearing at risk. Second, poorly designed headsets bring down the performance of any intercom system. Actually the noise you hear in your headset through the intercom is that generated by the noisiest microphone on the system! Some low priced headsets can induce so much noise into a system as to make it virtually unusable. That said, which inexpensive headset should you buy? For the cost conscious consider the Sigtronics model S-20 headset. The S-20 is Sigtronics least expensive headset but at the same time does not compromise on performance or quality.

Q. I have a military headset. How can I use it in a general aviation aircraft?

A. It is possible to use a military headset in general aviation aircraft but it is not practical. Military headsets are different than general aviation headsets in several ways. The first and most obvious is the plug. Most military headsets use a single four conductor plug (U-174/U type). General aviation uses two plugs. Sigtronics manufactures an adapter cable (p/n 900051) that converts between the two plug arrangements but that only solves the plug problem. Another difference is the microphones. Military uses a 5 ohm non-amplified microphone. General aviation mics are amplified and powered by the aircraft intercom or radio. The microphone level and impedance are vastly different in the two systems. Therefore to use the military mic in general aviation aircraft you would need to add a mic amplifier between the microphone and the aircraft. The last and final difference is the speakers in the ear cups. Military headsets use 8 ohm speakers while general aviation uses 600 ohms. This also can be converted with additional electronics. In fact over the many years Sigtronics has been in business we have seen many such converters/adapter cords that take care of all three problems. Our customers, however, tell us that none of them have worked very well. Another approach is to buy general aviation microphones and speakers and change out the military ones. This works but is expensive. The best approach is to buy a headset made for general aviation and save the military headset for the military aircraft.

Q. If I plug one headset at a time into our non-Sigtronics intercom they work individually. However, if I plug them both in at the same time only one or neither of the headset microphones works. Why is that?

A. Usually, this problem is not a headset problem. It is caused by a design shortcut used in inexpensive intercom systems. General aviation headset microphones draw power from the intercom or radio mic line they are connected to. Some poorly designed intercom systems hook all the headsets to one mic line. When this is done all the headsets must share the same power. Because different headsets have different power requirements one may draw just enough power so it works but there may not be enough left over for any of the others to function. With this type of design two identical headsets will work most of the time but get different types and models then you will have a problem. The only way to fix this is to modify or replace the intercom system with one that uses separate mic input lines. All Sigtronics intercom systems use separate mic input lines and as such do not have this problem.

Q. What happens if I plug in regular aircraft monaural headsets into a stereo intercom system wired for stereo headsets?

A. What happens depends on the design of the intercom system. At the very least you will get reduced performance. At most you or anyone else in the aircraft may hear music but will not hear intercom or aircraft radio reception. This latter situation is the reason that monaural headsets should not be plugged directly into stereo headset jacks wired for stereo. If stereo to mono plug adapters are used then the intercom and headsets will function correctly. Since the headphone plugs used on general aviation headsets are the same as that used in home stereo equipment, these stereo to mono headphone plug adapters can be readily purchased at your local electronics store.

The gory details: Stereo headset jacks have three electrical connections. A regular general aviation monaural headset has a headphone plug with only two electrical connections. When you plug this two connection plug into a three connection stereo jack it shorts two of the connections together (the RING and BARREL). If the RING connection of the jack is used, this shorts out that audio channel to ground so you will not hear it in your headset. You will still hear the music from the other channel in both ears of the monaural headset but you are only hearing half of the audio. Depending on how the intercom system is configured this may also prevent other headsets on the system from hearing that channel. Finally if the particular intercom system directly ties both channels together when aircraft radio or intercom traffic occurs both channels will be disabled since one channel is already shorted to ground.

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Helpful Hints for Communicating in Unusually High Noise Environments

The Sigtronics SPA-400N, and SPO-22N voice activated intercoms have been specifically configured to obtain the optimum "signal-to-noise" ratio for successful cockpit communications in the "noisiest" of aircraft. Aircraft applications will include Open Cockpit, Ultra-Lights, Warbirds, or Helicopters.

It must always be emphasized, however, that the intercom is only one component of the total communications system. The headsets chosen to be used are critical in determining the overall performance of the "System". For example, it is futile to attempt using any headset with a NRR (Noise Reduction Rating) below 24dB. The "stick-in-the-ear" or open frame type headsets are fine for the DC-10 cockpit environment, but are absolutely useless for the environment we're discussing.

For this environment, it is suggested that the Sigtronics S-20, S-45, S-58, S-68 or S-8 headsets be seriously considered for use. They are light weight for comfort (11.9 to 12.3 oz.), have a noise reduction of 24dB and are specifically designed for the most difficult noise conditions encountered in Military, General Aviation, and Airborne Law Enforcement. The M-80 / M-81 electret microphone is an essential part of the success of these headsets. If you prefer the S-40 headset, we can substitute the M-80 microphone for you. Also, we can retrofit the M-80 and our receivers to your helmet. All Sigtronics headsets are RFI and EMI immune and work superbly in fabric aircraft or in close proximity to antennas.

The following list of "Helpful Hints" is designed to maximize the performance of your Sigtronics Intercom System and allow "hands-free" cockpit communications where no previous intercom could!

1. If your current appropriate headsets have adjustable microphone gain settings, turn them down. This serves to improve the signal to noise figure of merit of most microphones. (Applies to David Clark H10-30, many models of Telex, and Peltor models with the amplified dynamic mic. Contact manufacturer for instructions.

2. In the Open Cockpit environment or when the boom mic is subjected to direct air blasts, large, foam type mic muffs should be used. We have a suitable muff available.

3. Correct positioning of the boom mic in relation to the mouth is important! Adjust so that the mic is to either side of the mouth (off center) but no more than 1/4" away (one can "kiss" the "talk" portion of the microphone.)

4. Most important of all: Adjust the Headset receiver volume control(s) for maximum volume. Adjust the Intercom volume control to the lowest setting that allows good communications.
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