Failure in the IR Pickup circuit, or anything between it and the MPU
inclusive of stuck or corroded tac switches. Due to the age even leaky
diodes or other components going bad will affect these operations.
Start by checking the operating potentials at the IR Pickup and
confirm indeed it is picking up and transferring the signals from the
remote. If so then trace that signal all the way to the MPU. Severe
clipping or loss of the Control Signals from the IR Pickup while in
route to the MPU will affect its functions. IMHO it is highly unlikely
that you have changed any data parameters when using the universal
remote that will result in this symptom.

OK, I'll try to measure the signals to see where the data is getting lost.
But I seriously doubt that the pickup circuit stopped working due to
physical components failing!

The failure to respond to IR commands happened * suddenly * and correlated
directly with use of a foreign remote control. It was working perfectly,
then universal remote was attempted, now 1 minute later and the TV no
longer responds to IR commands.

Unless there was a digitally triggered component failure - which is
possible but I think highly unlikely, I think that the seized up control
circuitry is not physically broken but is reading bad state information
from memory.

If there was an easy way to clear that memory, I would attempt that first.

--
Jem Berkes
http://www.sysdesign.ca/