TRANSISTORZED SMPS POWER SUPPLY TROUBLESHOOTING
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.
Now,
let’s come to the COLD side [secondary side] of the SMPS power supply.
The secondary side has the
output voltages to main board components.
There
are 6 voltage outputs:
+14VDC
> to audio output IC.
+5VDC
> to CPU side of the syscon chroma IC
An
other +5VDC > to tuner stage
+B110VDC
> to line output transformer [LOT] section.
+27VDC
> to vertical scan output IC
+8VDC
> to syscon-chroma IC, pins 17 & 49 of IC8873
Let’s
discuss all these supply voltages in detail.
+B110VDC: this voltage should be there all the time
when the set is powered ON; provided there is no fault to primary [HOT] side of
the SMPS.
+27VDC: this voltage too will be there all the time
when the set is powered ON; provided there is no fault to primary {HOT] side of
the SMPS
+14VDC:
this voltage too will be there, when the set is powered ON.
Then
comes +5VDC [CPU]: this voltage is the
output from a voltage regulator IC; 78L05.
It too will be, and should be there, as the input to this regulator IC
will be present when the set is powered ON.
Refer to the circuit diagram.
Input voltage to this regulator IC comes from pin-14 of the SMPS
transformer [after rectification].
The
+8VDC 7 +5VDC< {the voltage goes to pin36}are switched type, means these tow
voltages will be absent when the set it powered On. These voltages will be present only after we
press either the power button on the remote control handset or at the front
pane control of the TV set. Here more
description is needed I thinks.
Take
the case of these two voltages only.
Refer
the circuit diagram.
Consider
all these 3 voltages [+5V CPU, +5VDC, +8VDC]
The
source of all these voltages are generated by pin-14 of the SMPS
transformer,
and then rectified by (D604-FR104). The
rectified voltage is then fed to input of the regulator IC 67L05, which
in turn
outputs +5VDC CPU. It will be present
all the time, because this voltage is needed to CPU section circuit
inside the
syscon-chroma IC. Without this voltage,
the CPU section circuit within the IC wont work and there by the set too
will
not response to any external or remote commands. In short, the set will
be dead. This voltage is not switched type. Means it will always be
present. The rectified voltage is also fed to collector
of a NPN transistor [V507-C8080] and the collector of another NPN
transistor
[V508-C8050]; switches the +5VDC. Refer
the circuit diagram and try to understand what happens when we Switch ON
the set
first, after it is plugged to AC mains, and then we press the power
button; either
at the remote control, or at the front panel of the TV set.
When
we first Switch ON on the set:
+B110VDC
is OK - present
+27VDC
is OK – present
+5VDC
(CPU) is OK – present.
+14VDC
is OK – present.
But;
there will be no 8VDC & 5VDC. Both these voltages are by-passed by transistors V607
& V508 respectively.
Why? Here comes the point.
Note
the circuit with NPN transistor V507-C8080.
Its
base terminal is pulled down to 8.7VDC by using a Zener diode [VD8]. By no means the voltage at this pont will go
up, than the break-down voltage of this zener diode; ie 8.7V.
But,
from where this voltage comes from?
There
is 3.3KOms resistor connected to the cathode of this Zener diode; from the
27VDC voltage output.
Now
what happens?
When
we press Power button either on the remote control or at the front
panel control of the set; to Power On; there should be 8VDC to the base
terminals of transistor
V507 . But, it won’t be there now.
Why?
Refer
the circuit diagram once again.
The
base of this transistor is connected to [Power On] pin-64 of the syscon
IC. The voltage level of this pin will
be hight [=H]; when we switch On the set.
Just see, this high voltage is fed to the base of a NPN transistor
[Q210-2SC1815]. The collector voltage to
this transistor is fed from (+5V CPU), via a serial resistor CR292, and then a
LED (power indicator) and an other resistor R291.
As
we know, when the base bias of an NPN transistor is at high [H] level, it will
conduct and its collector voltage will drop considerably due to load. Here the emitter of this transistor is
grounded. So, when its base voltage is
high [H], its collector voltage sill be low.
In
fact, when we switch On the set by its AC main Power Switch, the system control
section voltages to the IC will be present, the voltage at pin number 64 of
IC8873 will be high[H], and the voltage at the collector of Q210 will be low,
and in turn the base voltage of +8V regulator transistor too will be low. So this transistor won’t conduct, and there
will be no +8V.
Let
it be.
Now
come over to the base of V508; the 5VDC controller transistor C8050.
Refer
the circuit diagram again.
Its
collector terminal get voltage from the cathode of D604 itself; through
resistors R572 & R571. The base of
this transistor is pulled down to 5V by using a 5V6 zener diode, and the base
bias comes from the emitter of V507 [+8VDC].
It
is clear that the base bias voltage to this transistor will be present only
after the +8VDC has been bypassed by V507.
In short, there will be no +5VDc without +8VDC.
When
+8VDC is present, +5VDC too will be there, and should be.
In
short; the Main Switching Point to these tow voltages are of pin-64 of the
system control IC.
When
the voltage level of this pin is high [H], there will be no +8VDC &
+5VDC. When we press the power button,
the voltage at pin-64 of the IC will go low[L], the collector terminal voltage
of Q210 will go up; and the base voltage of V507 too.
Now
the transistor V507 will conduct (bypass) +8VDC. The base bias of V508 will go up and it too
will start to bypass the +5VDc.
This
is the main switching process takes place, when we press the Power button on
the remote control.
