Ahhh... getting closer to the cause.? The voltage on the top of C406 should be 1/2 Vcc, set by the resistor divider of R401 and R402.? This voltage biases the RF signal through the trifilar transformer so it sits evenly in the middle of the 0-5V span, creating an even quadrature signal into the inputs of IC403.? If the bias voltage is getting pulled up to Vcc (and the resistors are still properly in place), then that would mean either the output of IC402 or the input of IC403 is shorting the voltage up to Vcc.? It's probably more likely for an output to fail shorted up, so maybe IC402?? You can probably measure such a short to Vcc with power off to verify which device it is.
Stan.
