Transponder failure

It was a beautiful day on the South Coast yesterday, and we flew circuits for a bit . Halfway through a funny thing happened. The guys in the tower directed  me to reset the transponder, telling me that I’d stopped showing up. (This isn’t as bad as it seems, because I was still on radar – they had me on radar, so they were only missing my altitude, and since I was in the circuit they knew what that was). Anyway, I recycled it and tested it and it was kaput. They told me to fix it later.
Lucky for me, one of my partners fixes transponders. What I thought was going to be a hassle wasn't a problem at all.
The first job is removing the transponder from the plane. If you’re only experience is installing car stereos, a NARCO AT 150 is a treat.

The transponder is the unit at the bottom, and you’ll see there is a little hole on the bottom right of it right under the IDENT button. Guess what? There's a screw in there.

You stick in the Allan key, and that unscrews a long retaining screw that attaches the transponder to the back of the instrument panel. You can see how nicely it slides out. Not at all like my ’72 Toyota Celica with the AM/FM 8 track.

You can see the attachment point at the end of the mounting box, along with the connector.

No wire harness or plug. Instead, it connects just like a card in a computer.

Once it was out we threw it on a few testers. Surprise, surprise, it worked fine. We cleaned the connector with alcohol and re-installed it. I powered it up, gave it a little bit for the tube inside to warm up, but no go!
Of course, there was something simple I should have thought of in the first place, possibly while in the air – the fuse. I pulled it, inspected it and found nothing wrong. I replaced it and miracle or miracles, the transponder light blinked blue. Good to go!
What was it? Gremlins?
Of course not. Remember, I was flying circuits on my way to soloing. Take off, turn out, do the downwind check “Primer in and locked, masters on, mags both, fuses and circuit breakers in….”
And that’s when I "broke" my transponder. Running my hands over the fuses must have been the cause. Lesson learned, and if I have another non-critical electrical screw up I’ll be sure to check the fuses early in the game.
Live and learn….