I need help with troubleshooting Sony GDM-FW900 24” CRT

[matthewmcconaughey]

Hello, I am the first owner of the monitor. I reached almost every repair store in my area, but no one even wanted to hear about troubleshooting an old CRT monitor. At this point, I have been trying to figure it out by myself, but I have no idea where to start or look for.

I wish to know how to think and locate a failing component. I will put all my efforts to learn and contribute.

1. It started with a bright and greenish picture upon starting a cold monitor.
2. About 2 years ago, only on the cold boot, the screen started to occasionally flash with a green picture with retrace lines.
3. About one year ago, when the screen was flashing on a high resolution I had to switch the input to only the OSD menu because in another case the screen would turn off and the diode would start to blink orange.
4. At this point, I started to turn on the screen with 640x480 resolution to the moment when he will warm up.
5. Always after the power station was turning on and off electricity, the issue started to progress. Basically, the electricity was cut off from the monitor a few times in a row in a very short period of time.
6. After a few episodes like that, the screen started flashing red instead of green.
7. After a few months, it was green and red in a similar proportion. The monitor had to be warmed up by switching INPUT 1/2 to calm him down.
8. At this time I have decided to keep him turned on all the time. He was working from one electricity shutdown to another.
9. After another episode with electricity, I wasn't able to power him on. I need to press the power button on and off for 20 minutes to finally succeed.
10. At this point, the only way to power him on was by pressing power on and off and changing the inputs from the front panel. He needs to stabilize and I won't do anything he will turn himself off like in this video
    
11. The last few times I was able to power him after heating his back with a hairdryer. When he was turning on he wasn't flashing red or green. He was warmed up.
12. At this point, I have stopped trying to power him on. There is a big chance that I would be able to turn him on but in the long run, it's pointless.

I have to fix my friend.

From the observations.

• When the monitor will luckily turn on there are absolutely no issues as long as he is displaying the picture. When the monitor will lose power or will go to standby mode it is not working and cannot be turned on.
• When the monitor will somehow turn on it is flashing green, red, or both. It depends on the room temperature.
• If he receives a high-resolution picture from a graphic card (higher voltage needed to operate) he is flashing green and red and goes to the standby mode and cannot be turned on again. He cannot stabilize
• It was required to keep him on 640x480 dpi until he will warm up
• If the back of the monitor was preheated with a hairdryer from the sides and turned on there is no green and red flickering and can instantly operate on higher resolutions with no issues. If he will luckily turn it on.
• When he cannot be turned on and there is no graphic card connected to the monitor the led is green and goes to standby mode. There is no OSD menu displayed at all or green INPUT subtitles on the screen.
• If he has a graphic card (first video input 1) connected, the monitor after a few seconds starts blinking orange - no light - orange no light infinitely. In the video below I am switching from INPUT 2 (no source) to INPUT 1 (VGA source). For BNC sources the same situation. There is no OSD menu displayed at all or green INPUT subtitles on the screen.

- I have dissembled him and could not find any burned, discolored, odd electronic elements.
- I couldn't spot any cold joints with a magnifying glass.
- I measured the output G board voltages upon turning on the monitor (green diode).
- I have corrected soldier joints around flyback
- Tube, Flyback, and caps are unlikely the issue (the guy who is having the same symptoms on the video had replaced every single cap across all of the boards)
- Something is shorting him heating up is not helping anymore

CN651
7V = 7V

CN650
STB5V = 5V
1W5V = 5V
ECO SW = 5V -> po chwili 3.5V
HTR SW = 4.9V
DGUSSW = 4.8V -> 0V
PWR SW = 5V

CN652
STBY5V = 5V
1W5V = 5V
+80V = ~15V for 1/2s -> 0V
+220V = ~90V-200V for 1/2s -> 0V
+12V = ~3.43V for 1/2s -> 0.04V
HEATER = 5V

CN653
+15V = ~1.36V for 1/2s -> 0.02V
-15V = ~2.43V for 1/2s -> 0.02V
+80V = ~3.8V for 1/2s -> 0V

CN653
H CENT H = ~4.3V for 1/2s -> 0V
H CENT N = ~6.3V for 1/2s -> 0V
H CENT L = ~7.2V for 1/2s -> 0V
+220V = ~15V for 1/2s -> 0V

- I have also measured the input on pins 1 to 9 on IC620 MCZ3001D SW REG CONTROL. They are not correct according to the Service Manual. I have voltages on GND and 0. The complete measurements in my playlist https://www.youtube.com/playlist?lis...rUeEXE58CKJpQU

First video from the playlist

At this point, I don't know where should I look for a failing component and how to proceed. If someone has an idea what should I check next and how it suppose to behave I would be grateful for any help. Please, I don't wanna hurt my friend, and it is hard for me to proceed on my own.

 

[matthewmcconaughey]

Thank you once again for getting back to me. The PSU output without heating the PCB
CN652
STB = 5V
80V = 79V
220V = 218V
12V = 12.0V
Heater = 5V (pin 9)

You might be right that it is not PSU anymore. I was blindly focused on PSU all the time and I wasn't thinking straight anymore. I understand it is about A board now and measuring G1 H1 H2 KG KB voltages. One probe GND one of those points?

Greatest signal I put gray picture full screen from PC?

I have read under a youtube comment with this monitor doing the same thing with green colors

(1) the green screen with retrace lines is due to a bad resistor near the flyback, it Is the "heater resistor" Look for one 3.3 -1. 5 ohm 1/2w.

and another person replied

(2) not a heater resistor. It’s a tube bias resistor and is to do with the G2 tube bias voltage. Nothing to do with the CRT heaters

this is all I know about this issue, I am an idiot, sorry. Where are those resistors located, the bias one?

Moreover, when I turn the monitor on and off 5x times in a row each time, he has less red/green.
After 3 times, there is no red color anymore only green.
The picture is black after 2-5 sec -> green.
After 5 - 7, monitors turn on in the row picture stays black and has no color issue.

After that colors are normal permanently, this issue occurs only when the monitor is starting up and warming up.

 

[Nigel Goodwin]

There is no heater resistor in this set, and even if there was it wouldn't cause this issue - and in my 46 years of experience, and repairing tens of thousands of TV's, I don't recall ever seeing one fail? (or hearing of one failing).

Tube bias resistor? - sounds made up, I've never heard of any such component. In this particular set the G2 voltage is rather strange as it's done in software, but is shown as 522V on the manual, but will vary depending on adjustment. So check that on the CRT base, and also check KG, KB and KR - again these voltages will vary depending on the settings (including that of G2).

As I've suggested all the way through this thread - the CRT is the prime suspect - particularly as it's red that has the issue.

You might haver noticed that most old CRT oscilloscopes have either green, or blue, traces - this is because these are the most efficient phosphors - red is FAR less efficient. So the red gun in a colour CRT is driven much harder than the other two, so red is the gun that usually rears out first.

But check the voltages on G2 and the three cathodes, and post the results.

 

[Tony Stewart]

It looks like a very precise design with only one pot ! That is on the video board for RV901 HV ADJ with the intention of board-level replacement. Bravo Sony! Very nice CRT 383 MHz pixel clock which can push 4k Res, but get it working on low res 1st and reduce brightness as much as possible to spare life of tube, supply and limit Xrays.

I am impressed with this design, and looks better than the old 24" Hitachi I had. THis TV is a great example for teaching how to design consumer products at industrial quality.
albeit tubes are passe now. This is a fine one.