Tower cannot see my KT76A

[dougmattson]

I have been using this transponder for about three years when the tower now says they cannot see it. I have had it benched checked and the power adjusted to 200 watts. It outputted all the mode a codes on the bench along with a couple of mode c codes. I have changed the antenna, the rf coax, and the short coax and rf connector on the tray. I have borrowed the neighbors kt76a and the tower still cannot see me. I had the avionics shop start a vfr transponder check and his power meter showed 100 watts out of the antenna on the plane. Before we could see why the power was down his meter quit. He thought even at 100 watts they should still see me within 10 miles. I have tried a different tower also. I have visually checked the power and encoder connector and it looks ok. I have not done anything with the encoder. The interrogation light flashes 3 times for every sweep with my transponder. Only once with the neighbors older model. He says he has never had any trouble with his. Anyone have any ideas?

[jpowell13]

Sometimes I get complaints from controllers also. My 76A is at the bottom edge of my panel. I've found that if I reach under the panel and push the connector in while pushing on the face of the radio the problem is fixed. (Autopilot helps when doing this little operation. ) The interrogation light blinks even when atc is not receiving, but tends to blink more rapidly after the fix. Seems to happen most often after I've removed and reinstalled the transponder. It sometimes happens after a full power climb out. John

[Walt]

You may try to find another shop that has some better test equipment, there are a number of things that can cause this all of which can be determined with the proper equipment. 125W is the minimum to pass the xpdr test but in reality 100W will still give excellent range so low power is not the problem.

[pilot2512]

To add to what Walt said. Those older units that use cavity tubes for xmiting are sensitive to having their freq pulled due to the coax lengths. Checking output freq is as important as power out. Regs call for +/- 3 Mhz but I have seen less deviation cause problems like yours.

[Billvt]

Since the KT-76A is a very commonly used xpndr, do you have a flying friend or colleague that you can swap the xpndr with.

Would be an easy check to see if it's the xpndr causing the problem.

Regards,
Bill

[robertahegy]

Could be the antenna or the lead or possibly a poor ground on the antenna.

Roberta

[dougmattson]

I have swapped transponders and replaced leads and antennas. Also antenna is grounded.

[az_gila]

I had the avionics shop start a vfr transponder check and his power meter showed 100 watts out of the antenna on the plane.


Was this with a separate testing antenna on a stand a foot or two away from the plane's antenna?

If so, you are radiating an RF signal, and the most obvious remaining error would be a small RF frequency shift as mentioned earlier.

[dougmattson]

Originally Posted by az_gila

I had the avionics shop start a vfr transponder check and his power meter showed 100 watts out of the antenna on the plane.


Was this with a separate testing antenna on a stand a foot or two away from the plane's antenna?

If so, you are radiating an RF signal, and the most obvious remaining error would be a small RF frequency shift as mentioned earlier.

Yes. But why did it not work when I put a different transponder in? The current setup worked for three years but could something in the rf transmission line cause this shift?

[Walt]

Originally Posted by dougmattson

Yes. But why did it not work when I put a different transponder in? The current setup worked for three years but could something in the rf transmission line cause this shift?

Yes a bad antenna or coax can pull the freq out of tolerance.