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    Old Tatung SMPS design, expert comment needed ...

    The PS in question is from an IBM 5153 CGA monitor which was made by Tatung in 1985. The PS is a separate internal assembly that produces 115 VDC @ 0.5 A for the CRT monitor chassis. It's housed in a metal enclosure with ventilation holes.

    The monitor is labeled as "120 VAC" (only) but I live in a 230 VAC country and would prefer to run it directly off the line without a stepdown transformer.

    Looking at the design (SAMs schematic,) since the primary DC rail runs fixed in full-wave mode and the smoothing caps are rated at 400 WVDC but only see 153 VDC at a 110VAC line, I think this design was originally intended to run at worldwide line voltages, and in fact ran just fine on 230 VAC for an hour until I became uncomfortable with the additional heat coming off the monitor, not present when on 110 VAC.

    I want to either reduce that heat loss by a design change, or at least improve the cooling.

    Under bench testing with a dummy load and the unit open in still air, using thermocouples I've narrowed the main heat source down to one power resistor, identified in the attached schematic. I don't understand enough about SMPS design so need assistance to understand if there is a minor design change that would reduce the heat losses off that part. The 136 C measured would not normally be of concern, but it will be much higher in the enclosure, inside the monitor. Other power resistors and the main switching transistor run at the same temp as they did under 110 VAC, and there are no additional noises that make me think the unit is under stress.

    Note, in the schematic the primary DC section is at the bottom and provides 153 V (or 306 V) to the switching circuit above via label "1".
    Attached Files

    #2
    Re: Old Tatung SMPS design, expert comment needed ...

    The easiest approach looks (to me) to just be to swap that resistor for a higher-power one, 10W maybe?

    Comment


      #3
      Re: Old Tatung SMPS design, expert comment needed ...

      Yes, thanks, I've done that (10w 15 ohm at right) and reassembled the monitor until I come up with some better ideas. It's still getting a bit toasty inside from the 5w 180 ohm as well as there is only passive cooling via small vent holes. I may try leaving the cover off (as shown in pic) internal to the monitor. Don't think that would hurt anything ...


      Attached Files

      Comment


        #4
        Re: Old Tatung SMPS design, expert comment needed ...

        So are you sure that the Flyback circuit is also made to be able to run over 300Vdc?
        Q801 is shown with 400V pulse with 150DCV supply, you will get a lot more than that running on 300VDC supply.
        Last edited by budm; 11-17-2016, 02:55 PM.
        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


          #5
          Re: Old Tatung SMPS design, expert comment needed ...

          Not entirely. The drive transistor is rated to 800V, the transformer 400V p-p and the nearest cap 1.6kV. I have a photo from a later revision of this power supply (one that is specified to run on 100-250VAC) that shows the same transformer part number, while R805 (mentioned first post) is twice the value I have. Other than that, it's fingers crossed.

          I've run it a total of about 2 hours without issue (other than heat) but I realize that doesn't mean it's working within its design envelope. I'm hoping to get more details of the newer revision so I can compare the schematic and part values. I suspect there are a number of updates, R805 just being one of them.
          Last edited by KiwiME; 11-17-2016, 03:12 PM.

          Comment


            #6
            Re: Old Tatung SMPS design, expert comment needed ...

            What about putting a small fan until you get a better answer

            Comment


              #7
              Re: Old Tatung SMPS design, expert comment needed ...

              Originally posted by KiwiME View Post
              Not entirely. The drive transistor is rated to 800V, the transformer 400V p-p and the nearest cap 1.6kV. I have a photo from a later revision of this power supply (one that is specified to run on 100-250VAC) that shows the same transformer part number, while R805 (mentioned first post) is twice the value I have. Other than that, it's fingers crossed.

              I've run it a total of about 2 hours without issue (other than heat) but I realize that doesn't mean it's working within its design envelope. I'm hoping to get more details of the newer revision so I can compare the schematic and part values. I suspect there are a number of updates, R805 just being one of them.
              If you can check that Amplitude of the pulse when running with the 230VAC supplying the unit instead of 120VAC, that will give you a clue as to what is going on. I would be afraid unless the designer has in mind the the unit will still operating properly by just changing that resistor alone.
              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


                #8
                Re: Old Tatung SMPS design, expert comment needed ...

                try heatsinking that resistor--

                but insulate the ends and put a fan on the heatsink first.
                Don't buy those $10 PSU "specials". They fail, and they have taken whole computers with them.

                My computer doubles as a space heater.

                Permanently Retired Systems:
                RIP Advantech UNO-3072LA (2008-2021) - Decommissioned and taken out of service permanently due to lack of software support for it. Not very likely to ever be recommissioned again.
                Asus Q550LF (Old main laptop, 2014-2022) - Decommissioned and stripped due to a myriad of problems, the main battery bloating being the final nail in the coffin.


                Kooky and Kool Systems
                - 1996 Power Macintosh 7200/120 + PC Compatibility Card - Under Restoration
                - 1993 Gateway 2000 80486DX/50 - Fully Operational/WIP
                - 2004 Athlon 64 Retro Gaming System - Indefinitely Parked
                - Main Workstation - Fully operational!

                sigpic

                Comment


                  #9
                  Re: Old Tatung SMPS design, expert comment needed ...

                  Originally posted by Drack View Post
                  What about putting a small fan until you get a better answer
                  Yeah, but there's no low voltage available in this monitor to run a DC fan, I don't want to use an external power pack and an AC fan ties me to one mains voltage. Easier just to continue to run it at 110 VAC off a stepdown transformer, or do nothing. Don't need much time using CGA these days

                  Originally posted by budm View Post
                  If you can check that Amplitude of the pulse when running with the 230VAC supplying the unit instead of 120VAC, that will give you a clue as to what is going on. I would be afraid unless the designer has in mind the the unit will still operating properly by just changing that resistor alone.
                  Unless I can do that using differential mode on a 'scope (400V isolation to ground) I don't have the kit to determine that. In any case, I know it's easily twice as high.

                  Originally posted by TechGeek View Post
                  try heatsinking that resistor--
                  but insulate the ends and put a fan on the heatsink first.
                  Per pic, R805 is heatsinked against the housing. The other hot resistor could be raised to touch the housing cover though.

                  Appreciate all the ideas, guys ... I'll focus on creating a schematic of a known-good 230VAC version using photos from a guy in the UK and modify my unit to match.

                  Comment


                    #10
                    Re: Old Tatung SMPS design, expert comment needed ...

                    Replace R801 3.3r 10w with double the value and wattage perhaps?
                    Better a pot resistor get hot than other parts.

                    Comment


                      #11
                      Re: Old Tatung SMPS design, expert comment needed ...

                      That's the inrush current limiting resistor, which is not getting too hot anyway since the current passing through at 230VAC is roughly half that at 110VAC.

                      Comment


                        #12
                        Re: Old Tatung SMPS design, expert comment needed ...

                        I replaced the entire power supply over 5 years ago with one fit for purpose.

                        Comment

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