stuck ident?

Bob Noel

Bob Noel

Touchdown! Greaser!
PoA Supporter
Joined
Jun 7, 2008
Messages
23,095
Display Name
Display name:
Bob Noel
Saturday I flew around for about an hour and was headed back to KBED
when ATC complained that my transponder appeared to have a stuck ident.
So I cycle the transponder and continued to KBED. Later the tower
said that my transponder appeared inop

I flew this morning and KBED tower complained that my ident was
stuck about the time I got to mid-field downwind. The controller
said the transponder was ok on departure.

Tomorrow morning I'll talk to my avionics shop but in the mean time,
any thoughts? Could it be overheating?

Thanks
Bob
 
The Narco AT50 in my plane has exhibited this behavior a few times. Each time, it was after the plane sat on a ramp for several hours on a hot day. Based on this, I believe that the AT50 is overheating.

Bob Noel said:
Tomorrow morning I'll talk to my avionics shop but in the mean time,
any thoughts? Could it be overheating?

Thanks
Bob
Click to expand...
 
If you have the KT79 transponder it will be the switch. Also the KT79 looses the VFR switch too. If they don't click, they are on their way out.

Kevin
 
Before you spend the bucks to have the shop look at it. Go to radio shack and buy a can of contact cleaner. With the master off, spray the cleaner around the button and push it in several times. If it is the switch, this might take care of it, if it doesn't, then take it to the shop.
 
If the feller what designed it had any smarts at all, (s)he'd have put a debounce digital circuit between the switch itself and the rest of the transponder circuitry. Since there doesn't seem to be any correlation between pushing the ident and having it stick on, my best guess is that the digital circuitry between the switch and the rest of the circuit has a goosey (that's a technical term, you'll get used to it) chip that is thermally sensitive and will latch on when either hot or cold.

On the bench, finding the thermally defective device is trivial with a can of liquid cold or a soldering iron / shrink gun.

Jim
 
Last edited:
The word from the shop fixing the KT-76 is that there was a bad capacitor.

I expect the transponder back soon.
 
> BTW are you flying with a loaner unit?

negative. :-(
 
Bob Noel said:
The word from the shop fixing the KT-76 is that there was a bad capacitor.

I expect the transponder back soon.
Click to expand...

Small world... I'm the guy that repaired your transponder. The capacitor was part of an RC circuit that controls the timer for the ident circuit. It also controlled the turn-on delay when turning the unit on. As the capacitor warmed up, it would fail and prevent the timers from completing their cycle.

Mike
 
mvferrer said:
Small world... I'm the guy that repaired your transponder. The capacitor was part of an RC circuit that controls the timer for the ident circuit. It also controlled the turn-on delay when turning the unit on. As the capacitor warmed up, it would fail and prevent the timers from completing their cycle.

Mike
Click to expand...

Well, Howdy.

Hopefully it'll be back in the airplane soon and the weather will
cooperate. It'll be a chore, but I guess I'll *have* to find
time to fly for at least a couple of hours, multiple times,
in order to fully evaluate the fix.

:)
 
mvferrer said:
Small world... I'm the guy that repaired your transponder. The capacitor was part of an RC circuit that controls the timer for the ident circuit. It also controlled the turn-on delay when turning the unit on. As the capacitor warmed up, it would fail and prevent the timers from completing their cycle.

Mike
Click to expand...

Mike:

Welcome to PoA!

We could sure use your perspective on issues, so stick around.
 
Thanks...! I do monitor several forums, including this one, on a regular (daily) basis. Always looking to learn something new. I'll try to post my two-cents worth whenever I can add some value to the discussion...:smile:

Mike
 
mvferrer said:
Thanks...! I do monitor several forums, including this one, on a regular (daily) basis. Always looking to learn something new. I'll try to post my two-cents worth whenever I can add some value to the discussion...:smile:

Mike
Click to expand...

If you're not already there, BeechTalk.com often has some fairly active avionics threads. In fact at one point a avionics specific sub-forum was being considered.
 
mvferrer said:
Small world... I'm the guy that repaired your transponder. The capacitor was part of an RC circuit that controls the timer for the ident circuit. It also controlled the turn-on delay when turning the unit on. As the capacitor warmed up, it would fail and prevent the timers from completing their cycle.

Mike
Click to expand...

I finally got to fly and check out the repair. After 1.8 hours of flying
ATC still had no problems with my transponder. So, it looks good.

Thanks
BOb
 
Very cool! An avionics guy! Thanks for checking in, Mike - lots of us want to learn more about electrons.

Care to share the basics of the diagnostics on this? I presume the cap. wasn't toasted (visually), if it still worked intermittently.
 
Glad to hear the repair is holding up. Hopefully, you'll get several more years of service.

Regarding the troubleshooting procedure, nothing magical but you do have to understand how the transponder works. In this particular case, the same R/C timer is used for turn-on delay and ident. The cap was simply leaky enough to prevent the voltage from rising high enough to signal a TTL logic 1. On first startup, it would work OK but after it warmed up, it would start to fail.
 
mvferrer said:
. The cap was simply leaky enough to prevent the voltage from rising high enough to signal a TTL logic 1. On first startup, it would work OK but after it warmed up, it would start to fail.
Click to expand...

Wow. I haven't seen a "leaky" cap since the days when they were made of tinfoil and wax. Was this a 'lytic?

Jim
 
weirdjim said:
Wow. I haven't seen a "leaky" cap since the days when they were made of tinfoil and wax. Was this a 'lytic?

Jim
Click to expand...
Quite certainly given that it was part of a long period time delay. And given enough heat most any electrolytic can fail.
 
Electrolytic... 220uf @ 16V. Transponder is 30-40 years old.
 
gismo said:
Quite certainly given that it was part of a long period time delay. And given enough heat most any electrolytic can fail.
Click to expand...

With most of the stuff we design into avionics having an 85°C (185°F) rating, the pilot should fail long before the capacitor. {;-)

However, an alternate freeze-bake cycle for forty years of winter-summer will take its toll on nearly everything, capacitors AND pilots.

Jim
 
You must log in or register to reply here.
Back
Top