Sony KVM1430A low contrast
 

Hi, I have a small Sony KVM1430A TV. A week ago, I replaced all the capacitors in the power supply, audio, and flyback transformer areas, as well as those on the board connected to the tube. Immediately after replacing the capacitors, the TV worked perfectly, but two days after replacing the capacitors, the TV seemed dimmer, less vibrant, and the image had shrunk slightly by 2 mm at the sides of the screen. I immediately fixed the screen width issue. I don't understand what happened. I tried sending a white pattern through a DVD player connected via RGB SCART, and as I suspected, the TV appears darker, with whites tending towards gray. I also sent a color pattern, and the colors are dark and not very vibrant, even with the contrast set to maximum. I tried measuring the tube filament voltage between H1 and H2 with an analog AC multimeter, and if the measurement is correct, it comes out to 5.4 volts. I think the voltage is low, it should be 6.3 volts, but I don't know how to fix it.

https://drive.google.com/file/d/1BPc...aZQWQBre1/view

https://drive.google.com/file/d/1RHf...Hsg4R3XHy/view

I redid the measurements again more accurately, with an analog multimeter with a connector mounted on the tube board, at the input of the H1 and H2 connector of the board it gives me 5.4 Volts and on the tube pins 4.3 Volts. With a digital multimeter 4.2 Volts on the connector and 3.86 Volts on the tube pins, instead measuring directly on the main board at the output by disconnecting the connector, therefore without the filament powered it gives me with an analog one by putting the positive lead on H1 and negative on H2 it gives me 5 volts, inverting the leads 5.6 Volts, instead with the digital one it gives me 4.5 volts and 4.34 volts.

Hi to all,
Hi @Bigboss80,

The CRT filament is powered by 15.6KHz pulse from a winding on the flyback transformer. Unless you use a meter capable of True RMS measurement, other types will give a +/- meaningless value.

Unless proved otherwise, your CRT heater is probably receiving the proper voltage/current. Is the neck glow a normal red/orange color?

I would investigate the main chopper power supply, on transformer T624's secondary, is the 120V DC line at the proper value?

Other members here probably have experience & advice about this Sony BE-2A chassis and ideas as to why your image is dim.

Best Regards
jhalphen
Paris/France

Hello and thanks for the reply, I took the measurements with a Sanwa XY360 analog multimeter, the color of the filament is dark red, not very bright, then I will try to measure the 120 Volt line, then I will let you know.

I just measured there are 128 Volts

I tried to measure the voltage present on pin 4 of the flyback transformer and from the diagram there should be 120 V or more because it should be the Vb, I put the negative to the negative of the frame and the positive to pin 4 and there are 128 Volts with the TV in standby and 122 Volts with the TV on but the board was disconnected from the tube, only the suction cup connected

I have found that these kinds of sudden issues that happen after doing a big recap or repair can be often caused by two things.

1) there are some broken solder joints on the PCB and you disturbed them when you did the repairs. you will not be able to see them with you bare eyes in most cases, they require a microscope to see them and even then they might not be visible until the board is running and heats up

-solution is to reflow the boards. pay special attention to connectors, heavy or large components and any area that looks burnt on the PCB

2) the G2 pot on the flyback is wonky and if you touch it, the brightness jumps around. easy to verify by poking it a bit or putting some upwards or downwards pressure on the screen knob while the set is running. be careful, if this is the problem, it will not take very much pressure to make it happen. be very gentle and don't touch it with your hand when you do this, use a long plastic thing

if this is the problem, it can sometimes get better with some deoxit or something like that, but it's usually unfixable. you can also sometimes get it to the right spot and fix it in place with a little loctite, but i won't promise it won't move after that. plan on looking for a new flyback. good news is that this is less common (but i've had it happen to me a couple of times)

I double-checked all the capacitors I replaced to make sure I didn't get any polarity wrong and they are all correct, then I also checked for any fractured tracks near the capacitors and I found only one, the one that connects the positive of the large 160 Volt 100uf capacitor, it wasn't completely disconnected but touching it disconnected it, I reconnected it properly but still nothing changed, I disassembled the screen trimmer and cleaned it with contact deoxidizer and tested with an analog multimeter and it works fine. I tried tapping the connectors but nothing strange can be seen, the image is always stable, I also re-soldered them with new solder, even the tube socket.

When I said the screen pot, I meant the one that is permanently attached to the flyback and cannot be taken apart; those get loose inside sometimes and become very flakey. You should check that it isn't wildly loose and that it doesn't cause the brightness to jump when you move it around a tiny bit.

Sorry that reflowing the connectors didn't help. It's a common cause of these kinds of sudden issues and is fairly low-hanging-fruit in terms of effort, so it's still best practice to do this.

Did you try to re-balance the brightness with the background and drive pots? If you can do that, then maybe it's just fine and you're worrying for nothing.

Thank you for your reply, I tried to move the red wire of the flyback transformer, but the image is stable, the wire that comes out of the flyback transformer is very fixed, I also tried to tap on the motherboard but nothing changes, I tried to adjust the potentiometers R G and SCRN but it does not solve the problem, by moving SCRN I can see the thin horizontal stripes and the screen becomes light blue, but it cannot solve the problem of low brightness, at first I also thought it was my eye that saw a bit dark, but then I realized by putting a white screen that I saw it tending to gray and the brightness did not push well, I have two other CRT at home not Sony and I saw that the white is powerful white and the colors are very bright on the other two TVs

I've been looking for another CRT TV I have at home, a small 14-inch Sinudyne from the late 1990s. I've used it a lot for over 10 years. It's been 15 years now since I've turned it on seriously, except once in a while to see if it works. I've put them side by side to explain the difference between the two, and until last week, the Sony was brighter. The white is the one that stands out the most. Before, the Sony was an ultra-bright, cold white, but now they're dull. The colors are nice and full and uniform, but they lack brilliance.

The Sony on the left, the Sinudyne on the right.

The white isn't so gray, but it's almost there; it's a bit distorted by the camera.

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

SCRN doesn't primarily adjust the brightness actually. It's kind of confusing unless you're already used to the way TV sets are designed, but it's backwards.

SCRN is the same as Brightness is the same as G2 on the CRT neck. It adjusts the Black point, not the Brightness (confusingly). You want to set that so that a black screen is totally black (with the user brightness set to the middle position). maybe a tiny bit grey, but JUST barely. NTSC black is actually 7.5, which is a very, very dark grey. I won't get into the details, but best practice is to wind in down until the screen is totally black, and then carefully turn it up until it's just riding the edge of black/grey.

You do not want to see the thin horizontal strips, that's bad. You need to back it down well past that until the screen goes black (and then up the tiniest bit).

The brightness of the image is actually controlled by the contrast control, also sometimes called "picture." It might be in a user menu accessed with the remote control on your set.

I'm beginning to think maybe the contrast/picture just got reset to the middle and it just needs to be adjusted.

Thanks, I have already tried to adjust SCREEN as the service manual says, but the maximum achieved are those results that I put in those photos on Google drive, there is no brightness in the colors and the white is dark, it is a contrast problem you are right, my TV does not have a service menu, the adjustments are all made via trimmer on the printed circuit board of the main board, the contrast that is set from the TV remote control however even set to maximum the result are the images that I posted before, there is some component that is not working well.

I found a resistor R812, a fuse resistor near the line transformer which should be 1Kohm but instead with the multimeter it measures 1.4kohm, is this an acceptable value or is it out of range and could cause problems, I saw from the diagram that it is connected in series to a D806 diode which in turn is connected to G2, could it be the cause of the TV problem?

It's out of spec and should be replaced, but it is unlikely to be the problem with your set. It's 70% of spec which is almost certainly not going to bring a whole portion and the circuit to its knees.

If there is something wrong with your set that it not just the CRT tube failing, then you will probably be able to find a problem with something in the service manual. There should be voltages listed all over the place, and possibly waveforms to check with an oscilloscope. Sony was pretty good with the waveforms, and a Sam's should have them too.

First order of business is to verify all of the power supplies. TVs tend to have a bunch of supply sources taken from both the initial power supply of the sytem, and also some derived from flyback pulses. Check all of them.

Once you've done that, start checking voltages on ICs and make sure everything is correct.

Then check all the waveforms.

At some point, you'll find an issue.

Keep in mind that voltages and waveforms are specified with a specific input condition, like a color bars input or a window input, etc.

What you have displayed on the screen will change voltages in many places, and waveforms in many places.

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.

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

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.

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.

I ran some tests on TDA3508 , first I measured the voltage on pin 19 soldered to the circuit and got 1.74 V with both maximum and minimum contrast; then I isolated pin 19 and measured only the voltage on the pad, getting 1.46 V. in the diagram it says that it should be 3.7 volts on pin 19 ,with the pin disconnected from the circuit, the image became completely devoid of contrast.
https://drive.google.com/file/d/170q...w?usp=drivesdk

Forgive me but I just double-checked everything and saw that I was measuring on pin 20, that's why it didn't change when touching the contrast, I redid the measurements carefully this time. On pin 19 connected to the circuit with contrast at maximum 2.6 Volts, contrast at minimum 1.99 Volts, instead on the pad with pin 19 isolated everything at maximum 1 Volt everything at minimum 1.4 Volts, then while I was at it I also measured 20 but this time adjusting the brightness because that is about the brightness, everything at maximum 2.2 Volts everything at minimum 0.9 Volts.

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

Perhaps I misunderstood. When you wrote this:

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.

I finally solved the problem, it was in the ABL circuit, R816 was interrupted and the system went into protection and therefore lowered the power of the tube.

https://drive.google.com/drive/folde...u4ufxRiVzpYrnV

Fantastic. Love to hear it. Sorry I couldn't be more help, but it looks like you didn't need it.


Based on those pictures your color purity is pretty bad though. It probably needs a good degaussing from a professional level coil device.

Thank you, regarding the purity of the color, the color seen live is perfectly uniform, there are no halos or defects, what you see in the photo is an effect that the camera does, I don't know why it does this, perhaps it is the refresh rate in conflict with the smartphone camera

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