Electro help: PHILIPS 21PT138A-94R – Troubleshooting - FROM My SERVICE TABLE

Tuesday, March 17, 2015

PHILIPS 21PT138A-94R – Troubleshooting - FROM My SERVICE TABLE - Today

Raster height is less with a horizontal line across the middle of it_Vertical Scan output section circuit fault – Philips 21PT138A/94R CRT TV

Fault: The screen narrowed [at about 7 inches in height at the center] (See the picture ) A horizontal line across the center of the screen and it expanded vertically to both top and bottom. Lack of vertical scan pulse amplitude detected with Fluke scope-meter.

Vertical scan output IC used is TDA3653. PT138A Chassis. Normally it seems to be a vertical scan output IC fault; but was not.
Low input voltage measured at Pin-9 of this IC. Instead of 26.0VDC, an extreme low voltage of 8.9VDC measured. Checked all the related components, including capacitors and resistors. On checking, the carbon film resistor [used as a fuse resistor of 1 oHms measured a value of about 50 oHms. That was the fault. Replaced that resistor, checked OK.

Resistor 3449, that connected between the coil 5449 which in turn connected to pin-6 of Line Output Transformer [LOT]; which generates the 26VDC to vertical scan output section circuit.
It took around 25 minutes to locate the exact fault location and to pinpoint the damaged component on circuit board.

Troubleshooting procedure that I've done according to my service experience. It might be different to an other person.

As the set has light at screen, and there is no unusual Hissing or cracking sound; it is clear that its horizontal section circuit is OK. Otherwise, there will be no symptom of light. Here the screen do not fill vertically; but it do fill horizontally; as you can see the attached photo of the faulty screen below.

Something wrong with the vertical scan out section circuit. Might be IC damage, loose solder terminal/s or any other reason.

With this set, vertical scan out section circuit is built around the IC TDA3653C. It has 9 solder terminals. Pin-9 is its main supply input, pin-5 is the vertical scan output and pin-4 is the Ground for vertical scan output section fabricated inside the IC.

I've checked all the solder terminals at the vertical scan output section circuit for any loose solder. All found OK.

Powered ON the set. Measured the voltage at the IC pins.

Measured the input voltage at pin-9 of the IC. Found very low; at about 9VDC or so, and it was fluctuating too. There should be around a steady 26VDC normally.

Suspected a short circuit within the IC.

Unplugged the set from AC mains.

Measured the resistance[multi-meter set to measure Ohms]; between pin-4 [GND] of the IC to pin 9 [+] voltage input.

No short circuit symptom.

Reversed the meter prod polarity, and measured it again. No problem at all. I used an analogue multi-meter [Sanwa YX360TRF] to detect the fault.

Sucked out all the solder at pin-9 of the IC; using a solder sucker; to isolate the connection to that pin from the rest of the circuit.

Switched ON the set.

Measured the voltage at the print foil of pin-9 of the IC. Was found OK. 26.8VDC now.

Disconnected AC.

Soldered the pin-9 again to circuit.

Switched ON the set. The voltage dropped to 9V again. Something wrong with the 26V supply line. When it loads, the voltage drops.

Concentrated to that supply line.

This 26V is generated by one among the winding at the LOT, at pin number 6 of it.

Checked the rectifier diode used there. Was found OK. No reverse leak at all; even at MegOhm range.

Checked the resistor connected in series. Measured an abnormal value. More than 50oHms.

Referred the schematic. It was marked as 1oHm 2W; Carbon Film [CF] type.

Replaced it with a new one. Checked OK.

Checked the heating effect of the IC, for an hour use. No overheating. Picture was perfect without any vertical rolling. Vertical height and linearity too was perfect.

That was all.

Voltages at pins of TDA3653; at perfect working condition.