RT-359A transponder mode C reading consent 500 feet

[blurceo]

So I have an issue with my RT-359A transponder. It does not read out mode C right and will only read out mode C. Which I think my local controllers are getting tired of dealing with it. I replaced the encoder and had the transponder bench tested. And it read fine on the bench but got it back in the plane and went for a flight about a week later and it’s still not reading right. I have been told those older transponders to take a while to warm up and then they start to work. But when I used it last I did a cross country and it ever read right. So I am leaning towards a new replacement to save the money from troubleshooting. Any recommendations? The transponder is in a 150 G that has had the pitot static system checked twice and it comes back good. Has anyone else has a similar issue

 

[arnoha]

So I have an issue with my RT-359A transponder. It does not read out mode C right and will only read out mode C. Which I think my local controllers are getting tired of dealing with it. I replaced the encoder and had the transponder bench tested. And it read fine on the bench but got it back in the plane and went for a flight about a week later and it’s still not reading right. I have been told those older transponders to take a while to warm up and then they start to work. But when I used it last I did a cross country and it ever read right. So I am leaning towards a new replacement to save the money from troubleshooting. Any recommendations? The transponder is in a 150 G that has had the pitot static system checked twice and it comes back good. Has anyone else has a similar issue

Have you had the cable between the encoder and the transponder checked? If the items on the ends aren't broken, all that's left is the wiring. RT-359A uses Grey code, I'm assuming, so if the cable is somehow broken...

500' would be high on C2 and B2 and low on all else.

 

[Capt. Geoffrey Thorpe]

Get it checked while installed. A bench check won't find the pin that is pushed out in your connector.

 

[N801BH]

You cannot get a transponder clean bill of health UNLESS it is installed and tested in the airframe anyway..

 

[blurceo]

You cannot get a transponder clean bill of health UNLESS it is installed and tested in the airframe anyway..

It's been checked out with it installed and it checked out fine for them. They also checked the lines running behind the panel.

I also check the wires liked arnoha suggested and from what I can tell the harness looks fine. Is it possible that the wires are not going to the correct pin?

 

[N801BH]

It's been checked out with it installed and it checked out fine for them. They also checked the lines running behind the panel.

I also check the wires liked arnoha suggested and from what I can tell the harness looks fine. Is it possible that the wires are not going to the correct pin?

A proper transponder check will use a device over the the static post to lower atmosphere pressure to simulate higher altitudes..... I wiring problem will show up instantly... As will a bad encoder......

 

[arnoha]

A proper transponder check will use a device over the the static post to lower atmosphere pressure to simulate higher altitudes..... I wiring problem will show up instantly... As will a bad encoder......

But where are they getting the reading from? The radio transmission itself or just a readout elsewhere? It's possible that if it's not the radio transmission being listened to, there's an issue in the radio encoder.

 

[blurceo]

But where are they getting the reading from? The radio transmission itself or just a readout elsewhere? It's possible that if it's not the radio transmission being listened to, there's an issue in the radio encoder.

The reading is coming from ATC when I pick up flight following. Is this radio encoder a separate piece of equipment from the transponder and encoder? Or is it built into one of them?

A proper transponder check will use a device over the static post to lower atmosphere pressure to simulate higher altitudes..... I wiring problem will show up instantly... As will a bad encoder......

I think I might go to another shop and have them check it out. And the encoder I had replaced as the one that was in there I believe was early 80s vintage and when he tested it was reading out bad.

 

[flyingron]

But where are they getting the reading from? The radio transmission itself or just a readout elsewhere? It's possible that if it's not the radio transmission being listened to, there's an issue in the radio encoder.

Of course, there's little point in the test if you're not looking at the output of the transponder.

 

[arnoha]

The reading is coming from ATC when I pick up flight following. Is this radio encoder a separate piece of equipment from the transponder and encoder? Or is it built into one of them?

I think I might go to another shop and have them check it out. And the encoder I had replaced as the one that was in there I believe was early 80s vintage and when he tested it was reading out bad.

Sorry, my post was ambiguous...I meant, what is the source of the reading when the test is being conducted on the ground? Were they actually listening to what was coming off the antenna, just like ATC, or were they actually physically connected to some test port that skips the actual broadcast part?

The radio encoder is just part of the internal electronics. Referencing the process of taking the static, parallel grey code on the pins and converting it into a serial stream of bits to send over the air.

 

[blurceo]

Sorry, my post was ambiguous...I meant, what is the source of the reading when the test is being conducted on the ground? Were they actually listening to what was coming off the antenna, just like ATC, or were they actually physically connected to some test port that skips the actual broadcast part?

No it was not, it was when I was airborne. So I think it's just coming off the antenna? Which makes we wonder if I have something up with my radios because I had someone tell me that my radio they could hear a pulsing noise in the background when I was talking.

 

[bnt83]

This is what you need done for the encoder error. If this test fails then its troubleshoot time to figure out what the problem is.

 

[arnoha]

This is what you need done for the encoder error. If this test fails then its troubleshoot time to figure out what the problem is.

Ah! That diagram answers my question about the test on the ground. It does listen to the signal from the aircraft transponder antenna.

Well, I got nothing more to add! (But I learned something.)