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:
|NotTaR of Television Sets : Brief comments on testing the HOT
1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.
I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.
<< Why do flyback (LOPT) tra.. |
| CRT rejuvenation >>
- 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.