Panasonic
14” CRT Color TV – AN5192K-B (Chroma)_MN152811 (System Control)_24C02A (System
Memory)_LA7837 (Vertical scan Output)_EHT Transformer (TLF15644F1)
Horizontal
thin line across the center of the secreen. Audio
is Ok; but have distortion.
Removed
the back cover and looked for any visual damaged components on board. Bulged or busted capacitors etc: Sometimes a visual check too might give us
hints of the fault.
I
didn’t find any such damaged components on board.
As
the Line is Horizontal, it is clear that the vertical scan section circuit has
the fault. There is no vertical scan;
that is why a horizontal line on screen here.
I
concentrated at the vertical scan output section circuit.
As
there is audio and a horizontal line on screen; it is clear that there is no
fault to SMPS and the LOT section circuits.
Here
I’ve to pinpoint the part that has damaged.
Sometimes it might take some time, but we have to do this patiently.
I
concentrated over to the vertical scan output section circuit, and the scan
output IC. Surely the fault should be
there. The vertical scan output IC used with this set is LA7837 SIL [Single In
Line] type. It has 13 solder terminals.
After
placing the PCB at a suitable position, I powered up the set. Checked all the 13 pin voltages present now
at the IC solder terminals; marked it for reference with the circuit diagram.
When
I compared the measured voltages with the schematics, I found abnormalities at
some pin voltages no present at IC terminals.
Pin
number – 8 – the supply input voltage – 24VDC – [OK]
Pin
number – 12 – the vertical scan out – 3VDC – [abnormal]. It should come around 13VDC.
Pin
number – 13 – boost voltage to vertical scan output inside the IC – 24VDC –
[OK]
Pin
number -6 – 0.8VDC – abnormal – it should come around 5.8VDC.
No
further check required. The vertical
scan output IC [LA7837] is defective.
But;
what might be the cause? We must look
for it. If there is any other cause,
surely the IC we will replace too will get damaged.
This
is the main point we have to be in mind; whenever we repair any electronic
equipment.
After
switched OFF the set, I de-soldered out the vertical scan output IC.
Now
switched ON the set, and measured once again the voltages present at the solder
foil terminals.
No
abnormality found.
Pin
voltages without IC in place:
1
– 11.7V – Normal.
2
– 4.2V – normal.
3
– now it is ‘0’V. here the voltage comes
out from the IC. We can get a voltage
here, after replacing the IC.
4
– 6V – normal
5
– 0.025v – normal
6
– 6V – normal
7
– 5V – normal
8
– 25V – normal.
9
– 0V – with IC replaced, it should be around 1.8VDC.
10
– 0v – normal.
11
– Ground.
12
– 0v – should come around 13VDC with IC in place.
13
– 24VDC – normal.
No
other abnormalities detected.
De-soldered out all the electrolytic capacitors used at the vertical
scan output section circuit and checked for any leakage or capacitance
deteriation.. All of it was fine.
Replaced
the IC with a new one.
Switched
ON the set. OK works fine.
This
IC will heat up while working. So we
have to give special attention to its mounting with heat sink. It should be tight. Otherwise, proper heat transfer wont take
place from IC to its heat sink. This too
is one among the cause to the failure of this IC. Use of heat-sink compound to its mounting is
a must.
Switched
on the set for 2 hours. No other problems.
The
set is ready to deliver.
FACTORY
MODE ADJUSTMENT
PRESS
THE Recall button together with Volume Down button. The TV in the normal mode changes to check
mode. “CHK” will appear on the screen.
Press
the Function button to select the required adjustment.
Press
the Normal button on the remote control Twice to return to normal mode.
HOTEL
MODE
Press
Channel ^ on the TV and ‘OFF Timer’ on the remote control together.
To
cancel hotel mode, press Vol (--) on the TV, and Off Timer button on the remote
control.
Click on the pictures to Magnify
