No Color. Black and White Picture

[Thirantha]

This is a Sharp CRT television. Model Number C-2091G . R1118C Chasis. It does not have remote control functionality. Now the picture is B/W. No color. I checked all the transistors, resistors and diodes in the chroma and video sections. They were all fine. Then I replaced all e-caps and pf caps in the chroma and video sections. I also replaced 4.43 MHz crystal and Chroma IC. But no any change. I also checked the Color control Variable resistor and spray some WD-40 into it. Now it is fine. There are some trim pots in chroma and IF sections. I also turned them and checked but No any change happened. Still the picture is Black and white. What causes this problem? Can defective power supply cause B/W picture? Do I need to check the tuner?

 

[kpatz]

Did you check the Color control? If it's turned all the way down (or not working) it could lead to a B&W picture.

 

[shrtrnd]

Yeah. It's possible you have a circuit problem, but it could just as likely be something simple like a knob.
I used to work in a TV repair shop during the CRT days. I'm trying to remember the quick things to check.
There was one pot that was the first thing to check on this type of a problem but I can't remember what it was.
'color burst', or something like that.
This can also be caused by one of the adjustment pots being fried.
No heat damage visible on the circuit board (boards)?
Like kpatz said, you might consider tweeking each of the external adjustments to make sure you just don't have the color turned-off.
DO NOT mess with any pots on the convergence board if it has one.
If I think of anything else obvious to check I'll send another post. You'll probably get other ideas here from other members in the meantime.
I'd look really carefully at the components and pots, just to see if any of them were heat damaged, to isolate the problem.
Good luck

 

[Thirantha]

Yeah. It's possible you have a circuit problem, but it could just as likely be something simple like a knob.
I used to work in a TV repair shop during the CRT days. I'm trying to remember the quick things to check.
There was one pot that was the first thing to check on this type of a problem but I can't remember what it was.
'color burst', or something like that.
This can also be caused by one of the adjustment pots being fried.
No heat damage visible on the circuit board (boards)?
Like kpatz said, you might consider tweeking each of the external adjustments to make sure you just don't have the color turned-off.
DO NOT mess with any pots on the convergence board if it has one.
If I think of anything else obvious to check I'll send another post. You'll probably get other ideas here from other members in the meantime.
I'd look really carefully at the components and pots, just to see if any of them were heat damaged, to isolate the problem.
Good luck

I checked the color control with multimeter. It had some problem. Then I sprayed WD-40 into it. Now it is working fine. There are no invisible burn on the circuit board or near pots. There are three presets in the crt board for R,G,B colors. When I turn them, the background color changes to Red, Green and blue color. Does it mean that chroma circuitry is fine? What area should I check?

 

[KrisBlueNZ]

The individual R, G and B trimpots (often on the CRT base board) control the drive and bias to the red, green and blue cathode driver transistors. Adjusting them will change the colour of the picture but this doesn't have anything to do with the chroma circuitry; that's all much earlier in the signal chain.

I don't know what to suggest to fix the chroma problem. It sounds like you've tried all of the likely problems. I suppose it could be a problem in the tuner or the IF because the chroma subcarrier is towards one of the edges of the IF filter response, but I don't know how to test for it.

Have you checked that the line sync pulse is present? This is usually taken from the line stage and is used by the chroma IC to time the detection of the colour burst, IIRC.

Do you have a service manual and/or schematic?

 

[Thirantha]

The individual R, G and B trimpots (often on the CRT base board) control the drive and bias to the red, green and blue cathode driver transistors. Adjusting them will change the colour of the picture but this doesn't have anything to do with the chroma circuitry; that's all much earlier in the signal chain.

I don't know what to suggest to fix the chroma problem. It sounds like you've tried all of the likely problems. I suppose it could be a problem in the tuner or the IF because the chroma subcarrier is towards one of the edges of the IF filter response, but I don't know how to test for it.

Have you checked that the line sync pulse is present? This is usually taken from the line stage and is used by the chroma IC to time the detection of the colour burst, IIRC.

Do you have a service manual and/or schematic?

No I don't have schematic or service manual and it is not available in the internet also.

 

[KrisBlueNZ]

Right, I couldn't find it either.

What is the part number of the chroma IC? Do you have a data sheet for it?

 

[Thirantha]

Right, I couldn't find it either.

What is the part number of the chroma IC? Do you have a data sheet for it?

Chroma IC is 1X0129CE. I replaced this IC.
Video IC is 1X0118CE. I did not replace this IC.

The following link show the service manual of Sharp C-2092SP Model. It's circuit diagram is much similar to the this model.

http://elektrotanya.com/sharp_c-2092sp_sch.pdf/download.html

 

[KrisBlueNZ]

Who is the manufacturer of the chroma IC?

Do you have a data sheet for it?

 

[Thirantha]

Who is the manufacturer of the chroma IC?

Do you have a data sheet for it?

Manufacturer is Sharp. Sorry I do not have a data sheet for it

 

[KrisBlueNZ]

Check the components and tracks in the path from pin 8 of the chroma IC to the flyback transformer. On that schematic they are R824 and R825, fed from pin 8 of the flyback.

If that's all OK, I don't know what else to suggest.

 

[Thirantha]

Check the components and tracks in the path from pin 8 of the chroma IC to the flyback transformer. On that schematic they are R824 and R825, fed from pin 8 of the flyback.

If that's all OK, I don't know what else to suggest.

Thanks for replying. I checked those two resistors you mentioned by removing them. They are fine.

 

[KrisBlueNZ]

And did you check the track for continuity all the way to the flyback transformer?

 

[Thirantha]

Yes. It shows the continuity to flyback pins

 

[KrisBlueNZ]

OK, well I'm out of suggestions :-(

 

[shrtrnd]

You probably have a circuit problem, but from your description I'd eliminate the pots as a potential source of problems.
I'd replace both the 'color' and 'tint' pots, and then see what you've got.
Like I said, a bad pot will result in your issue.
good luck

 

[ramussons]

Your picture tube is bust. Technically speaking, the "control grids" of the 3 guns have fused and the colour guns have lost their independent control.
Replacement of the picture tube is the solution.

 

[KrisBlueNZ]

Your picture tube is bust. Technically speaking, the "control grids" of the 3 guns have fused and the colour guns have lost their independent control. Replacement of the picture tube is the solution.

I'm afraid that's absolute rubbish. Please stick to advising people on subjects that you have some knowledge and experience with. The colours in a CRT are controlled by the cathode voltages, not the control grid voltages. In any case if you read post #4 you will see that this explanation can't be right.

 

[ramussons]

Yep, missed post #4. But it is my experience that the grid mesh of the 3 colour guns (ok, may not be colour control grids) fuse and the colour tube behaves like a monochrome one. The tube displays a B&W picture with just 1 colour gun (cathode) in operation.
Post #4 confirms that this is not the case here.

 

[KrisBlueNZ]

I have never heard of this happening. You may be thinking about heater to cathode shorts that can occur and can sometimes be cleared by applying a large current pulse. Have a look in the neck of a CRT sometime and you'll see that the control grids and accelerating anodes are too far apart to "fuse" together.