My service table today - Philips 21PT5437 – dead – TDA4864 Vertical scan output – No blinking indications

Philips 21” flat CRT TV – received for repair in dead condition.  Vertical scan output IC used is TDA4864

Set condition: Dead.
The party informed that the set went dead while watching a program.  Before going dead; the picture shrunk vertically at about 4” width across the raster; (a horizontal belt), then turned to a horizontal thin line across the center of the screen, and finally it has switched Off by itself.  After that, the set won’t switch On at all.
They also said this is the fourth time; the failure of this type occurs for this set within a month time.  Each time they called the technician at home and he has replaced some components and it was worked gain.
It was clear that the party might have lost their trust with the technician, and brought it at my service center.  Whatever it may be.  let me check the set.
I opened the back cover.  Looked at the circuit board for any visual damage to any of the component/s.  Nothing found damaged.checked the AC mains fuse.  OK.  Shouldn’t be no fault with SMPS supply circuit.

Plugged the set to AC line.

While looking at the front panel Power LED, switched the set ON, by its main power ON/OFF switch, at its front panel.

The power LED lighted for some seconds, and went OFF.  No blinking indications.

It is clear that there should be some short circuit at the main board.  Almost all Philips circuits will have a short circuit protection circuit (built in) which will make shut down the SMPS circuit; if any short circuit has detected by the main system control IC.

Switched OFF the AC mains switch.  Unplugged the AC chord from the wall socket.

Measured if there is any storage voltage retains in the DC main reservoir capacitor.  No storage voltage detected.

Now it is safe to use the multitester; to measure the characteristic of component/s.

How to go forward?  From where should I begin?

I did recall the fault information from the party.  From it, was clear that as they noticed a horizontal bar, and then a thin horizontal line along the middle of the screen.  If so, the fault should be at or around the vertical scan output section circuit.

In case of Philips circuits, the replacement component/s should be the exact number that they have used in their circuit board, otherwise the service work will fail; no doubt.  I’ve many previous service experiences. I did a visual check on the vertical scan output IC.  The IC is found bulged [broken] at its center part.  See the picture.

Clear that it has damaged. I did notice, there are some aluminum folding (screwed along with the heat sink of the IC.)

It was clear that the previous technician might have noticed an overheating with this IC; and to prevent it; he have fitted these extra pieces of aluminum.

If so, what make the IC overheat?  3 possibilities are there.

1] Overvoltage (110VDC) from the SMPS output.

2] Winding short at the vertical scan section winding at the deflection yoke.

3] the IC number replaced might be different.

Here I have to make sure about which IC was used by Philips to this particular model. 

I de-soldered out the damaged IC.  Checked its number. 

The replaced one is “TDA4864J”

I know, this IC number has 2 versions. “TDA4864J & TDATDA4864AJ”

Both of them are used at vertical scan output section circuit by Philips.  Both the number can be used, but should make some circuit alterations.  Let me explain.

Look at these circuits [typical vertical scan output circuits used by both versions of this IC by Philips]

CLICK ON THE SCHEMATICS TO ZOOM

1)With CS2 (typical value between 47 and 150 nF) the flyback time and the noise behaviour can be optimized.

2) With R5 capacitor CF will be charged during scan and the value (typical value between 150 and 270 Ω) depends on Idefl, tflb and CF.

3) R6 reduces the power dissipation of the IC. The maximum possible value depends on the application.

I did compare the present circuit on board.  It was clear that the circuit is configured for TDA4864AJ.

Double checked the circuit.  Yes.  I’m correct.

It is sure that the fault lies here.  Instead of TDA4864AJ, the previous technician replaced TDA4864J. 

Now I have to make sure that there is no other short circuit at the main board.

Please recall; now there is no vertical scan output IC on board, as I’ve de-soldered it out from the circuit earlier.

Here is another culprit.

For almost all Philips digital TV circuits, the set won’t switch On, without vertical scan output signal.

I did plug the set, and switched it ON; without the vertical scan output IC at slot.

The power LED began to light up; and remain lighted.  It does not go OFF.

If the set is in perfect working condition; this LED will light up fitst; within seconds it will go Off and the raster will appear simultaneously; and the set will work perfectly.  Here the LED remain ON, because there is no vertical scan output signal present due to the absence of TDA4864AJ.

Switched Off the set. Checked all the components connected to TDA4864 terminals for any damage. Nothing fond abnormal.

Yet, I replaced the 3Nos Electrolytic capacitors used at this section, as a precautionary measure.

Replaced the IC TDA4864AJ from my service stock components.

Screwed out all the extra heatsink fins used by the previous technician.

Once again checked all the component/s around the TDA4864AJ.

Plugged the set to AC mains, and switched it ON.

Ahh!!

The set works fine.

Kept at this condition for about an hour.  Checked the heat of the TDA4864AJ periodically.

The temperature raised to 40 to 45 degree C and remain noconstant.

No overheating, a moderate heat, and it remain stationary.

MY WORK FOR TODAY IS OVER

Let me relax for a while.

Have a good day to all.

  Both the TDA4864J and TDA4864AJ consist of a differential input stage, a vertical output stage, a flyback generator, a reference circuit and a thermal protection circuit.

The TDA4864J operates with a separate flyback supply voltage while the TDA4864AJ generates the flyback voltage internally by doubling the supply voltage.

Differential input stage
  The differential sawtooth input current signal (coming from the deflection controller) is connected to the inputs (inverted signal to pin INN and non-inverted signal to pin INP). The vertical feedback signal is superimposed on the inverted signal on pin INN

Vertical output and thermal protection
  The vertical output stage is a quasi-complementary class-B amplifier with a high linearity.  The output stage is protected against thermal overshoots.
For a junction temperature Tj > 150 °C this protection will be activated and will reduce then the deflection current (IV-OUT).

Flyback generator
  The flyback generator supplies the vertical output stage during flyback.
The TDA4864J is used with separate flyback supply voltage to achieve a short flyback time with minimized power dissipation.
  The TDA4864AJ needs a capacitor CF between pins VP3 and VP2 (see Fig.2). Capacitor CF is charged during scan, using the external diode D1 and resistor R5.
During flyback the cathode of capacitor CF is connected to the positive supply voltage and the flyback voltage is then twice the supply voltage. For the TDA4864AJ the resistor R6 in the positive supply line can be used to reduce the power consumption.
In parallel with the deflection coil a damping resistor RP and an RC combination (RS1 = 5.6 Ω and CS1 = 100 nF) are needed. Furthermore, another additional RC combination (RS2 = 5.6 Ω and CS2 = 47 to 150 nF) can be used to minimize the noise effect and the flyback time.