Set
dead. AC fuse blows up instantaneously the set is switched ON.
No
DC voltage across C507 [main filter capacitor]
The
type of SMPS here in discussion is composed of transistors. This type of SMPS is used with many brand and
models of TVs. Troubleshooting to this
type of SMPS is rather easy than related to STR based ones. If you get the set for repair in dead
condition; isolate the secondary +B voltage [110VDC] from the rest of the
circuit by anyway. This voltage is fed
mainly to Horizontals Output and Horizontal drive stages. Now proceed.
No
DC voltage across C507 [main filter capacitor] (100 to 250MFD 450VDC)
See
the circuit part. The AC mains voltage
is lead to the rectifier section circuit through a line filter choke L501 and a
serial fuse. This voltage is then
rectified and converted to DC voltage, by using 4 rectifier diodes. If any of these diodes has shorted junction,
the AC fuse will blow up as soon as the set is switched ON. Therefore if you see a blown up fuse; be sure
that some short circuited component/s at its main rectifier section circuit or
the rectified DC voltage line.
The
best way to detect this short circuit is to measure the DC resistance across
the main filter capacitor terminals. It
should not show a short circuit. If it
does, de-solder out the SMPS drive transistor and measure it again. If there is no short circuit detected now,;
be sure that the SMPS drive transistor has damaged [collector to emitter
leak]. Never replace it, and power up
the circuit now. There shouldbe a reason
for this damage to the transistor. Make
sure that there is no damage to other associated component/s with it. It is best practice to replace the other two
small signal transistors with new ones.
De-solder
out the other two transistors. Note
their numbers and position in circuit.
One of it is PNP and the other is NPN type. Now check the other components. Resistors,
zener diodes etc:- Unsolder up one end
of each component, and measure its characteristic. If found OK, solder it in place. You must be patient and concentrate on
work. Hurry up does not go.
See
the circuit. The rectified DC voltage
passes through the primary winding of the SMPS transformer; then to the
collector terminal of SMPS drive transistor. As we have already de-soldered out
this transistor from circuit, there should not be any short circuit to this
votage line to ground now. If it shows,
be sure that the SMPS transformer is damaged [winding short between its primary
and secondary]. But this kind of fault
is very rare. Sometimes 0.01% or
so. Can’t say never. Might be!
Now
check the DC resistance across the terminals of the main smoothing
capacitor. It should not measure any
short circuit now. Set the meter to Ohms
range, and measure this. Never set it to KOhms range. Suppose still the meter shows a short
circuit. See; there is no SMPS drive
transistor in place. Then which
component/s should be suspected?. Either
the rectifier diodes or the ceraminc capacitors connected in parallel across
each of it might cause this. Check all
the four diodes for reverse leak. If any
one among it shows a short circuit, replace all four of it at one time. It will be the best.
Here
one main point to recall is, the rectified DC voltage across the main filter
capacitor will be higher than the AC input voltage. If AC input voltage is 220VAC, this voltage
will come around 345VDC or more; and it should be. If not, be sure that the
rectifier diodes are faulty. Replace all
these four diodes along with the ceramic capacitors connected in parallel with
it. Never forget to check these ceramic capacitors. They too are likely to be faulty. All of it
might be 4.7KPf 630VAC. If all the
components are found OK, plug in to aC power; after inserting a fuse of
appropriate specs. 3.5Amps 250VAC. In
rare cases, when we power up the set, with degaussing coil connected, the AC
fuse blow up. If so, the degaussing
posistor is short circuited, and should be replaced.
Here
we can end with the troubleshooting to primary side of the SMPS circuit. There is not much more to check with this
section.
Beware: the primary section circuit will be always
HOT, irrespective of connected AC mains phase.
So, take care when you work with this section circuit. Use of a hand glove is recommended.
Troubleshooting
the secondary section circuit will be described by the next post here.
NB:
check the resistor and capacitor connected across the primary winding of the
SMPS transformer; for open or loose solder terminals. (R525 &C516). These two components are so critical. Damage to any one among it will result in the
failure (short) of SMPS drive transistor; at the instance of switch ON the set
to AC mains. So, look for any damage or
loose solder terminals to these components before power up this circuit. You won’t get any time to check this
fault. So be cautious.
