Chieftec CFT-750-14c

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  • pdavid
    Badcaps Veteran
    • Jan 2011
    • 279
    • Hungary

    #1

    Chieftec CFT-750-14c

    This psu was returned to me after I cleaned it. The owner said that his pc shuts down after a few seconds. Now I have to fix it since "I broke it probably" It turns on but soon after the 5v rail starts dropping to 4 something volts without load. All the other voltages are rock solid and within spec. With load 5V even gets lower like 3.6V and the psu turns off. I doubt only a recap would help on this one. The modular cable soldering is a mess, but there are no short or anything on the pcb. Has anyone experienced this issue with this psu. It's a cwt psh model.
  • pdavid
    Badcaps Veteran
    • Jan 2011
    • 279
    • Hungary

    #2
    Re: Chieftec CFT-750-14c

    I'm going to dissasamble this unit and rework the soldering. The controll board has a lm393, ps229 and a weltrend supervisor chips. I hope it's not one of these giving some trouble. It could be some of the tiny capacitors on this board...

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    • pdavid
      Badcaps Veteran
      • Jan 2011
      • 279
      • Hungary

      #3
      Re: Chieftec CFT-750-14c

      I checked the diodes for each voltage rails, changed the filter capacitors. The problem remains. I was playing around with the control board. "Unpluged" the 5 volt protection functions and the unit won't start up. Small capacitors measure ok. I guess that part works fine, but I'm not entirely sure how all the protections work. The problem is with the 5V regulation.
      Need more pics?
      Attached Files

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      • ben7
        Capaholic
        • Jan 2011
        • 4059
        • USA

        #4
        Re: Chieftec CFT-750-14c

        Does the PSU have minimum load resistors inside? If so, check those and see if they are all reading fine.
        Muh-soggy-knee

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        • pdavid
          Badcaps Veteran
          • Jan 2011
          • 279
          • Hungary

          #5
          Re: Chieftec CFT-750-14c

          There is only one 15Ohm 5W resistor on the 12V rail. It's the one on the supervisor pcb. The rest on the secondary are smd resistors. I wired the standy to the 5V voltage input on the ps299 and the symptom remains. All voltages are fine except 5V. The problem may be somewhere else.

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          • pdavid
            Badcaps Veteran
            • Jan 2011
            • 279
            • Hungary

            #6
            Re: Chieftec CFT-750-14c

            I figured that the C2 ceramic capacitor was shorted so I changed it to a 0.5 uF. Now 5 volt is 0.1 higher... but still something like 4.2V without load. Still regulation is screwed up so I'm tracking back things.
            Btw should I install the missing vr1 trimmer, if so what value?
            Attached Files

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            • pdavid
              Badcaps Veteran
              • Jan 2011
              • 279
              • Hungary

              #7
              Re: Chieftec CFT-750-14c

              I still haven't solved this 5V problem. I've been lectured by some pros that it's something with the feedback and that missing vr1 pot isn't important. There may be a faulty component on the primary...

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              • mscserbia
                Member
                • Dec 2013
                • 17
                • Serbia

                #8
                Re: Chieftec CFT-750-14c

                Use another core,new one for 5V rail and rewind it.I had the same problem with 3.3V rail.Try that.

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                • rtstorm
                  Badcaps Veteran
                  • Feb 2012
                  • 971
                  • Croatia

                  #9
                  Re: Chieftec CFT-750-14c

                  hi, i have the same problem. +5V go to 3.6V widouth load. Have you managed to get it working?
                  ...the quieter you become the more you are able to hear...

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                  • ben7
                    Capaholic
                    • Jan 2011
                    • 4059
                    • USA

                    #10
                    Re: Chieftec CFT-750-14c

                    Tried checking and/or replacing the 5V rectifier diode? It might be leaky too.
                    Muh-soggy-knee

                    Comment

                    • rtstorm
                      Badcaps Veteran
                      • Feb 2012
                      • 971
                      • Croatia

                      #11
                      Re: Chieftec CFT-750-14c

                      Ok does anybod have a schematic for this so i can find diode?
                      ...the quieter you become the more you are able to hear...

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                      • pdavid
                        Badcaps Veteran
                        • Jan 2011
                        • 279
                        • Hungary

                        #12
                        Re: Chieftec CFT-750-14c

                        Originally posted by rtstorm
                        hi, i have the same problem. +5V go to 3.6V widouth load. Have you managed to get it working?
                        I had much trouble fixing this problem on that CWT PSH. As far as I remeber it was something trivial. A bad smd resistor somewhere in the secondary.
                        There is no schematic for this psu platform yet. The 5V diodes are in the middle on the third heatsink.
                        I wish I could help more. Good luck!
                        Attached Files

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