Good day folks. A pretty simple job turned into a great confusion today. My boss sent me over to a friend of his to have a look at his washing machine which stopped working entirely. He opened it up himself and presented me with this lovely board before I got there. I took it back to the shop and instead of immediately hopping online and trying to find info on this board, I opted for a more hands-on approach and troubleshooted on my own.
The layout seemed pretty simple overall and it wasn't hard for me to spot the issue: the power supply in the lower-right corner had popped. I noticed the LNK304PN IC had a hole in it and also the 22ohm resistor was a bit dark, so I replaced these and called it fixed. I know, I know: the inductor is popped too, but the guy over at the store where I get all my parts from, who's a tech himself, told me I shouldn't bother with it as long as it still conducts....so I did just that
Trouble is I'm now hesitant to test it for one simple reason: the washer itself is still over at the guy's house. All I have is the board and I don't know where I should connect my mains input in the absence of the wiring loom which is still inside the washer. Adding to the problem is the fact that I don't know the exact model of the machine and the board pops up in many other models too. I tried comparing various service manuals, but they were of little help. Either I don't know how to interpret them or they indeed don't match the model.
There are contact pads on either side of the board which connect to various plugs on the loom, but it's not clear which does what, even with said service manuals on screen. This appears to be a non-isolated SMPS (a buck converter really) and this is the schematic which matches the best. The operation of this LNK IC is confusing to me. So you've got mains coming in on the L and N points - simple enough. The pad connected to N1 goes through two diodes, through the inductor, to the transformer (?) TF01 and the D pin of the IC. Is this a half-wave ? It must be - only one diode (two in series - same thing (???)
The pad connected to N is easily identifiable because the track going to R020 is clear as day right there, however I cannot find the other one (the track connected to L1). It seems to go to the S pins of the IC AND the VCC output. Logic would dictate it should be adjacent to the first pad, since I'm expecting L and N to be on the same two-prong connector on the loom, but it's not ! I also find this setup very weird: the L wire, which I cannot find, is somehow also connected to the VCC output....but so is the top winding of TF10. Meanwhile, there's a center tap which has a reverse-biased diode on it which "creates" a GND and a bottom winding too.....I'm having a hard time understanding how all of this works to be honest.
You'll immediately suggest tracking down the pad. I tried it: keeping one probe on the VCC point (so the S pins too) and running the other probe across all pads does eventually give me a beep where the L pin is connected, but it's nowhere near close to the pad where N was, plus it doesn't look anything like you'd expect a power track to look ! Thin tracks and mostly vias ! It seems to go to the push-buttons and the logic more than anything else ! Even though it makes SOME sense, because VCC IS essentially connected straight to main, I'm REALLY skeptical about connecting my mains plug straight on these random pads, just because they give me continuity ! Again: I'm not sure how it's possible to have the S pins of the IC and the VCC output connected together on the same "live" wire. Even after the fact, I cannot find something that looks like a power pad for the other leg of the mains....would they really split up L and N across two connectors like this ?
What should I do now ? Just go over to the guy's house and try it out ? Even so, I'm not sure I'd be able to plug the loom back in correctly, because from the short glance I took at it, some wires have the same color AND same number of pins and without a clear service manual, I run the risk of connecting them randomly :|
Any thoughts on how the circuit works and how I should test it on the bench ?
The layout seemed pretty simple overall and it wasn't hard for me to spot the issue: the power supply in the lower-right corner had popped. I noticed the LNK304PN IC had a hole in it and also the 22ohm resistor was a bit dark, so I replaced these and called it fixed. I know, I know: the inductor is popped too, but the guy over at the store where I get all my parts from, who's a tech himself, told me I shouldn't bother with it as long as it still conducts....so I did just that
Trouble is I'm now hesitant to test it for one simple reason: the washer itself is still over at the guy's house. All I have is the board and I don't know where I should connect my mains input in the absence of the wiring loom which is still inside the washer. Adding to the problem is the fact that I don't know the exact model of the machine and the board pops up in many other models too. I tried comparing various service manuals, but they were of little help. Either I don't know how to interpret them or they indeed don't match the model.
There are contact pads on either side of the board which connect to various plugs on the loom, but it's not clear which does what, even with said service manuals on screen. This appears to be a non-isolated SMPS (a buck converter really) and this is the schematic which matches the best. The operation of this LNK IC is confusing to me. So you've got mains coming in on the L and N points - simple enough. The pad connected to N1 goes through two diodes, through the inductor, to the transformer (?) TF01 and the D pin of the IC. Is this a half-wave ? It must be - only one diode (two in series - same thing (???)
The pad connected to N is easily identifiable because the track going to R020 is clear as day right there, however I cannot find the other one (the track connected to L1). It seems to go to the S pins of the IC AND the VCC output. Logic would dictate it should be adjacent to the first pad, since I'm expecting L and N to be on the same two-prong connector on the loom, but it's not ! I also find this setup very weird: the L wire, which I cannot find, is somehow also connected to the VCC output....but so is the top winding of TF10. Meanwhile, there's a center tap which has a reverse-biased diode on it which "creates" a GND and a bottom winding too.....I'm having a hard time understanding how all of this works to be honest.
You'll immediately suggest tracking down the pad. I tried it: keeping one probe on the VCC point (so the S pins too) and running the other probe across all pads does eventually give me a beep where the L pin is connected, but it's nowhere near close to the pad where N was, plus it doesn't look anything like you'd expect a power track to look ! Thin tracks and mostly vias ! It seems to go to the push-buttons and the logic more than anything else ! Even though it makes SOME sense, because VCC IS essentially connected straight to main, I'm REALLY skeptical about connecting my mains plug straight on these random pads, just because they give me continuity ! Again: I'm not sure how it's possible to have the S pins of the IC and the VCC output connected together on the same "live" wire. Even after the fact, I cannot find something that looks like a power pad for the other leg of the mains....would they really split up L and N across two connectors like this ?
What should I do now ? Just go over to the guy's house and try it out ? Even so, I'm not sure I'd be able to plug the loom back in correctly, because from the short glance I took at it, some wires have the same color AND same number of pins and without a clear service manual, I run the risk of connecting them randomly :|
Any thoughts on how the circuit works and how I should test it on the bench ?
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