With the multimeter and a steady hand, I've found the 3843B PWM controller Vcc pin is 12V when the supply is on, and 8.2V when it is plugged in but cold. Datasheet says it needs 8.4V Vcc to start-up so heating it is lowering its startup voltage or increasing the Vcc just enough.
To test the theory I supplied external 9V power to the pin, to "jump-start" the power supply. It worked!
So now, I think I need to find where this 8.2V is coming from so I can work out how to bump it just that little bit more, maybe by replacing a diode or something.
How this "start-up current" is usually generated on power supplies? Maybe from the pictures you can point to some probable locations I should look? What could be the cause of the low start-up voltage? Thanks in advance.
Okay, this is a weird one. First time on this Stack Exchange, work with computer science, and have only basic electrical knowledge, so beware of any possible misunderstandings below.
I have an "universal" laptop power supply, one you choose the desired output voltage with a switch, it goes from 12 to 24V. Since some time ago it started working only when warmed up. So to start using it, I had to warm it up with a hair-dryer and then couldn't leave it turned off for too long, or it would cool down.
I decided to fix this for good because it was getting worse over time (needed more temperature to start-up), it looks fun, and I am broke. So I opened it up, armed with a multimeter and started probing. It takes the 110/220V input AC, rectifies it and then passes it through a MOSFET (gate, I suppose, connected to some signal generator, the frequency of which maybe determined by the voltage selector) and onto the main coil, which gives our output voltage which is rectified again, filtered and then out. So the problem is, whatever drives this MOSFET wasn't working when cold. I narrowed the problem to 3 ICs and did the logical: warmed them one-by-one with a lighter to find which one was problematic.
TL;DR: The actual question
So I narrowed the problem down to an IC called "3843B" which turns out to be a "high performance current-mode PWM controller". So, heating this IC (or something on its close vincinity) gets the power supply going. After it starts working, then everything is OK. Below is a photo of it.
What could be causing this? Which of the suspect components in the photo could have a failure mode which makes it work only after it's hot? What can I do or which components do I replace?
Close-up of suspect