Sony KVM1430A low contrast

[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