TV Missing color diagnosis

You need to provide the rest of the troubleshooting voltage and waveform measurements at the crt for anyone to help.

too high.

No I did not say the voltage was too high. I said the bias or drive to the blue was too high.

What he's trying to say is that high cathode voltage reduces brightness, all other factors being equal. But you need all the other readings.

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This, however, is not fault finding. It is fault condition correction by bodging ...

( or botching if you prefer )

Arfa

I am trying to locate the actual drive circuit, but makes it quiet difficult without a schematic and if you don't know wtf you are looking for.

I have done some more testing and found that although i initial get low readings of Blue = 120VDC, Green =195VDC and Red =195VDC at the cathodes, if i leave the unit on for about 5mins the voltages are all the same. But still has a slight red tinge and green is still low.

Now these are the reading from the collector of the drive transistors and basically and cathode pins to the CRT. On readings i get on the otherside of the resistors for each gun is about 217VDC constant and equal between them all.

Now in this unit it looks like the main guns are red and green for intensity setting and the blue gun separate, as it has a slightly different circuit layout and components.

TEX

Art wrote:

You can see if it may be a driver problem by switching color drive feed wires to the crt board. As in switch the red with the green and see how bright the color is driven. This may eliminate the problem from the color output from the chassis and you can concentrate your effort to the neck board. If the green is bright after switching with red driver, then you will know the green driver is weak (not the output on the neck board). Jtt

Confirmed it is not a color driver feed issue. Found a schematic for another model TV and from that worked out which wires were the control/drive lines. 5 wire connection RBG gnd and 12v. All the input drives give value of around 3.1v. Swapped drives and confirmed they working.

So confirmed it is either a CRT fault or neck board fault. I was hoping that since the voltages i was getting at the driver transistors are the same weather the board is connected to the CRT or not, then it is board board.

Where to next??

TEX

James Thomps>> Been working on this 29" TV and cannot work out why the voltage on the one

After checking the CRT neck board again and found no obvious faults. I checked the driver signals and found that the Blue drive is double of that of the other two guns. As previously stated I am sure i noticed that the cathode voltage had equalised after a few minutes of running, but coloured hue was still present.

Will trace back further and see what else I can find. So maybe it isn't a CRT output drive board then...

TEX

TEX wrote:

Perhaps it has developed a small short do to oxidation in the picture tube. If you can find an old shop around with a picture tube rejuvenator, or if you might have one, hook up the tube and do a short test and zap the grids with it. Might not take much to get it back to good shape, since you say it work fairly well and only has a slight dulling to the green.

other

Worst case scenario ,,,,cut the trace to the pin and start adding 2M resistors,,,,a pot would be nice .I've done it on the screen voltage on monitors and corrected a BUNCH of them,,,,second thought 2Mohm might be a little excessive,,,,,,,;))) start w/whatever you have. Good Luck joe.

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Also check that the filament supply resistor has not gone high.