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.
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.
The Toshiba indicated here has 3 picture tubes---the set will blank or turn them off if there is a problem in either the deflection circuit (vertical or Horizontal) or if any of the voltages (supply and screen sources for the tubes are missing.
It means either a critical voltage is missing or a deflection issue causes the set to turn the tubes off so a mark (either vertical or horizontal) is not instantly burned into the screens of all three tubes.
Means for starters voltages and some other things need to be taken to determine where problem is.
If you have sound and feel the static on the front screen you have audio and high voltage---I would suggest one simple thing---with back off look carefully in the neck of the picture tube--should be 3 dimly lit lights there --the filaments for the picture tube--are they lit or not?
Rare for this model to have sound and not a picture or brightness on the screen--usually a shutdown circuit will activate.
Let me know what you find or if you have questions.
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..
Your set, with a dark screen or no picture but with sound is failing in the high voltage to the picture tube. It is caused by a failure in the horizontal deflection circuits which generate the high voltage (20,000 volts). The repair is expensive; at least $100 or more, most likely, depending on exactly what components have failed. Regards
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.