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    Matsui 175S microwave fuse replacement

    Hi,

    I recently picked up a used Matsui 175S microwave that was powering up (starts, turntable rotates) but not heating. From looking online the most likely culprit was the fuse, but unfortunately in this one the fuse had heated up and fused with the (badly designed) fuse holder and couldn't be removed. I guess this might indicate further issues beyond the fuse itself but at this stage I just want to replace the fuse and see how I go from there.

    Anyway since I couldn't get out the fuse itself the easiest fix was to replace the fuse holder assembly that runs between the power supply and the main capacitor but in my enthusiasm to get the job done I failed to make a note of exactly what terminal on the capacitor the fuse connected to, and there's two possible places it could've come from. So now I have a replacement fuse assembly but am unsure which of the two spare terminals it connects to. And I don't want to use the trial-and-error approach since this cap is rather large...

    I've attached a photo of the cap itself (Sang Nong 0.91uF 2100V) showing the two spare terminals; alongside with the schematic that came with the microwave in case it helps. Does anyone with more experience of these types of caps have any thoughts on which one the fuse should connect to? Thanks!
    Attached Files
    Last edited by drykid; 12-21-2020, 01:18 PM.

    #2
    Re: Matsui 175S microwave fuse replacement

    The red wire with black stripe goes the empty cap terminal, the other end of that red wire with black stripe goes to the fuse holder as shown in the schematic, the cap is non- polarized.
    Can we see the pictures of the fuse holder? Did you check the rectifier diode that is connected to the other capacitor terminal?
    BTW, make sure to discharge that high Voltage cap even though it has builtt-in bleeder resistor. You also need to check all the door switch/safety door lock switches to make sure they are still goes open circuit and close circuit when actuated/de-actuated.
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

    Inverter testing using old CFL:
    http://www.badcaps.net/forum/showthr...er+testing+cfl

    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
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    TV Factory reset codes listing:
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    Comment


      #3
      Re: Matsui 175S microwave fuse replacement

      Originally posted by budm View Post
      The red wire with black stripe goes the empty cap terminal, the other end of that red wire with black stripe goes to the fuse holder as shown in the schematic, the cap is non- polarized.
      Can we see the pictures of the fuse holder? Did you check the rectifier diode that is connected to the other capacitor terminal?
      BTW, make sure to discharge that high Voltage cap even though it has builtt-in bleeder resistor. You also need to check all the door switch/safety door lock switches to make sure they are still goes open circuit and close circuit when actuated/de-actuated.
      Thanks Your response confused me a bit at first but I went back and looked at the capacitor again and I think I was misunderstanding what I was looking at before. Each side of the capacitor has two "prongs" on it - for want of a better word - that can hold separate connectors - but it's just the same terminal so it doesn't really matter which of the two available prongs you attach the fuse holder to, am I making sense? I've attached another photo from a different angle as I realise the one before doesn't show what I'm talking about very clearly.

      I haven't actually discharged the cap manually as it's been switched off and unplugged for a week now and I'm assuming that it should have lost all of its charge in the meantime. (Also I don't really have the proper tool for discharging it.)

      I can see the diode you're referring to but I'm not really sure how to test it to be honest.
      Attached Files

      Comment


        #4
        Re: Matsui 175S microwave fuse replacement

        It looks wired wrong to me, the diode connects after the capacitor - not to the transformer.
        So I would move the red wire over and I'm not sure what the red/blk stripe wire goes to. It looks low voltage.

        P.S. Why is there so much dirt and food here yuck

        Comment


          #5
          Re: Matsui 175S microwave fuse replacement

          Originally posted by redwire View Post
          It looks wired wrong to me, the diode connects after the capacitor - not to the transformer.
          So I would move the red wire over and I'm not sure what the red/blk stripe wire goes to. It looks low voltage.
          The diode is connected to the cap itself, not the transformer. I've added another photo that shows it better. I'm not messing with what's already there; I've no reason to believe that there were any existing wrong connections when I got it. The red / black wire is the replacement fuse holder that I was asking about originally; although I think I've now concluded that as long as it goes on the right side of the cap it doesn't matter much how you connect it.

          P.S. Why is there so much dirt and food here yuck
          It's a nineties microwave; that's what 30 years of use does. I actually expected it to be worse. I think "out of sight, out of mind" is the key concept here.
          Attached Files

          Comment


            #6
            Re: Matsui 175S microwave fuse replacement

            Ok, I have repaired around 5 microwaves and stripped a few more. The only problems I have ever seen is the door switches wear out, or there is a loose connection somewhere. The door switches probably wouldn't blow/melt the fuse, but a loose connection could easily do so.

            Go over the wiring with a critical eye, look for loose spade connectors, especially on the board, on the transformer or on the magnetron. Especially look for darkened plastic covers on the spade connectors. Sometimes you can just replace the spade connector or tighten it with a pair of pliers. When I have fixed microwaves for others, I have just soldered the spades to the lugs or if it's a connection to the board, removed the spade connector and soldered the wire direct to the board.

            I have no idea why the connections get loose, maybe it's due to vibration, but it happens frequently.

            Edit: Of course you should be especially careful in a microwave, especially around the cap, and because there's very high voltage in there. You can easily kill yourself if you aren't careful.
            Last edited by clearchris; 12-22-2020, 03:37 PM.

            Comment


              #7
              Re: Matsui 175S microwave fuse replacement

              The door switches WILL blow the fuse, If the PRIMARY door switch stays closed when the door is opened, the MONITOR switch will short the ac line and blow the fuse to prevent the oven from operating with the door open. This is a critical safety feature of microwave ovens.
              If the main fuse was open the microwave would not have had any power, no turntable etc. Post a picture of the complete microwave components, not just the capacitor. Based on your picture. it looks like the white wire is on the wrong pin of the capacitor
              Last edited by R_J; 12-22-2020, 04:28 PM.

              Comment


                #8
                Re: Matsui 175S microwave fuse replacement

                Originally posted by r_j View Post
                the door switches will blow the fuse, if the primary door switch stays closed when the door is opened, the monitor switch will short the ac line and blow the fuse to prevent the oven from operating with the door open. This is a critical safety feature of microwave ovens.
                If the main fuse was open the microwave would not have had any power, no turntable etc. Post a picture of the complete microwave components, not just the capacitor. Based on your picture. It looks like the white wire is on the wrong pin of the capacitor
                +1
                Never stop learning
                Basic LCD TV and Monitor troubleshooting guides.
                http://www.badcaps.net/forum/showthr...956#post305956

                Voltage Regulator (LDO) testing:
                http://www.badcaps.net/forum/showthr...999#post300999

                Inverter testing using old CFL:
                http://www.badcaps.net/forum/showthr...er+testing+cfl

                Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                http://s807.photobucket.com/user/budm/library/

                TV Factory reset codes listing:
                http://www.badcaps.net/forum/showthread.php?t=24809

                Comment


                  #9
                  Re: Matsui 175S microwave fuse replacement

                  Originally posted by R_J View Post
                  The door switches WILL blow the fuse, If the PRIMARY door switch stays closed when the door is opened, the MONITOR switch will short the ac line and blow the fuse to prevent the oven from operating with the door open. This is a critical safety feature of microwave ovens.
                  If the main fuse was open the microwave would not have had any power, no turntable etc. Post a picture of the complete microwave components, not just the capacitor. Based on your picture. it looks like the white wire is on the wrong pin of the capacitor
                  OK I've attached a picture to show more, but just to spell it out the connections on the cap are as follows (when I say "left" I mean my left as I look at it):

                  Left hand terminal of cap:
                  - black lead to diode, red lead to magnetron (these two share the same connector)
                  - white lead to transformer.

                  Right hand terminal of cap:
                  - red-with-black-stripes lead to fuse holder.

                  But as I said originally my query was only about the fuse holder, which I unplugged from the right side of the cap without making a note of which of the two connectors it was plugged into. I haven't done anything to the ones on the left, so they must be right unless someone has taken it apart and played with it previously (but that seems unlikely, most people would steer well clear of microwaves; probably I should too but I've started trying to fix this now so aim to get it working...)

                  Also to be clear the fuse I'm referring to is the thermal fuse, there's no issue with the main fuse on the power board because - as you say - the microwaves powers on (clock, timer etc) when switched on.
                  Attached Files

                  Comment


                    #10
                    Re: Matsui 175S microwave fuse replacement

                    Originally posted by R_J View Post
                    The door switches WILL blow the fuse, If the PRIMARY door switch stays closed when the door is opened, the MONITOR switch will short the ac line and blow the fuse to prevent the oven from operating with the door open.
                    Interesting. I have only had a door switch failure on a commercial microwave (actually two identical ones), that box had around 7-8 door switches. I didn't trace them all out, I just replaced them all (they were all probably approaching MTBF and subject to all day, every day door slamming), seemed that one circuit just cut the power when the door was open, and some appeared redundant, but different, like one pair run in series the first microswitch would be pressed when the door was closed, the other switch would be unpressed when the door was closed, and they were wired so that both switches would conduct when in their proper position. Neither switch failure blew the fuse, it just failed to apply power to the magnetron IIRC. Undoubtedly, there was more than one circuit running though those switches though.

                    Good to know though.
                    Last edited by clearchris; 12-23-2020, 08:54 PM.

                    Comment


                      #11
                      Re: Matsui 175S microwave fuse replacement

                      The schematic does show the red wire connections, the white is the filament winding. So the transformer and HV fuse do connect to the cap right terminal like you've got it.

                      Are you going to test the diode and capacitor? You can also check for shorted magnetron.
                      A microwave oven diode has about a 10V drop, so they will not read using a multimeter's diode test function (unless the diode is shorted). These are just basics to save money blowing more HV fuses. The fan looks crooked.

                      Comment


                        #12
                        Re: Matsui 175S microwave fuse replacement

                        Originally posted by redwire View Post
                        The schematic does show the red wire connections, the white is the filament winding. So the transformer and HV fuse do connect to the cap right terminal like you've got it.

                        Are you going to test the diode and capacitor? You can also check for shorted magnetron.
                        A microwave oven diode has about a 10V drop, so they will not read using a multimeter's diode test function (unless the diode is shorted). These are just basics to save money blowing more HV fuses. The fan looks crooked.
                        Thanks.

                        Finally got round to trying this, tried it with the replacement fuse connected to both connectors on the right terminal in turn but no difference whatsoever to the outcome unfortunately. The microwave powers on and the turntable turns as it should do, but no heating takes place. Am assuming at this point that it must be either a failed magnetron, diode or cap, but not sure I have the troubleshooting skills to get much further with this.

                        Comment


                          #13
                          Re: Matsui 175S microwave fuse replacement

                          The most common failure is the magnetron ( open heater coil or no output from the magnetron it self ) the second most common failure is the capacitor ( open no UF reading value or a slightly shorted to ground) and the third most failure is the diode ( open or blown apart because it shorted )

                          The other most common failure is that the high voltage coming from the magnetron arcs inside of the microwave shell if this happens there is not much you can do about it ( a long time ago you could buy a special paint to fix this issue but it is not a permanent repair because the same issue will come back soon or later again )

                          The diode is very hard to test if you think it is open then just replace it
                          Last edited by sam_sam_sam; 01-03-2021, 10:08 AM.
                          9 PC LCD Monitor
                          6 LCD Flat Screen TV
                          30 Desk Top Switching Power Supply
                          10 Battery Charger Switching Power Supply for Power Tool
                          6 18v Lithium Battery Power Boards for Tool Battery Packs
                          1 XBox 360 Switching Power Supply and M Board
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                          6 De-soldering Station Switching Power Supply 1 Power Supply
                          1 Dell Mother Board
                          15 Computer Power Supply
                          1 HP Printer Supply & Control Board * lighting finished it *


                          These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                          1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                          2 Hem Saw Computer Stack Board

                          All of these had CAPs POOF
                          All of the mosfet that are taken out by bad caps

                          Comment


                            #14
                            Re: Matsui 175S microwave fuse replacement

                            The primary of the HV transformer is controlled by a relay, are the relay contacts closing and does the HV transformer primary get line voltage?

                            Comment

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