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Full Review of TKM Slide-in Replacement Radios

One of the most common questions we get at the shop every day goes like this, "Is there any way to slide in a new radio with all the new features where my old radio was without having to have the airplane rewired." The answer is... maybe yes or maybe no depending on what kind of airplane and avionics are already installed. If you are fortunate enough to own a 1967 or later model Cessna with original Nav/Coms or any airplane with NARCO Comms or King Nav/Coms from the 1970s, you're probably in luck.

Back in 1983, Bill Michel, an avionics engineer who specialized in the manufacture of avionics test equipment and blind encoders, saw the need for simple slide-in replacement radios. In order to make it viable, he had to research the production numbers of radios that were replacement candidates. He found the biggest numbers were in King, ARC, and NARCO radios.

The first radio built by his company, TKM, was a direct slide-in replacement for nearly all King KX-170 series radio. The Michel MX-170 by TKM was introduced in early 1984 and was an immediate hit. Without a direct slide-in option, aircraft owners would be forced to upgrade to another system at prices sometimes well over $3,000. There are tens of thousands of King KX-170A, KX-170B, KX-175, KX-175B type radios flying today and the MX-170 easily replaced them at about half the price. The latest iteration is the MX-170B which has improved technology, board compatibility, and the same price as other TKM NAV/COMS.

His second project was the MX-300. The MX-300 was designed as a slide-in replacement for most of the radios that were manufactured by the ARC (Aircraft Radio and Control) Division of Cessna in New Jersey from 1967 through 1977. These ARC radios were getting old and pilots wanted new features like dual flip flop displays and frequency memory. The MX-300 was to have all these new features.

One of the first problems was to design the radio so that it could be compatible with Cessna's solid state equipment of 1973 to 1977 as well as the larger mounting and different connectors Cessna used in their tube type radios from 1967 through 1971. To solve the problem Bill designed the radio so that it would instantly replace all "300" series solid state radios built from 1973 through 1977. Then, he designed a sleeve that the radio fits into that adapts it to the larger mounting and different connectors of the older 1967 through 1971 radios. Fortunately, all of the models built in these two time periods use only two types of mounting and wiring systems.

For 1978 and later model Cessnas with 28 volt electrical systems, TKM developed the MX-385. The MX-385 is a direct replacement for the Cessna RT-385 and RT-485 radios. They have all the features of the MX-300.

The MX-12 is the their newest slide in replacement for one of the oldest line of radios around: the NARCO Mark-12. However, there are potential problems with using these replacements. The "tube socket" style connectors these old radios use may develop intermittent contact and corrosion problems. A thorough cleaning of the old connectors and tightening up on the contacts is in order before mating to the new radio connectors.

The existing VOR indicator, usually a VOA-4, 5, 8, 9, 40 or 50, are as old as the Mark-12 radio itself. Problems with bad meter movements, bad capacitors, inconsistent course width problems and general old age can make using any these indicators questionable. Mating a new radio with this type of an indicator can be a bad idea. Another potential problem is the Glideslope interface. NARCO never used a standard Glideslope connector assembly, so interfacing the Glideslope receiver to a MX-12 may require the assistance of your local avionics shop. These problems aside, the MX-12 can be an excellent choice as a replacement radio.

For those with NARCO Communication radios who want to upgrade to digital, there is the TKM MX-11. This radio essentially replaces all the communication radio models NARCO built during the 1970s. Since the MX-11 is just the repackaged communication side of a TKM NAV/COM, performance and features are equal to other TKM radios.

All of the TKM radios have lots of nice features and excellent performance. The frequency displays are self-dimming, dual color LED. Red displays show standby frequencies and yellow displays show active frequencies. Somehow, I feel TKM got those colors mixed up. I always thought the active should be red and the standby should be yellow, but it's no big deal -- and this compares very favorably with a single color for both used in other company's products.

There are two frequency knobs. Unlike most radios, they control both the NAV and the COMM sides of the radio. Pressing the "N-C" button switches on an annunciator in the display tells you whether you are changing the NAV or COMM side of the radio. There are 30 memory channels each on the COMM and NAV sides. The squelch is automatic and works great.

We have found that all of the MICHEL/TKM radios have outstanding power output potential. Most units have transmitter output of between 10 and 14 watts and we have occasionally seen power in the 17 watt range. The 1 year TKM warranty is pretty standard in the industry but if you ever need after warranty service this company won't take you to the cleaners. Factory repairs seldom cost more than $100. You won't find a Made in Singapore or Made in Japan label on these radios; TKM manufactures their radios in Scottsdale, Arizona, USA.

In 1990, NARCO got on the replacement radio bandwagon. Their first introduction was the "Mark-12D Cessna" which replaced all 1973 to 1977 Cessna ARC "300" Series radios. It went head to head with the TKM MX-300 and is a top seller for NARCO. This radio is based on proven Mark-12D digital technology and is well known as a solid radio. For pilots with 1978 and later model Cessnas, they developed the "Mark-12E Cessna". This is a 28vt replacement based on their newer Mark-12E chassis.

A few years ago NARCO introduced a new COM-810R which replaces all of their communication radios of the 1970s. Based on digital COM-810A, this is another slide-in winner for NARCO.

Most all of these radio's slide-in and work the first time with no problems. However, some glitches have popped up over the years on replacement radios. However, we have found that most problems are installation related.

The most common problem is a poor electrical connection between the connector at the back of the radio and the mating connector in the mounting rack. This generally is one of three causes:

  1. Over the years the old radio in the old mount has developed a "set". When a new replacement is slid in, it doesn't make proper electrical contact and intermittent problems occur.
  2. Oxidation and corrosion on the mating connector in the old mounting rack causes intermittence.
  3. Small variations in the size of the replacement radio, usually its faceplate, can interfere with the ability to slide the radio into its mounting far enough to make good electrical contact. Remember too that these new radios may use more power than the radio's they replaced so new circuit breakers, fuses or wire may be required. Fortunately, these problems are rare and are easily solved by a good technician or mechanic.

Street prices are downright cheap on these radios even in light of recent price increases. The NAV/COMS range from about $1,600 for TKM to about $2,000 for NARCO. COMMS from about $900 for TKM to about $1,600 for NARCO.

Not every aircraft owner can benefit from the current collection of slide-in direct replacement radios, but those that can should strongly consider them. Slide-in radios won't make your avionics shop very happy but their relatively low cost leaves a lot more money to give them the business on that GPS or autopilot you always wanted.


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Written by Jim Kantor, President, Eastern Avionics Intl., Inc.
1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Eastern Avionics International, Inc.
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