Sony KVM1430A low contrast

[bigboss80]

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

[bigboss80]

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.

[jhalphen]

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

[bigboss80]

Originally Posted by jhalphen

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.

[bigboss80]

I just measured there are 128 Volts

[bigboss80]

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

[vol.2]

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)

[bigboss80]

Originally Posted by vol.2

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.

[vol.2]

Originally Posted by bigboss80

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.

[bigboss80]

Originally Posted by vol.2

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

[bigboss80]

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

[vol.2]

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.

[bigboss80]

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.

[bigboss80]

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?

[vol.2]

Originally Posted by bigboss80

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.