King KT76A + Narco AR-850 Encoder - Mode C Altitude Glitching

[50kttailwind]

Hi everyone,

We had this same issue a few years ago and ended up swapping out both the encoder and transponder. We did ourselves a disservice not narrowing down which part exactly was the culprit. I'm thinking the encoder should be responsible.

Does anyone think there's a scenario where the encoder is fine but the transponder is misinterpreting the data? What's happening is Mode C altitude reports 300-400' jumps basically every second.

Thanks for any help!

 

[Capt. Geoffrey Thorpe]

Vintage transponders get the information from the encoder through a parallel "Grey code / Gilham code" connection where you have one wire for each bit in the message. If you have two specific altitudes that are getting reported you may be able to determine which wire is loose in the harness connections.

https://support.michiganavionics.com/portal/en/kb/articles/transponder-gray-code-gillham-code-reference

 

[Chip Sylverne]

Ditch that thing and get a Trans-Cal digital. New, or csn find one used cheap as chips and are pretty much bulletproof.

 

[50kttailwind]

Ditch that thing and get a Trans-Cal digital. New, or csn find one used cheap as chips and are pretty much bulletproof.

I know these Narcos are indeed junk. I'm not familiar with the Trans-cal. Reliable? Will it interface with the KT-76A easily?

 

[Chip Sylverne]

I know these Narcos are indeed junk. I'm not familiar with the Trans-cal. Reliable? Will it interface with the KT-76A easily?

My KT76A and Trans-Cal ss120 make beautiful music together. Never had to have the encoder adjusted in several years of IFR certs.

The install manual:https://www.aeronautical.co.za/wp-content/uploads/2020/07/SSD120-30N-Owner-Installation-Manual.pdf

If you already have a digitizer installed, say for a GPS, if I recall the it's a pin for pin swap.

 

[50kttailwind]

My KT76A and Trans-Cal ss120 make beautiful music together. Never had to have the encoder adjusted in several years of IFR certs.

The install manual:https://www.aeronautical.co.za/wp-content/uploads/2020/07/SSD120-30N-Owner-Installation-Manual.pdf

If you already have a digitizer installed, say for a GPS, if I recall the it's a pin for pin swap.

Thanks a million! Looking in to this now. It definitely looks like the smarter approach.