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Failure modes of KT76A xponders???

Brantel

Well Known Member
I have a King KT76A xponder and all of a sudden I am not seeing any replies.

The reply light is good in test mode. The occillator is buzzing on power up. The ident button works but I am not sure it is actually xmitting. I just tried a new antenna...no joy!

Is it dead?
 
Try a ATC check

Hey Brantel,

Why don't you try a transponder check with ATC near you (airborne, of course)? By the way, your the electronics guy, I should be asking you....LOL

I'm interested because I am also using the VERY old KT76A.
 
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Mine is the old model as well.

I see no replies and I normally see them even on the ground at our field.

Pretty sure it is dead but I wanted a second opinion.

I found my stub antenna slightly bent so I thought that was it but changed it and no joy....

I will ask for an ATC check next flight..
 
The little orange "I am being interrogated" light is activated solely from pick up of the RF component of the output circuit. There is no physical connection to the output circit

If that is pulsing in flight then you are at least generating some RF signal. If it lights when Ident is pressed then the tube is not totally dead.

If the tube is dead, then replace the antique technology with a new solid state one...:)
 
I get no reply light in flight unless I press the ident button.

I hate to spend any money on xponders until this ADS-B stuff flushes out.....
 
KT76

The KT76 remains my favorite transponder. Why?? It is very visible at night even when located far out of my line of sight. The ident button is easy to find, even without looking. I have yet to see another xpdr that I like as well including the latest Garmin and KT76C.
The old Collins microline is a close second as far as user friendly. I can buy three or four KT76's off ebay for the price of one new Garmin. So technology be damned, I will stick with the KT76 as long as they are legal and can be supported.
By the way the only failures I have seen with the KT76 in thousands of hours is the last two digits being out of alignment with what ATC is showing. This has happened twice, no other failures ever.
 
I would first try "re-seating" it to see if that helps, I see lots of problems with the old rack connectors on the KT76A. I'll do a free bench check for you if you want to send it to me. Sometimes they are repairable, it just depends what's wrong.
 
Tried that today Walt. No joy!

Anyone got a schematic in e-format?

I may take you up on that Walt. Problem is I don't want to spend much on a repair of a $500 box.

I think the tube is good if what Gil says is true. I get a really good bright reply light when I press the ident button.
 
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with 4k hours on mine i have had no issues. replaced an encoder years ago. with my small zaon i can check the encoder readout and squawk code.
 
Tried that today Walt. No joy!

Anyone got a schematic in e-format?

I may take you up on that Walt. Problem is I don't want to spend much on a repair of a $500 box.

I think the tube is good if what Gil says is true. I get a really good bright reply light when I press the ident button.

A schematic isn't going to do you much good unless you are also willing to build a test harness and have a test set to interrogate it and check the reply.

It does sound like it's not recieving, have you also checked the coax for continuity and shorts?
 
I agree. After learning much on the web last night about how these things work, I am leaning toward a coax or connector problem since I get a good strong reply light during test mode. This apparently injects a fake interigation signal into the receiver and the reply light shows that the RF output section is working. The occilator whine says that is working. That leaves the antenna, coax or connectors.

I just put a new antenna in.

Will check for shorts and continuity today....

I would still like to have a schematic and even better the maintenance manual if for nothing more than education. Walt you got this in PdF?

Thanks for the help!
 
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would still like to have a schematic and even better the maintenance manual if for nothing more than education.

I found the RF info for the reply light at an avionics shop while checking why my light was weak.

As the tech was looking at the schematics, it was apparent that there are several different schematics for this transponder based on the serial number.

Make sure you can get the correct schematic....:)
 
The little orange "I am being interrogated" light is activated solely from pick up of the RF component of the output circuit. There is no physical connection to the output circit

If that is pulsing in flight then you are at least generating some RF signal. If it lights when Ident is pressed then the tube is not totally dead.

If the tube is dead, then replace the antique technology with a new solid state one...:)

The reply light is driven by the same logic that also intiates a xpdr reply. As far as I can tell (I'll have to dig into the schematics a bit more) the reply light in the "test" mode only tests the reply light circuit and does not actually cause it to generate a reply or test the receiver. This also makes sense as you would not want the xpdr generating actual replies during the test function.

From the KT76 manual:
This circuit switches the reply lamp on and off for each time the ANNUNCIATE line from the LSI chip goes high. A cockpit light level sensor automatically dims the reply lamp for lower cockpit lighting levels. Additionally, placing the function switch in the TEST position turns on the REPLY light.
 
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The reply light will light when it receives an interrogation and is an indication that the transponder has decoded the signal and is replying. However, it does not indicate if the transmitter is actually transmitting anything. In fact, the reply light will still function with the cavity tube removed. Same situation with the Ident.

Hearing the T-whine from the power supply is good but not a guarantee that all the proper voltages are present.

If the antenna and coax are OK, and the power supply voltages are correct, I'd suspect a problem with the receiver front end or the LSI chip.
 
The reply light will light when it receives an interrogation and is an indication that the transponder has decoded the signal and is replying. However, it does not indicate if the transmitter is actually transmitting anything. In fact, the reply light will still function with the cavity tube removed. Same situation with the Ident.

Hearing the T-whine from the power supply is good but not a guarantee that all the proper voltages are present.

If the antenna and coax are OK, and the power supply voltages are correct, I'd suspect a problem with the receiver front end or the LSI chip.

On the schematic I saw the reply light was driven by a small wire pick up next to the RF cavity. A diode turned it into a dc signal to drive a physical lamp, with a light sensitive resistor providing the variation due to ambient light. I remember being impressed by the simplicity of the circuit.

Are all generations of the KT-76A wired as you say, or did I read the schematic wrong?

Isn't the KT-76A transponder old enough to have early versions pre-LSI chips?
 
On the schematic I saw the reply light was driven by a small wire pick up next to the RF cavity. A diode turned it into a dc signal to drive a physical lamp, with a light sensitive resistor providing the variation due to ambient light. I remember being impressed by the simplicity of the circuit.

Are all generations of the KT-76A wired as you say, or did I read the schematic wrong?

Isn't the KT-76A transponder old enough to have early versions pre-LSI chips?

All KT-76A and 78A transponders have the LSI chip. The reply/ident light is driven by a pinout on the LSI chip. The older KT-76 and 78 did not use the LSI chip and the reply light was triggered by discrete decode circuitry.

I've worked on most all of the GA transponder and I don't recall any that uses an RF pickup method to drive the reply light.
 
Checked the cable and connectors and all is fine.

Gil, looked at the maintenance manual/schematic and Mike speaks the truth....

Looks like I need some test gear to go any further.

Dang it I wish it had been something simple!
 
76A install question

I just installed a kt76a I got off eBay with a high serial number(126394).
I wired it with 14v power to both pin 11 and 12 in case I ever wanted to use a 76c.
The transponder works OK except the reply light flashes like normal even if the power switch is off. Should I have left pin 12 open?
 
To finish the story on my xponder failure. I ended up going to Stein for an all digital replacement. It should arrive early next week.

I decided that my future with ADS-B is not with Mode S but with a stand alone UAT instead because i want FIS-B weather so I decided on the Garmin 327.

Now a couple hours upside down under the panel!!!
 
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Thanks Walt. If I leave pin 12 open is it still a slide-in replacement for a 76c?
Will it hurt anything to leave it hooked up?
 
I had to go to the schematic to find pin 12 on the KT76A, its a maintenance test point for 13.75v (out), by hooking it to power you are supplying 13v (in) and bypassing the on/off switch, so the xpdr will be on all the time :eek:

Even though its not listed in the install guide pin 12 could be used for switched 13v out for encoder power.

The KT76C receives power on pin 11 and 12 (tied together), so the C model will work just fine with 12 not connected.
 
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ATC reports my 4-digit mode 3A is alternating between two codes, the set 1200 and another in the 2000s that was not consistent.

36 inch coax to a stick and ball antenna is located forward right cockpit floor. Same faults toward and away from the ATC radar site location.

Was not in close formation/single ship and it was not 1200 and then 2400. Spun all 4 knobs around a few times during a recycle.

No change.

Recent cert and bench check history, no issues.

Thoughts?
 
Any way debris moved somwhere causing the issue that can be removed with contact cleaner?

Is this easy for a tech to find/repair on the bench?
 
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