TROUBLESHOOTING - CRT TV - NO PICTURE - AUDIO OK - WHITE SCREEN DISPLAY - SINGLE COLORED SCREEN DISPLAY - RASTER WITH RETRACE LINES etc: Contn

ONE COLOR PREDOMINANT SCREEN [RASTER WITH RETRACE LINES - BLUE]
                                                             Click on image to Enlarge

Before performing any repair work, a close look to all the solder terminals at the video output card is necessary.  In most cases, loose solder terminals can cause severe faults that can’t be predictable.  In this section, most of the components will get heated while set works.  Repeated heating and cooling might make the solder terminals to get loose contacted due to expansion and contraction of component leads.  It is advised to re-solder all the suspected solder terminals by applying a little more fresh solder to each terminal, without making any solder bridge short in between adjacent solder terminals.  Always give preference to this.  In most cases, the cause can be this.  Go further only after this check.

As discussed earlier, let’s concentrate on Blue Color Output.   Here, see an NPN transistor [2SC2688] is used.  All other color output section has used this number transistor, and the circuit configuration is the same. Come closer to blue color output circuit.  The collector of the transistor is connected to [+ voltage], generated by the line output transistor.  It is fed to the collector via [L601 & R604].  Here R604 acts as a current limiting resistor.  The emitter of the transistor is connected to Ground via R601 and AC is bypassed by C601.  The base of this transistor gets its bias from the chroma IC, and a variable resistor VC602 controls it.  The emitter of the transistor is also fed a [+ve] voltage to balance the color intensity (for short). 

Let’s examine the situation here.  We have no screen at all now.  Only sound output is there. No screen or picture.

 Check the voltage at the collector of Q601.  If yes [at about 200VDC], measure the voltage at its base.  Should be a small voltage [about 2V or so].  If not, suspect the chroma processor IC.  If yes, suspect the transistor. If transistor is opened, or there is some loose solder terminals either its collector or emitter, it won’t conduct.  The voltage at its collector will be high, and there will be no emission from the blue cathode.  But, if this is the case, there will be picture or screen, but the intensity of blue will be less or absent.

Here we have not screen at all. Assuming that the [200VDC] for video output section is OK.  If so, the line output transformer and line output section circuit are performing well.  There will be Focus voltage, screen voltage and EHT.  Then what might be the cause for no screen?  Surely there should be high voltage at all the three cathodes of the picture tube.  If the voltage is low, you will see either a plain white raster with thin retrace lines all over across it, or a color predominant screen with retrace lines.  Here the voltage is high, means, the transistors are not working.

Unplug the connection pin {R-G-B].  Switch On the set.  No effect?  Replace Q640.  It might be open.  But before replacing it, de-solder it out from the circuit, and check whether R641 & R642 are OK.  This might be the cause.

But, if any of the coil connected in series with the collectors of any color output transistors, or the resistor in series has opened, you will get a color predominant screen with retrace lines.  If Red color output transistor loss its voltage, you will see a red screen, if green output transistor loss its voltage, it will be green  and if the blue loses will produce blue.

                      White screen with retrace lines.  [Absence of  video output drive     

voltage or defective picture tube]

                                                      Screen with red retrace lines 

                                                  Green screen with retrace lines

                                                     Blue screen with retrace lines,