No picture dark screen,sound and system operational, tubes lit,no screen and focus voltage vertical circuit has complete negative and positive supply,is it possible if one of the crt's are deffective may cause similar problem?
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Re: no picture,sounds and system operational
The vertical circuit may have its positive and negative supplies, but is the output waveform correct? I have seen many of those TDA vertical outputs open up and cause exactly what you are seeing. The + and - supplies are there but there is no output. Replacing the IC usually cures the problem.
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EHT (Extra High Tension) usually reefers to the 24kv to 30kv, focus and first anode (A1) voltages required to operate the crt (picture tube). These voltages are supplied by the line output stage, primarily the line output transformer (often refered to as the flyback transformer). This transformer steps the voltage up to 8kv and an embedded rectifier trippler circuit tripples this to 24kv dc to supply the final anode voltage to the tube (Hence the big rubber cap and thick cable.
The line output transformer is driven by a smaller transformer (line driver transformer) via the line output transistor.
The driver stage is fed by the hoizontal oscillator circuit.
So we can see that a failure in any part of this circuitry will most often result in no picture. The line output transformer is the heart of the system, it not only supplies the EHT voltage it also has to
supply an LT voltage for the vertical output stage but also the sound output stage. A Fault in the A1 circuitry would result in a very dark picture but sound ok. Or the picture badly out of focus.
The line output transistor going short circuit results in no picture and no sound with the set shutting down to the standby mode or
hic upping (making a ticking noise. This problem would be amongst the most common and is easily confirmed with an Ohm meter. The line output transformer itself can develope an internal short with the same results (less common). Failure of the line oscillator and driver stages are most easily confirmed with an
oscilloscope. A fault in the power supply will also give these symptoms. To confirm line output stage is functional look to see if the tube heaters are glowing, this supply also comes from the line output transformer. Can you feel the static if you run the back of your hand across the front of the screen (often feels like running your hand through cobwebs. this would indicate that the EHT is present and the fault is elsewhere. Understanding how something
works enables you to find out why it doesn't. If you have occasion to replace a short circuit transistor, be sure to reslder all pin connections to the line driver transformer. I've found dry joints here
to be a cause for random inttermittent failure of this transistor,
Best regards from Vintie.
<p>How to check the colour cathodes of a picture tube for emission<br />
<p>It is very easy to cehck the emission of the cathodes of a colour picture tube. Assuming all other circuit parts are working, including LOT [Line output Transformer] and video output section.<span> </SPAN>There is proper screen voltage; focus voltage and EHT [Extra High Tension]. Locate the three cathode points on the base card, usually marked as Gk, Bk and Rk.<span> </SPAN>Keep the brightness, contrast and colour control to zero position.<span> </SPAN>Take a wire of suitable length, say 50cms or so, tie its one end to TVs chasis [negative].<span> </SPAN>Take one resistor of value 100K 1W, tie its one end to the other end or the wire, thus one end of this resistor is now connected to negative chasis and the other end of it is free now.<span> </SPAN>Hold this resistor with with some electrically noncunductive material [using hand gloves will be better]; switch on the Tv, wait a minute or so to get it warm up.<span> </SPAN>Just make a touch at each cathode of the picture tube base card solderings.<span> </SPAN>Just one touch will give you a colour of the respecive cathode on the screen.<span> </SPAN>Never hold it for long time. This process can be used for checking cathode emission of any type of Cathode Ray Picture tubes. <br>
Its a pretty good set, If no brightness on screen I suspect you will find either the three tubes are not firing up (lit) or it has no high voltage and there are not too many things a do it yourself person can do unless they have a volt meter and know how to check voltages and resistance.
This model does have a light box which means all the working part of the set less the speakers can be removed--called a box as it looks like one when removed.
Most of these have convergence problems which will either result in a dead set with blinking power light or a 3d picture.
It is possible it has a short in the sweep circuit but that also triggers shutdown and the power light blinking.
About the only simple (not real simple) thing might be the voltage for the filaments in the 3 tubes may be missing, If high voltage was present and those not lit it could cause your problem.
In addition the set like most is well protected, if no vertical or horizontal sweep is detected the set blanks (turns off) the 3 picture tubes to prevent them from instantly burning the mark into the face or screen of the tube.
What I am saying is there are several possible reasons all of which require some skill to find much less fix.
Either (unless we have a tech in the family) time for a shop or time to retire the set and go shopping.
There are many cause of dark picture symptom of standard television.First thing to do is try to adjust the screen control adjustment located at the flyback at the lower parts.There are two flyback adjustment the focus and screen adjustment.The upper level is focus and the lower is screen control.
If check also the CRT socket board section.This CRT board is compose of high voltage terminal coming from the high voltage supply.Check if there's a cold solder in CRT socket terminal and other parts around the CRT socket.The heater supply,the RGB cathode supply,and the screen supply are the supply of picture tube to be lights up.So try to resolder those parts of the TV.Avoid short circuit in the PCB board.Hope it will help you.Thanks..
check the high voltage with the screen control turned up high where there is no raster. suspect problem within the video circuit where there is no high voltage or defective picture tube.some time when the screen is black and the screen control advanced,you can see if the tv has video or vertical or horizontal problems on the screen.service the high voltage circuit if there is no raster or high voltage is seen.no raster can be caused by the no heater or filament in the CRT.----
a defective picture tube can cause many problems, such as, poor brightness ,missing colors,intermittent picture,poor focus,a single-colored raster,arching in the gun assembly,retrace lines in the picture,negative picture,chassis shutdown,noheater or filament lit and no raster to name a few.an open filament or heater can cause no picture or no raster symptom.a defective CRT can have an extremely bright screen.loose particles of the cathode element can lodge between the grids and cause an intermittent black and white picture. simply tap the ends of the CRT gun assembly and notice if the picture begins to flash off and on.----
with a leaky picture tube after turn on the raster dims and get extremely bright.a dim picture with no green in the raster can result from a bad green assembly. a heater-to-cathode short can cause a raster to change colour or a result in no raster at all.replace the picture tube if there is severe arching in the neck of CRT.
excessive brightness with retrace lines and a vertical collapse to a thin line can be caused by a bad coil on the neck board of the picture tube.rejuvenate the picture tube when there is very bright green screen.
check for the resistance change on the resistors of the CRT neck board for an excessively bright picture with retrace lines.excessive dust inside the CRT spark gaps within the picture tube socket can cause the very bright raster and the chassis shutdown.a defective video amp,luminance buffer,or reference transistor can cause a bright picture with retrace lines.
for getting any required parts to replace you can get it from
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What you are experiancing is loss of High voltage. The line is called a retrace line. It is usally caused by failure of the flyback circuit or aging of the TV tube. This is the single most common failure with CRT based display systems. The high voltage can be tweaked up, to accomidate for the age of the tube. Its a short term solution. Because of the high voltage nature of this circuit, suggest a professional repair.
First you can up the screen voltage, by finding the HV transformer. It has Screen and Focus control on it this it the Transformer that is in back of set and has an anode lead going to Picture tube. Come up on the screen control slowly and if the picture brightens with vertical lines then it's too high, The red problem is a small transistor that maybe located on the CRT board. It's probably shorted. There are about 3, each going to the different colors Red, Blue, Green.
This sounds like the vertical output IC has stopped working. If the vertical pulses are missing at the yokes then the picture shuts down to protect the CRT's from being burned by the intense thin white line through the center of the picture. I have seen many of those TDA vertical output IC's fail.