Failure of the horizontal
output stage to produce deflection, and the major byproduct high
voltage usually results in a dead set because the supplies needed
for signal processing are also absent. Often the failure results
in a blown fuse. For example, there is little to limit current from
the power-line operated supply if the H.O.T. is shorted.
If a primary-supply fuse has blown
check the H.O.T./damper diode for shorts, and replace if necessary.
If the fuse holds when power is reapplied, but there is no deflection
or high voltage, check for the correct waveform at the base of
the H.O.T. This might require a substitute bench power supply for
the horizontal oscillator circuit to make this check, as the normal
supply for this circuit is also produced from the flyback pulse,
and a momentary starting voltage is used to get things going. If
drive is present, check the yoke/flyback transformer for opens.
If the primary fuse blows again, the
cause may be an additional load on the flyback transformer or failure
of the transformer itself. First inspect the transformer and yoke
for signs of arcing or char. If there are none look for loads that
might pull the H output circuit down. Disconnect temporarily any
hold-down circuit. Also lift aux. supplies, one by one that are
driven from the flyback to see if any had been responsible for
excessive load. Disconnect, or check the high-voltage multiplier
to see if it is part of an excessive load. The idea is to eliminate
all loads down to the flyback itself. The transformer represents
a high replacement cost as well as the labor involved in replacing
it, so it's a good idea to be sure the trouble has been definitely
traced to the transformer before replacement is attempted.
Failure of the flyback transformer
can be due to opens, leakage between windings or shorted turns.
Opens can be found with ohmmeter checks. Inter winding leakage
can be checked with a Hi-pot leakage tester. Shorted turns are
the most difficult to find as they have little effect on total
winding resistance, but a single shorted turn acts like a shorted
transformer secondary and so acts like a heavy load.
Shorted turns can be identified by
conducting a "ringing" test. This is done by establishing,
and then interrupting a dc current in the primary, and then checking
for the number of sine waves (rings) at the natural frequency of
the inductance of the unloaded primary and the total shunt capacitance.
If turns are shorted the rings die out very quickly. If a DSO (digital
storage scope) is available this check can be set up quite easily
using a low voltage dc supply or a battery, Set the scope to trigger
when the supply is interrupted and check for the number of rings
before their amplitude falls to some preset value, like 50%. Go/no-go
criteria can be set by checking a known good flyback.
Ringing tests are also facilitated
with Sencore instruments such as the Model TVA92 TV VIDEO ANALYZER
and CM2125 COMPUTER MONITOR ANALYZER. These units employ a patented
system that pulses the flyback at low level and counts the number
of rings that exceed a reference amplitude.
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