Announcement

Collapse
No announcement yet.

Power supply problem VS 940bx

Collapse
X
Collapse
 
  • Page of 1
  • Filter
  • Time
  • Show
Clear All
new posts
Previous template Next
    #1

    Power supply problem VS 940bx

    This board is not outputting 13V. 5V is ok. I haven't done a lot with power supplies yet, other than replacing caps. I tested the transformer before removing it from the board with a ring tester, and the primary side was showing a short. After removing it from the board it tested good. So I think there is something shorted on the primary side. Test around and can't find anything.

    I happen to have a replacement transformer that was working in another board, so I try it. Before I removed transformer B from board B, it rang good. So I begin soldering transformer B to board A. Still trying to locate the problem, I solder 1 pin at a time, and check the ring tester to see where the short is introduced. When I solder the third pin from the left on the secondary side (see diagram) then the ring tester shows short. I have all the other pins soldered in and no short, but when I solder that last pin it shorts out (ring tester shows no lights).

    This seems really strange to me because the two center pins feed the 5V which is working... And even without that pin soldered the 5V still works. The only difference is I am getting a little less than 1 V on the 13V pin, where I was getting 0 before.

    Any ideas?
    Attached Files
    Tags: None
    #2
    Re: Power supply problem VS 940bx

    Shorted diode on the 13V output?

    PlainBill
    For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

    Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

    Comment

      #3
      Re: Power supply problem VS 940bx

      I cannot remember the last time I saw any transformer,in this position on a Monitor actually cause a problem.As Plain Bill said the fault would seem to be on the output side of the 13v supply,and have had failure of one or other of the diodes.

      Comment

        #4
        Re: Power supply problem VS 940bx

        I pulled the diode at D201. The transformer rings ok now. I think this is a schottky diode and when testing it the left side is always open circuit, which is different from the other side. So it appears to be toast.

        I found a datasheet that matches the part number, Attachment 1. Digi-key recommended the part in attachment 2. It has some different specs and I can't tell if it will work or not. If someone could take a look and let me know I will get one on order.

        Thanks for the help with this. I have had a few boards with similar problems and I thought they were transformer issues and I did not repair them. I will have to look thru my stack and see if I still have any of them and take a second look.
        Attached Files

        Comment

          #5
          Re: Power supply problem VS 940bx

          Originally posted by sceva View Post
          I pulled the diode at D201. The transformer rings ok now. I think this is a schottky diode and when testing it the left side is always open circuit, which is different from the other side. So it appears to be toast.

          I found a datasheet that matches the part number, Attachment 1. Digi-key recommended the part in attachment 2. It has some different specs and I can't tell if it will work or not. If someone could take a look and let me know I will get one on order.

          Thanks for the help with this. I have had a few boards with similar problems and I thought they were transformer issues and I did not repair them. I will have to look thru my stack and see if I still have any of them and take a second look.
          The replacement suggested by Digikey is a slight upgrade from the original. It should work without any problems.

          PlainBill
          For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

          Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

          Comment

            #6
            Re: Power supply problem VS 940bx

            I have put it on order. Thanks again for the help.

            Comment

              #7
              Re: Power supply problem VS 940bx

              I have replaced the diode and it is working!! Well sort of. I had not noticed that the lcd was cracked before getting the backlights on. Luckily I had another lcd panel to swap it out with. Then I started getting 2 sec 2 black... the ccfl's on this panel were bad, so I swapped the ccfl's out of the cracked lcd and finally it works!!

              So this diode converts the power from the transformer to 12v. Can you use a dmm to test the voltage between the secondary of the transformer and the diode?

              Thanks for the help.

              Comment

                #8
                Re: Power supply problem VS 940bx

                Originally posted by sceva View Post
                I have replaced the diode and it is working!! Well sort of. I had not noticed that the lcd was cracked before getting the backlights on. Luckily I had another lcd panel to swap it out with. Then I started getting 2 sec 2 black... the ccfl's on this panel were bad, so I swapped the ccfl's out of the cracked lcd and finally it works!!

                So this diode converts the power from the transformer to 12v. Can you use a dmm to test the voltage between the secondary of the transformer and the diode?

                Thanks for the help.
                I'm not sure what you are asking. Certainly you can use a DMM to read the voltage on the transformer side of the diode, but that won't be very informative. You're getting a series of pulses at more then 12V, the width of those pulses will vary with the load.

                PlainBill
                For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                Comment

                  #9
                  Re: Power supply problem VS 940bx

                  Originally posted by PlainBill View Post
                  I'm not sure what you are asking. Certainly you can use a DMM to read the voltage on the transformer side of the diode, but that won't be very informative. You're getting a series of pulses at more then 12V, the width of those pulses will vary with the load.

                  PlainBill
                  I had tried to check voltages in this area before and got readings that I did not understand. Just trying to figure out why.

                  Another thing I don't understand is if I test the AC volts at the plug or fuse on a power supply I don't see 110-120V. It usually is somewhere in the 60's.

                  Comment

                    #10
                    Re: Power supply problem VS 940bx

                    That makes me wonder if there is a problem with your meter.

                    One very simple test is to measure the voltage at a normal AC outlet. It should read 110 VAC to 125 VAC. (If the nominal line voltage is 117 VAC, as in the USA.) If it reads something else, either your technique is wrong, or the meter is defective.

                    For DC, I usually measure the voltage of a new 1.5V dry cell. It should read about 1.6 volts.

                    PlainBill
                    For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                    Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                    Comment

                    Previous template Next
                    Working...
                    X