(Avionics) Narco AR-850 Altitude Encoder Teardown

[KG7AMV]

Found this sitting on the shelf today! A couple years back my Narco AR-850 Altitude Encoder Failed (Stopped reporting Altitude) on My Cessna 150. The Marco Company is Defunct no one "but me" can legally repair it! Yes, I can fix it and have it checked by a avionics tech aka repair station. At the time I had no time to do so it was cheaper to replace than travel home get tools and diagnose and in aircraft parts pricing replacement at wholesale it was cheap under $200.00! So I replaced with a modern Unit!


If anyone has any info on this unit please post would love to diagnose and repair it! I did find the Narco Patent Info rather limited info.
https://www.google.com/patents/US5247465?dq=Narco+Altitude&hl=en&sa=X&ei=FHh1VN7GI6_UiALHpIGQBA&ved=0CB4Q6wEwAA

On the photos right click view then back link for larger version. My little point and shoot cam sucks! I know but have not seen any info on these on the net so said why not!


Narco AR-850 Altitude Encoder


Narco AR-850 Altitude Encoder Side Profile...


Guts (Little Plug for Snap-on haha)


PCB Bottom, Lots of flux! (They Need to learn to clean the board!)


PCB Front, Lots of flux!


PCB 1 Caps are RMRM no telling who that is!


PCB Side Profile


Heater, What Usually Fails from what I Here from avionics tech's ELSEN WA098 POCU97C temp sensor on the bottom is cooked there is another part in the heater from the traces will disassemble and see what it is.


Chips, As soon as I find the box I have my Xeltek Superpro V in I will dump the 2716 eprom.

[SeanB]

Well, it uses a 7106 in ratiometric mode to read the pressure sensor, and likely the eprom is there as cheap glue logic, taking some of the digit outputs ( and the backplane) and simply does a lookup to 8 bit altitude data. The ULN2003 buffers then feed this Grey coded data to the transponder as altitude that will be sent on demand. Good enough for non pressurised flight use, as it will read from sea level to 25600 feet. Daughter board will contain the pressure sensor, with the transistor heater to keep it stable. If the voltage regulator IC's are not dead ( one will give 9V for the converter, the other likely is a 5V one for the eprom) then the fault will be the heater has died and cooked the sensor. Easy to check all those, the sensor will be fed to the 2 opamps to get a larger signal, both likely are used as gain blocks to feed the ADC with a 2V signal.

When I saw the title I was expecting one of the units I used to work on occasionally, where you had an aneroid capsule, gears and display with an optical block that used a coated glass grey coded disk and a small lamp and phototransistors to get the data encoded for digital use.

[KG7AMV]

SeanB,

We have 2 regulators 7805 5V & 7808 8V (When I disassemble it further I will check them)

Daughter Board indeed has 2 LT1008C Op Amps, 

Not sure what is inside the pressure sensor on the daughter board. The Patent says MPX100A. I will take it apart to find out!

MPX100A Data Sheet. http://pdf1.alldatasheet.com/datasheet-pdf/view/146713/MOTOROLA/MPX100A.html

I will assume the 2n6387 transistor is used as the heater for the pressure sensor.

[SeanB]

Probably will be that. What was the original fault? No reading or zero or full scale output, or a varying output. My bets are on the regulators being faulty, the rest is pretty robust, though if the 8V regulator dies it might have killed the 7106. Varying reading with good supplies likely a capacitor leaking or an opamp that is noisy or drifting.

[KG7AMV]

If I have some time while dinner is cooking I will apply power and check the regulators. Happy Thanksgiving!

[KG7AMV]

I got around to playing with this. The Voltage Reg's checked out! The 4.7uf 50V cap and the 47uf 50V bottoms were swollen the 4.7uf on the 7808 was leaking as you can see in the photos. It now has a combo of what I had in the drawers. Panasonic 4.7uf 50V & Nichicon 47uf 63V 

[Howardlong]

Somewhat coincidentally the mechanical encoder broke in my Cherokee earlier this year, above about 6,000' the readout was intermittent (as opposed to erratic, it just didn't report sometimes), and then above 8,000' nothing at all was reported.

Unfortunately not being an accredited tech, and not having the pitot gear to test it anyway, I left it to a commercial shop... who concurred with my findings, and completely disconnected it and sent me a quote for either $750 for a direct mechanicsl replacement or $1,500 for an up to date electronic unit. Regretfully I went for the latter.

Unfortunately, here in the Uk once a certified tech finds a fault, it becomes reportable and they have to disconnect it completely even though it was working for the vast majority of cases: here in the South East Uk, the opportunities for flying VFR much above 4,500' are pretty limited due to massive swathes of class A airspace above.

I wish I'd taken a look at it myself after the event, who knows, a simple bit of contact cleaning on the mechanical encoder might've been all that was needed. Next time I'll know better, these days your average shop won't be interested in a repair.

[KG7AMV]

Howardlong,

I have Delivered a bunch of aircraft to the EU and UK they have some strict rules on that side of the pond!

 

[amyk]

Bad electrolytic caps in avionics? That's a bit... scary.

[KG7AMV]

Some of the older Avionics do not even have solder mask on some of the boards! I grab a photo of my King KR-86 ADF next time I can get one!

[Howardlong]

Howardlong,

I have Delivered a bunch of aircraft to the EU and UK they have some strict rules on that side of the pond!

I am more than aware of that, without going into any particular details, many avionics problems with 20 odd year old kit can be resolved mechanically with a bit of internal switch cleaning or other similarly benign repair, but nobody's interested these days in doing that, it's a replacement 4 figure unit.

Sometimes it's a case of see no evil, hear no evil, so I am told. Cough.

On the basis that I temporarily had a completely airworthy aircraft with absolutely no radar altitude reporting capabiltity at all, rather than one that worked 99% of the time, I would say it was rather absurd, but there ya go.

[Howardlong]

Bad electrolytic caps in avionics? That's a bit... scary.

It's not that big a deal to be honest, there's so much redundancy built in that it becomes little more than a minor inconvenience when something fails.

I've had two complete radio failures in a year, it's a pisser, but you work around it. That's what 7600 is for on your transponder, it wakes up a lot of people with feet on their desks, that's for sure,  it's always nice to be greeted by fire trucks.

Edit to add, the radio failures were not in my a/c!

[LabSpokane]

Bad electrolytic caps in avionics? That's a bit... scary.

It's a Narco.  And it's from the olden days.   Narco was always a budget brand typically used in VFR light aircraft.  A flaky encoder is pretty common on older planes and the flight controllers will generally have the pilot just call their altitude.  No biggie. 

I'll let the avionics techs tell you about the differences in the guts, but the better brands like Bendix/King are built to much better standards. You could literally hear the difference between the old King and Narco radios. This isn't a slam on Narco, it's what we had in our plane.

Don't worry about too much modern avionics. That stuff is great nowadays.

[SeanB]

There is a big difference in quality and an even bigger difference in price. Bendix would use essentially hand made brass mechanisms to make instruments, and those are repairable with the right equipment. The electronics are also ultra reliable, with glass sealed capacitors, gold plated connectors and every board fully conformal coated with the heavy parts tied down or bonded with a blob of flexible compound. All IC's are glued down as well, and you have all enclosures hermetically sealed with a very good seal.

The major problem is the electronics have to be repairable for a decade or three, and often the parts will be obsolete and hard to obtain.

[KG7AMV]

LabSpokane,

Ummm Personal Experience having a Garmin G1000 Total Failure (Black Screen) over the North Atlantic in a 172! Partial Panel IFR over the ocean and Greenland icecap is not fun! Only radio I had was my HF! With a DH of about 4,000' AGL and ceilings of 3,000OC makes for a fun approach following a Sikorsky rescue helo till runway in site! 

[LabSpokane]

Steven,

We flew war surplus.  My idea of ultra modern is a KX155.   

And who the hell puts a glass panel in a 172?!  I thought you just duct taped an ipad to the panel for that? (Kidding!)  Ferrying over the ocean single engine IFR is ballsy shit. Respect.

Just as an aside, have you considered putting in a BNC jumper to the antenna so that you can plug in a handheld as a backup?  I know it's "wrong" but I used to do that with my ELT antenna and it worked well enough.