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LC TDK power supply starts when it feels like it


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I ordered some supplies so I can redo my soldering on my Mac LC motherboard the right way. In the meantime I figured I'd focus on the power supply, which is the typical TDK one.

 

Of course, it had crapoed capacitors, so I replaced all the output-side capacitors and cleaned up the mess. The power supply now provides correct voltages, but... only when it feels like starting.

 

Generally, if I mess with it for a while, it will happily start, but if it's been left alone for several hours, it won't start, just doing nothing at all, no clicks, nothing.

 

I thoroughly cleaned the board, desoldering some components to clean up some more capacitor gunk, reflowing all solder joints, but the problem persists.

 

Something interesting I found out while probing around with my voltmeter: the moment I probed between the microcontroller's OVP pin and its ground, that kickstarted the PSU. As if something about the overvoltage protection circuit was bad.

 

In the meantime I desoldered the controller, cleaned it and put it back. We'll see.

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Well, I spoke too soon. The damn thing is being flaky again. I replaced the microcontroller and the optoisolator, to no avail.

 

I could try going for the shunt regulator but I doubt that is the issue - when the PSU is in non-working state, there is no current at all in the output side. However, in that situation, the microcontroller is getting 13-14V instead of the normal 12V, which I think is what trips the overvoltage protection.

 

Mains current goes through R2, R3 and R4 before powering the microcontroller.

 

R2 is a black thing that looks like a tantalum capacitor, but the board says R2 with a resistor symbol, so... well. Says 16D-9 on it. It measures at around 16 ohm.

 

R3 and R4 are both 150k ohm resistors. I measured them, they give 149k and 151k respectively. I will try measuring them again after they cool down, to make sure they aren't drifting or anything weird.

 

I had already measured a bunch of other resistors on the input side and they measured good.

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Well, I did a few hunts for leftover cap gunk. Basically a visual inspection, but also heating up solder joints -- any bubbling through the liquid solder is a telltale sign of cap gunk.

 

I thought I had thoroughly cleaned the damn thing, guess I wasn't thorough enough, because I did find quite some leftovers. For each affected component, I desoldered it, cleaned it and the PCB, and put it back.

 

I'm going to see if it works better now, but not right now -- waiting on some new components, due to some idiocy on my part. If that doesn't do the trick, I will try replacing the shunt regulator, and checking resistors and capacitors and whatnot.

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  • 4 weeks later...

Lil' update on this. I replaced the few resistors I'd blown, still waiting for a replacement MOSFET (original one went bad too). I took a 2SK2611 MOSFET from another power supply, which is electrically compatible. The power supply now seems to work reliably, however the 2SK2611 is bigger than the original 2SK1024, so I can't quite put the thing back together. I'm waiting for a proper replacement (still).

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  • 2 weeks later...

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