Transponder failures

the400kid

the400kid

Pre-takeoff checklist
Joined
Sep 2, 2013
Messages
131
Display Name
Display name:
FLAAV8R
Over the past 2 years I've had two transponder failures. The original KT76A bit the dust and I got a used KT76C because I didn't want to have new wiring and tray installed. The 76C worked great for about a year then crapped out. I then had a brand new Trig TT31 installed and it stopped reporting Mode C last week to the point where ATC can't see me, although the unit is very much alive based on the display and pressure altitude reading. At this point I'm thinking that it's the antenna or coax and am taking the plane in next week to be looked at.

Question: Could a bad antenna or short in the coax cause a transponders circuitry to fail?
 
I have no qualifications or specific knowledge about this... I'm a mechanical ME guy, not an EE

but just for brainstormin' purposes..... The first thought I had was power, the second thought I had was antenna.
because more than likely these were shared with all of the failures.
noisy power, low voltage, high voltage, spikes, that kind of thing.... maybe intermittent and hard to spot...
and on the antenna side...like you suggest...maybe a short that causes some sort of funny business with the transmitter power
 
1) I’m not an A&P or Avionics guy
2) Maybe a bad or poorly connected blind encoder (depending on the symptoms)?
3) Maybe a lot of oil, etc on the antenna?

I may be wrong here but I think the Trigs actually have an internal encoder even though they still need to be connected to an external one. I can’t see that in the specs or install manual but I seem to recall it coming up when dealing with either my Trig in my gyro or, more likely, the control head for my FreeFlight RANGR (which uses a Trig head). That could explain why the Trig could be reporting an altitude on the screen even if not broadcasting one to ATC
 
Yes.

A short or high resistance in the coax/antenna could cause the transponder to fail. The transmitted power has to go somewhere and if the coax or antenna is limiting/blocking/shorting the output, then damage can occur to the transponder (radio).
 
Llewtrah381 said:
2) Maybe a bad or poorly connected blind encoder (depending on the symptoms)?
Click to expand...
I believe the Trig displays the PA that it is trying to broadcast to ATC. If the display looks correct, then I think this #2 can be ruled out.
 
If you had that many transponder failures, you better look closely at your antenna and coax/ connectors. Daleandee is right on.
 
Daleandee said:
Yes.

A short or high resistance in the coax/antenna could cause the transponder to fail. The transmitted power has to go somewhere and if the coax or antenna is limiting/blocking/shorting the output, then damage can occur to the transponder (radio).
Click to expand...

This pretty much confirms what I suspected. I'm trying to find a shop that can fit me in, which isn't easy these days. Thanks for the reply.
 
I have had a few transponder fails due to antenna crimps. If you still have RG-58 coaxial, I would have that replaced first and see if the problem goes away. Since you have a Trig 31, it may be using the same tray that you had with the 76A since they can all use that tray so you may have some connection issues from the tray. You cannot legally remove your transponder; only an A&P can do that, but using some ACF-50 on the connectors may help too.
 
did they install a new encoder with the trig? or did they put a new transponder in with the old piece of crap encoder?im willing to bet the encoder is a old piece of S>>>t ack that gave up the ghost. if so, get a decent new unit, and have them replace the coax and maybe the antenna also. sorry if this sounds harsh, but pilots try to cheap out and install used and old equipment to save a buck and not replace things with new units. its a false economy. it cost more in the end than if you just bite the bullet in the first place. I have seen in happening a lot lately with guys that went with a tailbeacon or wingtip ADSB solution and kept and old piece of crap tube transponder. a year so later, they are now replacing that old transponder. it would have been cheaper in the long run to just put in a new adsb transponder in the first place. I had an old at-50 in my warrior, I didnt even debate it, it got a garmin 335.
 
unsafervguy said:
did they install a new encoder with the trig? or did they put a new transponder in with the old piece of crap encoder?im willing to bet the encoder is a old piece of S>>>t ack that gave up the ghost. if so, get a decent new unit, and have them replace the coax and maybe the antenna also. sorry if this sounds harsh, but pilots try to cheap out and install used and old equipment to save a buck and not replace things with new units. its a false economy. it cost more in the end than if you just bite the bullet in the first place. I have seen in happening a lot lately with guys that went with a tailbeacon or wingtip ADSB solution and kept and old piece of crap tube transponder. a year so later, they are now replacing that old transponder. it would have been cheaper in the long run to just put in a new adsb transponder in the first place. I had an old at-50 in my warrior, I didnt even debate it, it got a garmin 335.
Click to expand...

Actually the encoder is about four years old, but that wouldn't explain why the display on the Trig is giving an accurate pressure altitude reading. However, I did have somebody take a look and they reported that the connector at the antenna end is crimped and in bad shape.
 
You must log in or register to reply here.
Back
Top