WM SL & WM valves. Checked hi-fi wire connectors, checked splice box, removed field wires, ohm meter showed 33 ohms, fired valve from Progressive, worked, ran 24 volts from clock, cleaned field wires, installed new King blues, fired-up, worked. Deduced bad zone wire connection, though it still could be a bad coil, as they sometimes don't short until they heat-up. Did notice SL automatically removed "Short" from the display. Will check tomorrow after zone has it's normal run.