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The SL40 760 Channel VHF Comm

First let me give you some background on the (16747 bytes) IIMorrow has been in aviation for years. They pioneered the easy to use loran "C" units such as the 612B, 618TCA and many other friendly units. Several years past, IIMorrow was purchased by a company called UPS. You may have heard of them. My guess they wanted IIMorrows technology to track their trucks. I do wonder if the folks working in Salem Oregon have to wear those brown outfits. From this dealers point of view, IIMorrow has grown from a medium size company to a large organization. Years prior, I used to know just about everyone who worked there. It's definitely different there nowadays. I remember the days when a real person would answer the telephone! I've found my representative, Wayne McGee, to be very helpful. I still hate their phone system because I feel the first person you talk to should at least have a pulse but that's my view. Many years ago IIMorrow came out with a VHF com but it didn't sell too well because of the market in those days. They tried to compete with King Radio, Narco and Terra. Back then those manufacturers had the market sewed up because they had a great product and excellent dealer/owner rapport. There just wasn't enough market for the new IIMorrow com. Also, the early com IIMorrow had didn't really have any bells and whistles such as the SL-40 has. Things are different today. In my opinion, Narco has alienated itself from the dealers and the customers. Terra has just been purchased by Trimble, we will see what happens there. King still produces a great product when you can get it. Lead-times are terrible on most King products. Also, King hasn't changed their nav/com market since 1981 as far as features go. Units in stock, new features and fair prices are what the aircraft owners want and IIMorrow seems aimed in that direction. My crystal ball tells me IIMorrow is thinking about a full line of avionics for the average piston aircraft. Who knows what great things those squirrely software engineers are dreaming of at night.

The SL40 arrived the same day the customer showed up to install the VHF com. The aircraft was a Cessna 152 with a 28Vdc charging system. The factory RT-385A nav/com had been a problem radio for the three years he owned the aircraft. In fact radio repairs were running more than the maintenance. This doesn't surprise me, I know Cessna radios. I convinced the customer to remove the SPA-400 ICS which seemed to be a problem child. With different manufacturer's headsets the audio level would drop too low to hear each other on the headsets. The SPA-400 seemed to work marginally with same manufacturer headsets. Also, the ICS seemed to pick up alternator noise at low RPMs. So out came the radio and the ICS. We removed all the wiring down to the circuit breaker and removed the aircraft jacks. We were getting ready to install the SL-40 with a 2001 re-worked GPS. The VHF com antenna and coax were replaced, proper circuit protection was installed and the GPS antenna was installed. Now was the time to open up the SL-40 box.

The packaging was received in good condition, courtesy of UPS. We found the SL40 to be well protected inside of anti-static wrapping. IIMorrow provides a "Package Contents" that tells us everything that should be inside the box. Fortunately, everything was there. Our measurements showed the SL40 to be 1.30"high, 6.25" width and 11.45" long. The width is industry standard but the unit is small. My first impression was would I be able to see the display of this tiny thing. Us shops love little avionics, we can install more! Weight was a stingy 2.1 lb. I've eaten hamburgers that weighed more than this, "after cooking" of course. The SL40 meets every TSO and FCC regulation I could find. Next was a general over view of the installation manual. The installation manual is one of the best on the market. I have no doubt IIMorrow hired a avionics installer to write the manual. The print is big and in plain English. The wiring diagrams are excellent, easy to read. Maybe part of the reason for this is so any home-builder or A&P could easly install the SL40. I quickly noticed that shielded wiring was required where it should have been in the wiring diagram. The wiring diagrams and installation steps were so easy that I tried to find mistakes but I failed. One would have to really try to screw up the installation. The pins in the "D" connector are the crimp type and a special tool is required to do the crimping. I've found the crimp pins are best and all avionics shops have the proper tools to crimp the pins. If the installer was in a situation where the crimp tool wasn't available, a solder "D" connector could be used. The solder connector is not provided and only should be used as a last resort. IIMorrow even supplies the Allen tool you will need to install and remove the radio from the rack. The rack assembly is strong and high quality.

I couldn't help but tear the radio apart. Off came the covers and the face plate. What I found inside was "typical" IIMorrow. The technology and quality was there. It appears to be one of the best assembled radios on the market. The knobs feel like knobs should, the buttons feel like buttons. If the SL40 ever fails it would be best to send it back to IIMorrow. The insides of the SL40 isn't the place to be working with a blow torch and nail. Back together with all the parts. I noticed the SL40 had plenty of inside room and provisions for some more connectors. You can bet IIMorrow is going to be stuffing a GPS receiver in this jewel later. It goes together as easy as it came apart. So far I haven't broken anything (which is unusual).

Next step was to hand it to my chief installer, Tom Knoll. Tom doesn't like anything new so it was going to be interesting on his opinion of the way the SL-40 wired and integrated with the 2001 GPS. Within a three hours he had the complete harness wired including the 2001 and ready to install in the Cessna. Next he mounted the SL40 and 2001 racks, antennas and ran new coax throughout. Power was run to the circuit breakers, single point grounding was incorporated. We didn't install cooling, IIMorrow said it wasn't necessary, something about the thing is so efficient very little heat is generated. The SL40 uses about third the power of other coms in the same league. Someone has done their homework on designing a high tech com. After the installation was complete, the APU was plugged in and the switches were turned on. My thoughts about the display being too small were soon gone. I love the display, it's easy to read and clear. The LCD display coms aren't even in the same league. I seldom get excited, especially over a display but you would have to see the SL40 display to understand. It's the best on the market. I personally like it better than the gas discharge displays that the upper end radios use. The little com has a photo cell that darkens the display when needed.

Now what features does the SL40 have? Grab a cup of coffee, the feature list is long. The display shows the active and standby frequencies at the same time. You push a button and the frequencies will "flip/flop, just like the high end radios. You turn the large knob and the MHZ will channel, turn the small knob and the KHZ will channel. The com transmits and receives from 118.00 to 136.975 Mhz, 760 channels. This is typical of any modern VHF com I did find the power output was 8 watts across the band. This is plenty of power for any VHF radio. The power/volume/squelch knob is all in one. As you rotate the on/off switch clockwise, the volume will increase. Pulling out the knob will open the squelch. Another neat feature! You can set the squelch threshold via software! No more bench adjustments (more on this later). If one presses the (EC) button, 121.50mhz is loaded into the standby frequency slot and the monitor function is enabled. Oh, did I tell you about the "Monitor" feature? Sit down for this one; this is one of the highlights of the SL40. As with most radios, the receiver listens to the active frequency. If you push the (MON) button the receiver will now monitor the standby frequency. If any activity is on the standby frequency you will hear it. Should activity begin on the active frequency, the radio will automatically switch to the active frequency. A small (m) is presented by the standby frequency when the monitor mode is enabled. A small (<>) will point to which frequency you are listing to. This feature is a great idea in a single radio aircraft. One could stay with tower and listen to ATIS at the same time. As far as I'm aware, this is the only general aviation panel mount VHF com that has this feature. By pressing the (MON) button again, the monitor feature was disabled.

If you press the (RCL) button and turn the large knob you will display the auto stack list . Now turn the small knob and you will see the last eight frequencies you stored by pressing the (MEM) button. This is a easy way to call up frequencies you use a lot. I personally found it was faster just to manually tune in the frequency in the standby slot. Another highlight of the SL40 is it's ability to receive the weather band. Just press the (RCL) button, turn the large knob until it says "WTH" then turn the small knob to the proper frequency. Of course you can't transmit on the weather channel but you sure can hear the local weather. I really liked this added feature. I don't know of any other panel mount radio that has the weather band. Another feature we found is that the SL40 will read the frequencies of the airport database in the 2001 GPS. While this is a neat bell and whistle, I found it easier to read the frequency off the GPS and tune it in manually on the SL40.

We did remove the SPA-400 ICS and wired the SL40 to use it's internal intercom. I was really interested on seeing just how well the ICS would work. Most com ICS systems in the past have been poor performers. We had to install a switch in the aircraft so the intercom could be turned on. When we flipped the switch, the ICS worked better than I expected. Standby frequency monitoring is disabled when the ICS in functional. An "I" is displayed in front of the standby frequency. I found in the hanger with the engine off, the ICS worked great and the squelch was set properly. We tried keying the microphone to see if the stuck mic function worked. Around 35 seconds the transmitter quit and the display showed "StuckMic". Pretty neat feature. The SL40 seemed to work as expected on the ground with the engine off. But would it perform in the air?

The floor is vacuumed, the APU unplugged and the little Cessna is preflighted. We start the little Lycoming and turn on the SL40 and 2001 GPS. The transponder goes to standby at this point. We try the weather channel and pick up ATIS before contacting tower. I noticed the weather band and ATIS were at the same listing level, we didn't have to change the volume level. The reception was very clear but then again, we're still on the ground. We gave tower our spill and got permission to taxi out. I asked tower how the transmission sounded and they replied "5X5". Once in the run up area we tried the intercom function. I was pleased with the ICS. The squelch level was perfect with the engine at idle. We did notice a major change after our installation of the SL40 at this point. As I mention earlier, there was always an alternator whine in the headsets with the SPA-400 ICS but when using the SL40s ICS feature, the alternator whine was not present. Maybe a good built radio or maybe shielded wiring solved the problem, who knows, but the alternator whine was gone and that was a big plus. Tower says "go" so we let the Cessna develop all the power "and noise" it can. I'd forgot what it was like to fly in the 150 series. The noise is like being in a jar of bumble bees. At full power we found the ICS wanted to ever so slightly break squelch. A couple pushes of a button and we changed the squelch threshold. This fixed the problem, now the ICS only worked when we spoke. As we reduced power for straight and level, I found the ICS squelch was still good. The ICS was clear and worked better than I expected for an ICS that's inside the radio. The King KY-96A does have an ICS, well sort of, but it's a hot mic system and is poor at best. The SL40 worked and it worked better than I expected it would. True, it's not a NAT or some other top end ICS but not bad for the bucks. The ICS is limited to two stations and no stereo inputs. If you want to listen to Willie Nelson or need more than two ICS stations, then don't use SL40s ICS, install a separate ICS. The system can be easily wired without the intercom. During the flight we found the com squelch, receiver and transmitter worked great. During our testing we found at high engine RPMs and a great distance out, tower would say they heard a lot of back ground noise. Of course the Cessna 152 cockpit has about 97Db of noise. We found if we unplugged either headset the back ground noise would go away. It appears both mic audio inputs are bused together so the transmitter sees both mics when either is keyed. I wish IIMorrow had installed boom mic isolation inside the SL40 but it appears that isn't the case. This isn't a big problem but I wish it had been addressed. In a quieter aircraft the problem wouldn't have been noticed. The knobs and push buttons worked excellent. They felt natural to use even in turbulence. The display was easy to read in all lighting conditions. We tried the frequency storage the SL40s has and this works along with the down load of frequencies from the 2001GPS but I found it easier just to manually crank in the frequencies. I was really pleased with the little SL40 after we flew with it. I'd never sell anything I haven't flown and I will sell the SL40 in the future.

Once on the ground we installed a "boom mic isolator" to cure both mics being on at the same time. The next test flight took care of the back ground noise. Tower commented that we now sounded like an "airliner"! Funny what a $30.00 part will do. What do I think of the SL40? Will I push the product? To answer both questions, I'll sell the heck out of the thing. It's great for fleet owners who have 12&24Vdc aircraft. It works great, everything feels like it should and it's fairly easy to install. Expect to pay around $1,300.00 for the SL40 but this is excellent value for product. There are some other small features I didn't go into because of space but this product is the wave of the future. UPSAT hit a home run with the SL40. I'd love to see them come out with a complete radio stack that has the quality of the SL40. If they ever fire the guy who wrote those fantastic installation and owners manuals, I'd hire him in a heart beat
This article by Tom Rogers originally
appeared on and
is republished here with permission.

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