Vendex VGA CRT Monitor Issue

[chjmartin2a]

Hi,

I have a VGA CRT monitor that I want to use. Unfortunately, it randomly turns off and then back on, without any predictability. I removed the power board and saw a bulged capacitor. I replaced all 7 caps on the power board. I left the main board alone. The picture seems brighter when it is turned on but it still randomly shuts off. I tested the switch which is just a mechanical switch and I have relatively high confidence that the switch is fine. When I saw shuts off, the LED on the front shuts off and you get that squeaky familiar CRT turning on or off sound. My next move would be to replace the switch and also replace the other electrolytic caps on the main board, but now I am just shooting in the dark. Any feedback on what I should test or do to try to isolate the issue better?

Thanks,

Chris

[thchardcore]

Did you inspect the flyback joints and all vertical ICs? Anything mounted to a heatsink is usually the first to exhibit problems with solder points from my experience.

[Josh128]

^
Agree. Sounds like a solder joint problem. As the unit runs, cracked solder joints that are barely making contact expand causing them to open. When they cool from the unit turning off, they may make contact again and power the unit back on. Without much else to go on , I would pull the mainboard and inspect closely with a magnifying glass all the solder joints. Reflow everything that looks even remotely suspect.

I think its very unlikely that its an IC or cap itself causing this type of issue, more likely their solder joints.

[chjmartin2a]

Thank you. I am going to try that. Now that you have said it, it makes sense.

[chjmartin2a]

I am pretty sure that this is something happening on the powerboard as when it shuts off the voltages coming off of the power board are all far too low. I thought it was that the rectifier had a cold solder joint because it felt like if I moved it that it would go back on, but after reflowing the solder it didn't fix it. I think I am just going to reflow every connection. When it does go off I convince myself that flexing the board, or tapping on it makes a difference, but I am not sure. Do you think that if when it "turns off" that the voltages coming off the board are too low is a good indication that the problem is with the power board and not the main board?

[chjmartin2a]

If I had to be a technician to make a living then my family would starve. I reflowed every bit of solder on the power board. I need to take it apart further and go to the main board.

Ugh...

[chjmartin2a]

Power Supply Board Schematic:

https://drive.google.com/file/d/1684rMP ... sp=sharing

Power Supply Board Inverse:

https://drive.google.com/file/d/1xMR4TV ... sp=sharing

Here is the schematic and back side of the Power Supply board. I replaced all of the caps. I reflowed all of the solder connections. I started randomly checking resistors. I tested the primary transition (Q901 I think.) The board has normal voltages when it is working and then all of the supply power gets wonky once the monitor warms up and it has switched off. Then a bit of time goes by and it comes back on. I haven't done anything outside of the power supply. I thought of writing down all of the voltages when it is working and then see which ones do match and don't match when it isn't, but that felt like a whack-a-mole strategy. Can anybody tell me where I should focus my search based on the schematic? Like I said, I can meter any voltage when it is "running" versus when it switches off. There is a difference between the "switched off" and truly no power. I am about to just reassemble it, give up and move on to the next thing. It is clearly beyond my skillset, but I am trying to learn.

Chris