I just fixed my iiyama 24" monitor (E2407HDSD) that went off with a bang last week.
It started buzzing while I was using it but this only lasted 10 seconds before the whole thing went pop. During the buzzing there was no noticeable side effect but I didn't get a chance to turn it off before I realised what was wrong.
When it popped an ember shot out of the bottom of the power supply. I believe this was from the fuse which really struggled to interrupt the failure current. Probably a cheap ass fuse. Tearing apart the fuse revealed the sand that it was filled with had glassified, so a substantial arc likely formed within it.
Tearing it down, found the primary switching transistor shorted, no surprise there. It's a simple flyback so that or the bridge rectifier is bad. The driver IC was popped as well - and the part number was unreadable. Thankfully, I found a schematic of a Viewsonic monitor with the same power supply (likely the same OEM as iiyama); PSU board was JT240xx
https://cdn.badcaps-static.com/pdfs/...ce119f6336.pdf
So I was able to order a replacement IC and a few replacement transistors. Tried it with my bulb tester and to my dismay whilst there was no light the PSU still did not work.
Checked a few things and found that the source resistor for the MOSFET was bad. This was 0.27R, I did not have an exact value but I had 0.22R so that was subbed in its place. However upon doing this I was greeted by a glowing bulb and only 12V dc across the main filter cap terminals.
Testing across there with ohmmeter revealed a 2k ohm "leak" to ground. Removing various parts I eventually found my new MOSFET was leaky. How odd. I am not sure if the open source pin with a driven gate caused this. But it did seem odd. Replacing the transistor fixed this.
So now I tested the unit. No flash from the bulb and only 250V DC across the filter cap terminals... Odd. However the 5.4V main output is OK and it's enough to make the standby LED glow. I decided to investigate the filter cap after I notice that the cap discharges very quickly once the monitor is unplugged - in less than a second. I am used to this monitor being able to remain in standby for up to 30 seconds after unplugged so this is very odd. I discovered that the positive lead of the very high quality Elite cap had completely corroded and as such the cap was no longer connected.
This must have been the source of the original failure, as the cap going bad would explain the buzzing. I imagine then the flyback was desperately trying to keep the output under regulation with very low line voltage at times and so working much harder than normal. To its credit I did not notice any flicker on the LCD despite it losing its main source of power. Must have been putting the smaller output caps under some stress! I replaced the 150uF 400V cap with a Vishay BC 220uF 400V cap that I had spare.
All in all an interesting chain of failures. I suspect the reason the source resistor opened was because the fuse was very slow, and the MOSFET killed the driver IC, all because of a 400V cap went bad.
And the kicker? I never replaced the 400V cap when I first got the monitor as a freebie - it had two bloated low voltage caps but the 400V one "never goes bad", right? Now I know better! (that red wire on the board is one of the jumper wires I put in to repair some damage I did when I first repaired this monitor. Early days, I was bad at soldering!)
It started buzzing while I was using it but this only lasted 10 seconds before the whole thing went pop. During the buzzing there was no noticeable side effect but I didn't get a chance to turn it off before I realised what was wrong.
When it popped an ember shot out of the bottom of the power supply. I believe this was from the fuse which really struggled to interrupt the failure current. Probably a cheap ass fuse. Tearing apart the fuse revealed the sand that it was filled with had glassified, so a substantial arc likely formed within it.
Tearing it down, found the primary switching transistor shorted, no surprise there. It's a simple flyback so that or the bridge rectifier is bad. The driver IC was popped as well - and the part number was unreadable. Thankfully, I found a schematic of a Viewsonic monitor with the same power supply (likely the same OEM as iiyama); PSU board was JT240xx
https://cdn.badcaps-static.com/pdfs/...ce119f6336.pdf
So I was able to order a replacement IC and a few replacement transistors. Tried it with my bulb tester and to my dismay whilst there was no light the PSU still did not work.
Checked a few things and found that the source resistor for the MOSFET was bad. This was 0.27R, I did not have an exact value but I had 0.22R so that was subbed in its place. However upon doing this I was greeted by a glowing bulb and only 12V dc across the main filter cap terminals.
Testing across there with ohmmeter revealed a 2k ohm "leak" to ground. Removing various parts I eventually found my new MOSFET was leaky. How odd. I am not sure if the open source pin with a driven gate caused this. But it did seem odd. Replacing the transistor fixed this.
So now I tested the unit. No flash from the bulb and only 250V DC across the filter cap terminals... Odd. However the 5.4V main output is OK and it's enough to make the standby LED glow. I decided to investigate the filter cap after I notice that the cap discharges very quickly once the monitor is unplugged - in less than a second. I am used to this monitor being able to remain in standby for up to 30 seconds after unplugged so this is very odd. I discovered that the positive lead of the very high quality Elite cap had completely corroded and as such the cap was no longer connected.
This must have been the source of the original failure, as the cap going bad would explain the buzzing. I imagine then the flyback was desperately trying to keep the output under regulation with very low line voltage at times and so working much harder than normal. To its credit I did not notice any flicker on the LCD despite it losing its main source of power. Must have been putting the smaller output caps under some stress! I replaced the 150uF 400V cap with a Vishay BC 220uF 400V cap that I had spare.
All in all an interesting chain of failures. I suspect the reason the source resistor opened was because the fuse was very slow, and the MOSFET killed the driver IC, all because of a 400V cap went bad.
And the kicker? I never replaced the 400V cap when I first got the monitor as a freebie - it had two bloated low voltage caps but the 400V one "never goes bad", right? Now I know better! (that red wire on the board is one of the jumper wires I put in to repair some damage I did when I first repaired this monitor. Early days, I was bad at soldering!)
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