Sony KVM1430A low contrast

[vol.2]

Originally Posted by bigboss80

I don't have an oscilloscope, nor a color bar generator, I use a DVBT with color palette as a generator, from what I understood by reading R812 it is part of the ABL (Automatic Brightness Limiter) circuit, but I don't know if I misunderstood the diagram

It's out of spec and should be replaced. It's not likely that being that much out would mess things up that much. It's 70% of value, and the typical tolerance is 80%-120%. So it's a little low, not a smoking gun.

Even if you don't have a scope, you can use your DMM to check all the power rails shown in the service manual to start. If you can verify those are good, you can start checking voltages around transistors associated with the circuits that might have an effect on the brightness.

[bigboss80]

Originally Posted by vol.2

It's out of spec and should be replaced. It's not likely that being that much out would mess things up that much. It's 70% of value, and the typical tolerance is 80%-120%. So it's a little low, not a smoking gun.

Even if you don't have a scope, you can use your DMM to check all the power rails shown in the service manual to start. If you can verify those are good, you can start checking voltages around transistors associated with the circuits that might have an effect on the brightness.

However, I didn't even solve the problem with the resistor, I found two 516 ohm 1 w ones, I put them in series but it's still the same, but I noticed something that I had never stopped to look at, as soon as I put the white image and the decoder that uses the charge on the screen, the image is clearer, then once it goes to full screen it becomes gray, I also noticed that the white writing on the colored background is white, even the red overlay menus of the decoder when they are small are much brighter and more vivid, however, putting the menus in full screen turns off the brightness, as if it lacked power to keep the image bright at full screen, yet the voltages are correct, on pin 4 of the flyback transformer there are 122v, at the output of D805 near the flyback transformer there are 199v.

[bigboss80]

Originally Posted by bigboss80

However, I didn't even solve the problem with the resistor, I found two 516 ohm 1 w ones, I put them in series but it's still the same, but I noticed something that I had never stopped to look at, as soon as I put the white image and the decoder that uses the charge on the screen, the image is clearer, then once it goes to full screen it becomes gray, I also noticed that the white writing on the colored background is white, even the red overlay menus of the decoder when they are small are much brighter and more vivid, however, putting the menus in full screen turns off the brightness, as if it lacked power to keep the image bright at full screen, yet the voltages are correct, on pin 4 of the flyback transformer there are 122v, at the output of D805 near the flyback transformer there are 199v.

I took other measurements, I'll have to find something wrong ��, so, on + of C824, according to the diagram, 24 V should be coming, but I find 16.3 V. Then, continuing with D807 in parallel with C820, I find 3.8 V. I tested D807 disassembled and the multimeter is fine. If I read the diagram correctly, D807 and C820 go to pin 9 of the flyback .
The 24 V line that goes to C824 comes from pin 7 of this thing that I don't know what it is, I have to locate it on the board.

https://drive.google.com/file/d/1MI5...w?usp=drivesdk

[vol.2]

Originally Posted by bigboss80

I took other measurements, I'll have to find something wrong ��, so, on + of C824, according to the diagram, 24 V should be coming, but I find 16.3 V. Then, continuing with D807 in parallel with C820, I find 3.8 V. I tested D807 disassembled and the multimeter is fine. If I read the diagram correctly, D807 and C820 go to pin 9 of the flyback .
The 24 V line that goes to C824 comes from pin 7 of this thing that I don't know what it is, I have to locate it on the board.

https://drive.google.com/file/d/1MI5...w?usp=drivesdk

That's the vertical output IC. They tend to run very hot, and they tend to dry up capacitors close to them physically on the PCB. If you are getting weird readings around there, you should suspect caps in that area.

However, that usually leads to vertical failure, and not a dim picture (in my experience)

This might mean that you are dealing with multiple issues. There might be bad caps in the vertical, but there might also be something wrong with whatever is that 24V source.

"as soon as I put the white image and the decoder that uses the charge on the screen, the image is clearer, then once it goes to full screen it becomes gray, I also noticed that the white writing on the colored background is white, even the red overlay menus of the decoder when they are small are much brighter and more vivid, however, putting the menus in full screen turns off the brightness, as if it lacked power to keep the image bright at full screen, yet the voltages are correct"

Oh, I think this would be very helpful to figure out what is going on. I wish I was as good as the older guys on this forum. Hopefully one of them will chime in to make a suggestion as it sounds very solvable now.

[vol.2]

Originally Posted by bigboss80

as soon as I put the white image and the decoder that uses the charge on the screen, the image is clearer, then once it goes to full screen it becomes gray, I also noticed that the white writing on the colored background is white, even the red overlay menus of the decoder when they are small are much brighter and more vivid, however, putting the menus in full screen turns off the brightness, as if it lacked power to keep the image bright at full screen, yet the voltages are correct, on pin 4 of the flyback transformer there are 122v, at the output of D805 near the flyback transformer there are 199v.

I am not good enough to figure it out from the symptoms you previously described, but I feel pretty confidant that, if it's not a CRT problem, that you have a power regulation issue.

If the image suddenly get darker like that, then I would assume your power supply is getting swamped.

I think your best hope here is for one of the other more experienced guys to chime in and offer a suggestion. You might want to keep your comment focused on this information.

If anything, I would run more tests with on-screen information and document the conditions under which you see the "correct" brightness and when you don't. Maybe even a little video you can post somewhere and make a link to, showing how you are able to achieve the proper brightness under those conditions, and when it starts to fail. That would give people a better idea about what is happening, and they might see something that tips them off that you wouldn't recognize.

[bigboss80]

Originally Posted by vol.2

I am not good enough to figure it out from the symptoms you previously described, but I feel pretty confidant that, if it's not a CRT problem, that you have a power regulation issue.

If the image suddenly get darker like that, then I would assume your power supply is getting swamped.

I think your best hope here is for one of the other more experienced guys to chime in and offer a suggestion. You might want to keep your comment focused on this information.

If anything, I would run more tests with on-screen information and document the conditions under which you see the "correct" brightness and when you don't. Maybe even a little video you can post somewhere and make a link to, showing how you are able to achieve the proper brightness under those conditions, and when it starts to fail. That would give people a better idea about what is happening, and they might see something that tips them off that you wouldn't recognize.

I tried to make a video but you can't understand anything from a film, the only thing you can understand is by looking at those photos that I uploaded to Google Drive where you can see the difference between the two TVs, you can clearly see that the Sony has contrast problems, in fact measuring on pin 19 with the contrast at maximum I measure 2.6 volts, instead there should be 3.7 v as the service manual says.

https://drive.google.com/drive/folde...jgWwl3hniQjBD0

[vol.2]

Originally Posted by bigboss80

you can see the difference between the two TVs,

Perhaps I misunderstood. When you wrote this:

as soon as I put the white image and the decoder that uses the charge on the screen, the image is clearer, then once it goes to full screen it becomes gray

I assumed that you meant you can get the proper white color in full brightness to display on your "broken" TV in certain conditions. Is that not the case?

Based on that assumption, I was suggesting that the power regulation was getting swamped when you displayed images with more bright content on them.

If that's not the case, and you were just comparing it to a different, properly working TV, then maybe I'm wrong there.