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The SL70 Apollo Transponder
Transponders have been in general aviation radio stacks for decades. Basically they all do the same thing and that is show ATC on a screen our location and altitude. The SL70 is still what we call a (4096) transponder, meaning it has the capability of displaying up to 4,096 different codes in mode A operation as well as mode C altitude outputs. By the way, the SL70 mode C has the capability of reading from -1,000ft up to 63,000 when used with the proper encoder. We will not be testing the upper limits of the SL70, our Cessna 182 doesn't fly well at 63,000ft. Guess we will just have to take Apollo's word for it.
Is the SL70 just another transponder? Well, yes and no. Yes, it
works off the same principals that transponders have for years but the SL70 has
some features that no other transponder has today. Of course the SL70
meets TSO-C74C which is the highest TSO for general aviation transponders. The SL70 has built-in
diagnostics that constantly monitor the
operation of the internal electronics. 
No other transponder that I'm aware
of has this
feature. Just how good is this feature? Well listen to this.
We recently removed a tired old Narco AT-50 and installed the SL70. The owner
elected to install a new transponder instead of continually dumping money into
the dinosaur Narco. Seconds after the SL70 was turned on, it read
"TX Failed" in the window. Of course my first thoughts were the
transponder had failed. fortunately, I had another SL70 in stock and
installed that one. We had the same results, the display showed "TX
Failed" again. Now I was fuming! How dare IIMorrow ship me two bad
transponders. The design engineer at IIMorrow said the problem was
probably in the antenna system. He went on to say the transponder
transmits a signal down the coax cable to the antenna and looks for a
certain amount of reflected power. Personally, I thought he was blowing
smoke but I agreed to check out the system. Sure enough, the engineer was
correct. We found a badly corroded coax BNC fitting on the antenna.
Replacing the bad BNC fixed the problem. I wouldn't have believed it if I
hadn't seen it with my own two eyes. Now the SL70 passed
"self-test" and was ready for action. The SL70 performs all
types of self-test during flight just to make sure everything inside is up to
speed. We later found that we could read out the fault and figure just what
went wrong in the transponder system or transponder. Another unique
feature of the SL70 is it's ability to accept standard "Gray Code",
which is supplied by older style encoders or "Serial Data" input,
which is less prone to problems and easier to maintain. Gray Code takes 10
lines to hook up and Serial Data takes two. In this case, less is
better.
The SL70 has a Serial Data Output. If Gray Code "normal mode C" is going into the SL70, it has provision inside to transponder this code in to Serial Data and transmit it back out. Where would you use this? As you may be aware, IFR GPS units require altitude information from the encoder; some only want Serial Altitude such as the SL50/60. The SL70 can provide that Serial Data for the GPS. Now don't get too excited, yet. Even though the SL70 has Serial Output, it will not work with all GPS units. We found this out the hard way. Garmin and some other GPS's will not work with the SL70 Serial Output even though the baud rate is adjustable. We found out the problem had something to do with packets, whatever that is. No doubt the SL70 Serial Output will work with all of their products. Be sure to discuss this issue with your avionics professional before breeding it to other equipment such as the GPS or TCAD.
IIMorrow's SL70 input voltage is 10-35Vdc. This is one of the few
transponders that do not need a dropping resistor when using 28Vdc. 
We
found on the bench the SL70 drew only .65 amps at 27.5Vdc, which is about a
third of the current needed for most transponders. The SL70 is 1.3" high,
6.25" wide and 11.45" long including the connector. It weighs a
mere 2.64 lb. and does not require any external cooling. One thing I
really like about IIMorrow, all of their products are well engineered.
All draw very little current, work off of 14 or 28Vdc and are small.
Output power measured a whopping 295 watts on the bench. This is the
highest I've ever seen out of a general aviation transponder. The SL70
does not incorporate a cavity "tube" like older transponders do.
Cavities are prone to failure after five years or so and often need
alignment. The SL 70 is ready for operation within a few seconds after
turning on. Keep in mind though that most encoders need five minutes or
more to warm up. The bright LED display is easy to read and intensity
adjusts automatically via the photocell located in front of the transponder.
SL70 performance on the bench is the best I've ever seen and quality is
everywhere you look; from the rack to the metal bezel.
Let's go over the some of the ":Push Buttons" on the faceplate. To turn on the SL70, simply rotate the on/off knob. There's nothing new with the "Ident" button. The "Ident" button and the Reply lamp will go on full bright for around 20 seconds and a rely pulse is sent to ATC when ever you press the "Ident" button on the panel. The SL70 can be externally wired for remote indent should you desire. By pressing the " SBY" button, the SL70 will remain in the standby mode until another mode is selected. A LED digit above this switch will be illuminated when in the standby mode. Even though the squawk code and altitude are displayed during the standby mode, none of the data is being transmitted to ATC. If you press the "ON" button, guess what. The SL70 now transmits information if interrogated by ATC. Press the "ALT" and you can turn off your Mode C if requested by ATC. There is an LED lamp over this button showing you when you have "ALT" turned on. Now here's a nice feature. Finally ATC gives you the word to "Squawk VFR". With the SL70 simply press the "VFR" button and the transponder automatically displays and transmits 1200. You don't have to fiddle with the unit to get the famous 1200 code to be displayed. The next button over is labeled "HLD". Here's how it works. Press the "HLD" button to set your current altitude as the hold altitude whenever you level off. The LED over the "HLD" button will light, thus the pilot knows he or she is in the Altitude Hold mode. Not only does the SL70 display show the pilot the normal Mode A codes, it also shows the altitude readout of the encoder based on pressure altitude. When the "HLD" button is pushed, the altitude display now shows a value that is relative to the hold altitude in 100ft. increments. In other words, if you drift up 100ft, the SL70 display now shows +001. The same holds true if you get below your altitude that was displayed when you hit the button. The SL70 Altitude Hold feature has a buffer. Simply press the "HLD" button two seconds or longer and turn the small knob to change the buffer value. Factory default is 300ft but the buffer can be anywhere between 200-2,500ft. If you need a larger buffer than 2,500ft, I'd suggest you get some dual time before blasting off again. When your altitude exceeds the threshold of the buffer, the display will flash. Selecting a squawk code is somewhat awkward. Here's how you do it. First, rotate the large knob all the way to the right of the transponder one click. The first character of the squawk code will flash. Now rotate the small knob to get the number you desire. Rotate the large knob again to move to the next digit and again rotate the smaller knob to select the proper number. Continue the same procedure for your next two selections. I wasn't impressed with the way the SL70's mode A squawk code is entered. It took me three times as long to enter the same code as compared with a mechanical transponder that has four knobs for selection of the proper mode A code. I'll agree, once the code is placed in the transponder you seldom have to change it. Squawking VFR couldn't be easier, just push the "VFR" button and the SL70 display shows the famous characters "1200". At this point I was anxious to see just how easy this transponder would operate in the air.
The first thing I noticed was anytime power is interrupted to the
transponder it will revert back to the standby mode. You have to press
the "On" and "ALT" button each time you turned on the
avionics master. For us pilots who are used to leaving the transponder on
and expecting it to be in the same position next time, does take some getting
used to. You will probably want to add "Turn on transponder" to
your check list. I'm sure you use your check during each flight... During
my flight I found the "reply" lamp was hard to read. I'm used to
the huge reply lamp like is in the King KT-76A transponder. Sure the SL70
lamp blinked but you have to look for it. The display is easy to read and
dims automatically via a photocell. I found no problem reading the display
under all lighting conditions or using polarized sunglasses.
The altitude
display was great. I enjoyed seeing the transponder displaying pressure
altitude; this way I know my encoder is working. By the way, how many of
you remember how to convert your altimeter setting to pressure altitude?
Finally I leveled the Cessna 182 off at 3,500ft. I set the altitude buffer
for 300ft and pressed the "Altitude Hold" button. As I changed
altitude the display did in fact show me the difference in the altitude of when
I pressed the "Altitude Hold" button and the altitude I was at the
time. In other words if I climbed to 3,700ft, the display showed a
+002. When I deviated more than 300ft the display on the SL70 flashed
warning me I had exceeded my programmed threshold. The blinking stopped
when I came within the threshold setting. While this is all well and nice
it isn't the greatest thing since sliced bread. Normally when we bust an
altitude it's because we are busy with other cockpit chores or fighting up and
down drafts. During these situations the pilot does not have the time to
look at the transponder. If I had time to monitor the transponder altitude
alerter then I would have used that time to look at the altimeter in the first
place. What the SL70 needs an audio output, a large hammer or some other
means to notify the pilot that he/she has violated the altitude threshold and
they had better straighten up and fly straight. During my four hour flight
I played with the altitude hold feature but was somewhat disappointed in what it
did for me, the pilot. Another disappointment I experienced was trying to
select a squawk code in turbulence. The knobs are small and the knob
friction just isn't enough when it gets rough in the cabin. It's too easy
to over shoot the code or position you want to crank in . I found the only
way I could get a code in the SL70 during moderate turbulence was to hold my
right hand steady with my left hand along while muttering a few choice words.. Now
try doing this without an autopilot, it's a real joy of a job. The knobs
need more friction and should be larger if you are going to use this type of
entering method. I agree, you seldom have to change codes during a flight
but still... The Beech A-36 driver with the radio stack all the way
to the right of side of the panel would have a real challenge changing the codes
on the SL70 on a bumpy day. But then again, a Beechcraft owner would probably install a
autopilot to cure this problem...
Clearly the SL70 has some great features such as 10-35Vdc input, low
current draw, gray code or serial data input and a serial data output port just
to name a few. Quality for the dollar you'll be hard pressed to find
a better transponder than the SL70. Cockpit ergonomics and pilot ease of
operation could have been better in my opinion. The SL 70 can only be
sold by the installing dealer unless it's for a "Home-built"
aircraft.