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Help with LNK304PN power supply (Whirlpool auto washer)

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    Help with LNK304PN power supply (Whirlpool auto washer)

    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 ?
    Attached Files
    Wattevah...

    #2
    Re: Help with LNK304PN power supply (Whirlpool auto washer)

    Return to the guys house, let him plug it back in, he removed it.

    Comment


      #3
      Re: Help with LNK304PN power supply (Whirlpool auto washer)

      Originally posted by diif View Post
      Return to the guys house, let him plug it back in, he removed it.
      Yeah, that's won't quite fly with him being pals with my boss and all....plus, I'm not entirely sure he's the one who pulled it out - he may've had another tech look at it beforehand.
      Wattevah...

      Comment


        #4
        Re: Help with LNK304PN power supply (Whirlpool auto washer)

        You've potentially fixed the board but it has to go back in the machine to verify.
        Didn't you have a look/take a photo of where it's fitting giving a chance to work out where all the connectors go, perhaps they have been marked up.

        Comment


          #5
          Re: Help with LNK304PN power supply (Whirlpool auto washer)

          Originally posted by diif View Post
          You've potentially fixed the board but it has to go back in the machine to verify.
          Didn't you have a look/take a photo of where it's fitting giving a chance to work out where all the connectors go, perhaps they have been marked up.
          No, I haven't - the machine was already pulled apart when I got there. My only hope is that the shape the wires have taken over time and general direction they tend to bend in will hint where each one goes.

          I was planning to at least power it up to see if the LEDs come on on the bench...provided the schematic is correct, I could just stick one leg of my mains on the resistor and the other straight on the VCC point, but again....that's seems a little iffy to me. Not sure how it's implemented inside the washer....

          I know how a buck converter works in general, but this one seems to "create" the negative rail, while passing the positive one unchanged straight off the mains, if that makes any sense in layman terms. All DC-DC buck converters I've seen, pass the GND unchanged and "shape" the positive....this one seems to be the opposite. The "transformer" is also not a simple "coil" like you'd find in a regular buck - it has a primary winding too. My guess is this gets "charged" when the S pin of the IC pulls pin 5 of the transformer low. This charges the coil, then when the S pin cuts out, the energy is dumped into the secondary. Here, I was expecting a forward-biased diode in series with the secondary winding (pin 10 of the transformer ?) to rectify the output to DC, while the other end of the winding is the GND like in all SMPSs, but apparently it's flipped: the negative is what's "rectified" (the center tap), while the positive is "common", going all the way back to the mains input....I think this is how this works, by a long shot :|

          I also noticed the bottom winding puts out a negative 12v rail as well...couldn't tell you what that is used for, but the principle should be the same as in an ATX supply where the diode is simply placed backwards, as opposed to forwards to get a positive output...
          Wattevah...

          Comment


            #6
            Re: Help with LNK304PN power supply (Whirlpool auto washer)

            The choke L003 often burns out, so the circuit should help
            Attached Files

            Comment


              #7
              Re: Help with LNK304PN power supply (Whirlpool auto washer)

              Just in case someone is stil watching this thread and is curious how the repair worked out in the end, the guy had been out of town for the past week, that's why it took so long to try it out, but today I finally got round to paying him a visit, as I was driving round town for my work anyway and managed to put the machine back together. Long story short: I plugged it in with a bit of trepidation and it works - case closed

              It was here I also managed to finally understand the wiring harness diagram in the service manual and what all those numbers and colors and tags refer to: the letters are actually stamped on the plastic shell that encases the board, not on the board itself, but I didn't have that, so that's why it made zero sense when I had just the board in front of me. Also, it turns out not all the connections on the board are populated ! About half of them are unused in this model !

              Another big concern of mine as I was piecing the machine back together was the plugs with the same wire color and same number of pins on them: it turns out there's a small plastic prong protruding out from the connector shell ever so slightly, which ensures it can only mate with the correct hole in the plastic shell, so you can't get them wrong, even without a manual, unless you brute force them like an a$$ and break that plastic prong.

              Another thing that ensures you can't screw up, unless you're a complete fool, is that some connectors have a dash of color paint on them, which again cleared the mystery of what those colors and numbers in the manual refer to: they indicate what color the stripe should be for a particular connector.

              All in all, a sweeping success
              Wattevah...

              Comment


                #8
                Re: Help with LNK304PN power supply (Whirlpool auto washer)

                Originally posted by Dannyx View Post
                Another big concern of mine as I was piecing the machine back together was the plugs with the same wire color and same number of pins on them: it turns out there's a small plastic prong protruding out from the connector shell ever so slightly, which ensures it can only mate with the correct hole in the plastic shell, so you can't get them wrong, even without a manual, unless you brute force them like an a$$ and break that plastic prong.

                Another thing that ensures you can't screw up, unless you're a complete fool, is that some connectors have a dash of color paint on them, which again cleared the mystery of what those colors and numbers in the manual refer to: they indicate what color the stripe should be for a particular connector.
                I have seen this before I wish manufacturer would do this more often that if you are going to use connectors that you use the ones that can only be plugged into one way or the board has cutouts for the connector and is keyed differently so it can not be put back in the wrong place or direction it is much easier to put back together

                But it was nice that the manufacturer also noted this in wiring diagram this is not done very often most of the time you have to guess what wire is what or what connector goes where

                “ unless you brute force them like an a$$ and break that plastic prong” you take all the fun out of it you want to see factory smoke coming out of it ( just kidding )

                I was once working on plasma power supply controller board that several connectors on it and they were all the exact same color wire depending on what the board function was depending on which connector was used this board got fried because I was working on it and then another more important issue came up and to fix that issue when I went back to it well which connector was used for this function I guess wrong and it did not go well for this controller board

                Also to make this problem worse is that the wiring diagram is not very clear on which connector to use and it not very easily following the wires coming from the connector

                Good repair by the way
                Last edited by sam_sam_sam; 08-28-2021, 05:07 AM.
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