Saturday, June 06, 2015

PANASONIC TC1420C – CRT TV 14 Inch – From MY SERVICE TABLE Today

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