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Mode C Problem

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chasb(at)satx.rr.com
Guest

Posted: Sun Feb 15, 2009 12:04 pm Post subject: Mode C Problem

Mode C problem 



My Mode C is intermittant and I cannot figure out why. 

 

I have an Apollo SL-70 transponder hooked to a Trans-Cal 120-30xx solid state 

encoder. The two are hooked together via an Approach Systems cable system. The 

mode C seems to work as advertised on the ground, but becomes more and more 

intermittant as the airplane goes to altitude. At altitude, the Mode C quits for 

a couple of minutes, works for a minute or so and continues to cycle at a 

variable rate. When the Mode C is transmitting at altitude, it sends correct 

altitude data. My static system is tight as confirmed by a pressure test that 

stays constant for nearly 30 minutes. The cable connections seem tight and 

wiggling the cable and other wires behind the panel has no effect on the Mode C 

operation on the ground. I had the same problem with an AmeriKing AK-350 encoder 

which was recently replaced by the new Trans-Cal encoder. 

 

I am beginning to suspect the SL-70 though the Garmin (formerly Apollo) folks 

insist the problem is with the encoder. 

 

Any suggestions would be appreciated. 

 

Charlie Brame 

RV-6A  N11CB 

San Antonio

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floyd wilkes

Joined: 11 Jun 2007
Posts: 39
Location: spring branch, tx

Posted: Sun Feb 15, 2009 12:35 pm Post subject: Mode C Problem

Charle,



If you are using grey code, it sounds like one of the pins at the 

transponder is bent.

I had the exact same problem and it was a bent pin.



If you are using serial, disreguard the above.



If you need help, come see me, I am next hangar to Brian and Jim at Kestrel.



Floyd Wilkes

601XL



Do not archive

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_________________
Floyd Wilkes

n744bh(at)bellsouth.net
Guest

Posted: Sun Feb 15, 2009 12:53 pm Post subject: Mode C Problem

Charlie,



I had a somewhat similar problem with my transponder and found out it was a 

bad finger in the tray connector.  It caused me to pull my hair out finding 

it but when I looked closely at the connector with the transponder out I 

noticed one of the fingers in the connector didn't look quite right and when 

I checked with my avionics shop he looked at the pin readout and told me 

that that particular finger could certainly cause the problem I was having, 

which was the mode C was intermittently going way off altitude.  Once I 

pulled out the connector and replaced that finger the problem was solved.



Try pulling out your transponder and take a careful look at the tray 

connector.  This may not be the problem but it's one more place you might 

look at.



Bill,

Glasair SIIS-FT



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skywagon

Joined: 11 Feb 2006
Posts: 184

Posted: Sun Feb 15, 2009 2:38 pm Post subject: Mode C Problem

Is the transponder antenna "clean"....?, And well connected, grounded, etc.

Engine oil can cause power emission troubles as it is a bit conductive at 

those high frequencies.

D

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