Reminds me of something I did on my plane. As an electronics designer, this would be too embarassing to tell this story, but hopefully it will save someone else from doing the same thing.
When I bought my M2 I knew that it needed a transponder test before I could operate mode C legally. Wanting to make sure it at least worked before I brought it to a shop to pay for a static test I thought I would check it first. Went up a few thousand feet and called a tower to confirm that they could read my code. Then switched on Mode C and they had no altitude report. Tried again the next week and same thing even though the guy I bought it from said it worked fine. Pulled the transponder and brought it to a friend that had the test equipment and he confirmed it was fine so must be the encoder. No problem, I had a spare. Put it in the plane, called a tower, turned on Mode C, no altitude. Arrrgh. Bought a used one from Wentworth and tried it, no luck. Had them send me another, still no good. Transponder and all wiring good so I decided that I must be overlooking something so I built an encoder test set (fairly easy with just resistors and LEDs to read the grey code). Hooked it up with tubing through a T going to the encoder and an altimeter so I could read the code and verify it was correct. Powered up and altitude read something like 200' and would not change on all three encoders, weird. While pondering what could be wrong all of a sudden the altitude changed and I verified by pulling vacuum with the syringe that it was correct. Turns out that the old style encoders need the temperature to stabilize for about two minutes before they start sending altitude. Not wanting to be reporting without knowing if it worked I had been turning the encoder on just before asking for an altitude check and the unit was not warmed up yet. Live an learn. Anyone want to buy three tested good encoders? Brian Kraut
