SER FAQ: TVFAQ: Brief comments on testing the HOT

S.E.R. FAQ

NotTaR of Television Sets : Brief comments on testing the HOT
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Brief comments on testing the HOT

For a TV with no blown fuses that will not start, here are two quicky checks to see if the HOT is good and has power and drive:

HOT tests - check across each pair of pins for shorts (preferably removed from the circuit board). No junction should measure less than 50 ohms or so. Lower readings almost certainly indicate a bad HOT. If in-circuit, however, the reading between base and emitter will be near zero due to the secondary of the driver transformer. See the document: Basic Testing of Semiconductor Devices. Don't be confused by internal damper diodes and B-E resistors.
Power - measure across the collector to emitter with a multimeter (with the HOT removed or if there is no deflection, this is safe with it in place). There should be solid B+ - typically about 100 to 160 V (115 VAC sets - possibly higher for 220 VAC sets). If this is missing, iether there is a problem with the power supply or the emitter fusable resistor has blown (probably in addition to the HOT) and there is no return.
Drive: put an oscilloscope on the base - there should be pulses around 0.7 V for most of the scan (~50 microseconds) and probably going negative a couple volts at least for retrace (~12 microseconds). If drive is weak or missing, determine how startup is implemented as there may be a problem in the startup power supply or deflection IC.

WARNING: use an isolation transformer for the oscilloscope tests (and whenever you are probing a TV in general)!!! This part of the circuit, in particular, is usually line connected. See the sections on safety.