Altitude encoder anomaly

I mentioned in another thread that I had been experiencing incorrect
mode C reports when the airplane was cold. Since a few people offered
suggestions, I'm following up with a report.

Installation: PA28-180, ACK A-30 mod 1 encoder, KT 76A transponder.
Observation: Mode C reports in error in winter time but ok in summer.
In winter reports became accurate after 15-30 minutes of operating
time. Mode C appeared, as monitored by MRX PCAS, to be correct if
encoder was preheated.

I built an encoder test box and tested the encoder output, both on the
aircraft, and in a bench test. For the bench test the encoder was
cold soaked overnight in the freezer at approx +5 deg F. It was then
powered and the current draw and altitude code were monitored.

Time zero current 0.37 A. No altitude report.
4 minutes 0.40 A. No altitude report.
10 minutes 0.35 A. No altitude report.
11 minutes 0.30 A. 22,100 ft.

After about 13 minutes the current stabilized between 0.24A and 0.26A
and the reported altitude continued to reduce. (Current includes
monitor LED current so varies with altitude)

21 minutes 18,200 ft
27 minutes 9,500 ft
33 minutes 6,000 ft
39 minutes 4,400 ft
46 minutes 3,300 ft
59 minutes 2,200 ft
77 minutes 1,700 ft
100 minutes stabilized at 1,500 ft / 1,400 ft (as expected for my
elevation)

The data seem to indicate that the heater is working but shutting off
before the sensor is heated to working temperature. I suspect the
problem is in the temperature feedback circuit.

Anyway this kept me amused for a while but now I have to send it back
to be fixed or buy a replacement. Either way no airplane driving for
me for a few days. (unless someone in SW USA would like to lend me an
ACK A-30 for a week or two)

Just because you have a transponder with a current certification don't
assume it's sending good data. Spot check with ATC, PCAS, or some
other monitor.

Andy