Your TDR-94/94D transponder communicates what is happening on the inside of the unit through error codes presented on the display. Many require little or no action on your part. However, there are Level 1 Failure codes that, if ignored, are detrimental to the unit and will cost you more in the end.
If you receive a Level 1 Failure Code, you are instructed to pull and send in the unit for evaluation. Below are examples of two Level 1 Failure codes you do not want to ignore.
Error Code 12—70v DC power supply is out of tolerance.
The flag warning on the transponder could be intermittent or on all the time, often occurring at startup but going away after a few minute warm-up.
This is indicative of something going bad on the high voltage power supply board. The unit will still work for a while, but don’t ignore it. It will cost significantly more to replace the board than to replace electrolytic capacitors.
Error Code 32—Synthesizer isn’t working or low power output.
The TDR-94/94D transponder system has a transmitter circuit and a receiver circuit. These circuits are run by the unit’s synthesizer. If the synthesizer has a low output, the receiver and/or the transmitter may fail.
Error Code 22/23 – Top/Bottom antenna low power output.
The power output of each antenna line is continuously monitored to ensure a proper signal is being transmitted to ATC (Air Traffic Control). If the power falls below the operational limits, then your transponder may become invisible to ATC. This error could be caused by a defective synthesizer, as mentioned above, or it could be simply that the transmitter is in need of an alignment.
How do you prevent Level 1 failures?
Replacing electrolytic capacitors in the high voltage power supply during overhauls has proven to help prevent error code 12 faults from occurring. Error codes 22, 23, or 32 would require more extensive troubleshooting but could end up needing nothing more than an alignment to repair the issue.
Aircraft Compass System Alignment Formulae