An economical solid-state TDR with octal plug-in base, the 339 maintains excellent repeat accuracy despite wide voltage and temperature variations/even after long periods of down-time. Two models are available. Each has six dial selected ranges from fractions of a second to as long as 10 hours. Each model has a selectable on-delay or interval timing mode. OPERATION: Timing beings when the start switch is closed. This starts an oscillator which runs at a frequency determined by the time setting. A fixed number of counts from the oscillator determines the end of the time cycle. The time required to accomplish this depends on the oscillator frequency. During timing, a LED located on the dial face blinks. For the first ten percent of the cycle, the LED repeatedly blinks once followed by a pause, for the second 10% it blinks twice and so on indicating the cycle progress. It flashes rapidly and continuously after time out.