KT-76A fuse blowing?

psween

psween

Pre-takeoff checklist
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psween
KT-76A transponder inop, shop replaced an internal fuse and recalibrated ($170). Now a few hours later it's inop again. The panel fuse is good, so I suspect the internals again. What could cause repeated fuses to blow? I would like to keep the 76A limping along for another year or so until I stick a Stratus in for ADSB out. I'm an A&P, but know next to nothing about avionics. Thanks!

Patrick
 
You’ll spend more hours on board level troubleshooting, IF you can find an avionics shop that can even do that anymore, than on a used replacement.

We fiddled around letting THREE supposed avionics techs look at a similar vintage King DME that was also blowing its internal fuse. Not one could figure it out.

I damn near took it to an old friend who worked for King back when they were actually King and knew something about their own electronics.

Realistically, with the state of the avionics world having mostly gone to people who are box-swappers and know nothing about electronics, other than the last remnants of traditionally trained older techs (who are busy doing ADS-B installs and don’t need hours of bench time to make their mortgage payments at all right now, since good shops are booked out six months or more), it’s time to put the new one in.
 
What is the internal fuse rated at versus the circuit breaker? Also, have the antenna wire checked with a SWR meter to be sure there isn't a problem.
 
There are a couple of low voltage power supplies internal to the KT-76A. I can't find the overhaul manual for that gear. But, I suspect the "internal" fuse is for one of those power supplies. If the fuse keeps blowing, but doesn't blow immediately on power up with new internal fuse, then either A) there's something loose internal to the transponder (not always discovered on a routine bench test), or B) pin 11 on the transponder edge connector is seeing a large transient.

Is your aircraft power 12vdc, or 24vdc? Do you use a "avionics master switch"? And, if so, is it a breaker style switch, or is it a low current switch controlling a relay?

IMHO, these are the questions someone you hire to work on your gripe should be asking.
 
Have a KT76A with an 8130 yellow tag I can let you have pretty cheap.
 
wilkersk said:
IMHO, these are the questions someone you hire to work on your gripe should be asking.
Click to expand...

Most avionics shops have people who don’t know their ass from the hot end of a soldering iron.

Doesn’t help much that good electronics techs don’t go into the lower end of the avionics biz anymore because to sign off on anything they’d need an A&P certificate.

They can make a lot more money elsewhere. Cellular, two-way radio, manufacturing, you name it... they really have to want to work on avionics to take that up front cost hit learning stuff that has nothing to do with electronics troubleshooting.

FAA needs a separate certificate for modern avionics work that has nothing to do with learning to set the timing on a 1930s designed air-cooled tractor engine.
 
Hard to find a better old school transponder than a KT76A. Plus they are as cheap as dirt on the used market and easily replaced, I’ve seen them in good working condition for $50 to $100. Not to mention they are cheap to repair when compared to modern digital transponders.

Check out this place for repairs: http://www.transponderrepair.com/
 
Little more info, 12v system in a 150, and it does have an avionics master. The avionics switch is just a switch, not a breaker. There is a fuse in the panel dedicated to the transponder, I think 3 amp but would have to check. Not sure when it blew, partner had it repaired over the winter and it was inop again when I got the plane back last month. Thanks for the ideas so far. Swapping in a working one to buy me some time might be the most cost effective solution.
 
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