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Switching power supply repair

Discussion in 'General Electronics Chat' started by lynx, Apr 10, 2014.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Well, the current sense resistor could have blown because of an overcurrent, but i would think it would blow OPEN not to a near SHORT (or zero). This makes it hard to say if the IC will blow again or not, but usually the way power supplies are tested is the input is raised little by little checking everything with a scope and if anything looks wrong it is fixed before continuing. Once we get up to the full input voltage with no problems then we assume it's ok. This includes checking for spikes across the main transistor(s).

    But as i said before, this is a difficult thing to fix without having it right in front of me where i can examine many aspects of it, and even then it may be too hard to fix. Could there even be something wrong with the thing it is powering.
    That's another reason why i strongly suggest building your own power supply. Yes you have negative voltages, so what, that's what a power supply does :) You probably only need one negative voltage and then regulate it down using regulators chips. The current probably isnt much but you can measure it at start up to see if it goes too high, then upgrade that line to a higher current. At least this way you have a definite way to fix it.

    It's still up to you of course, but you may never get this fixed unless you get lucky.
     
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  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Resistors can blow both ways. Carbon composition had a tendency to increase value. Metal films can do just about either, including pudde.
    Wirewounds tend to blow open.

    Metal oxides tend to blow completely open by design.

    The current limiting resistor could be the cause of all of the problems and could also be a design deficiency and it's a good reason why the supply wont start. Sometimes those resistors are metal oxide so it acts as a fuse.
     
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  3. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Hi Guys

    I do not normally stick my neck out and get involved in the middle of a discussion...but its quite and and I am totally bored.

    So, here is what we know:

    1. No parts are blowing so far(good).
    2. The Current Sense resistor has gone from 0.47 ohms to open (MegaOhms). So it is blown.
    3. I would say it blew when the original Chopper IC failed.

    What I would do is the following:

    1. Replace the Chopper IC
    2. Replace the 0.47 ohm Current sense resistor
    3. Remove and check D128 to see if it is OK (no reverse leakage). If it reads faulty, the Optocoupler is possibly damaged too.
    If the above are suspect, replace them.
    If the above are OK....be brave and feed the PSU Mains and see if it magically works.

    It's very hard to repair things that are not in front of me.

    Regards,
    tvtech
     
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  4. dave

    Dave New Member

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  5. lynx

    lynx Member

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    it magically works! :) i've replaced the R104 awhile ago (a friend had that value but it was 1/4 watt so we used 3 of them)

    BUT NOW I AM WHERE I STARTED! and i wrote at the first post.. "the LM317T gets quite hot (120c) and the dvd doesn't spin..."

    between the D108 and the ground it's 23.20VDC while the schematic says it should be 15VDC, so i guess that's the reason the LM317T gets that hot, but what causes that?
     
    Last edited: Apr 20, 2014
  6. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Good

    Leave it for now. Turn all off and I will try to help you Tomorrow..Deal :)

    Regards,
    tvtech
     
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Congrats:

    Take a look at the voltages on the transformer which have values. They might all be high.

    The 5V source is what's regulated directly by the SMPS IC. Everything else, get's post regulated.

    I can't remember if the frequency relationship is linear, but, I think, there was a 67 kHz or so version. FWIW: 22*67kHz/100kHz is about 15V.

    You could resort to a transformer supply for the 12V supply.
     
  8. atferrari

    atferrari Well-Known Member

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    All together? In parallel or series?
     
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  9. lynx

    lynx Member

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    thanks :) i will...although at some point in the past (before the IC blows up) that issue
    with the hot regulator was fixed by itself for awhile and everything! was working pefect...
    i told you about it in past pages...so now i don't know why it returned to tha state.

    in parallel, we tried to match the 0.5 watts we used three just in case 0.5w is little, but probably i'll replace it again as soon as i find a single resistor because i don't like it the way it looks on the pcb, it might be funny what i said..but yes when i fix something i like it to be as it was, tight and clean.
     
    Last edited: Apr 21, 2014
  10. alec_t

    alec_t Well-Known Member Most Helpful Member

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    :eek: That means the current limit for the FETs inside the IC will be ~ 3 times the designed value. Your IC probably won't last long :(
     
  11. lynx

    lynx Member

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    i wanted to say... a friend had that value but it was 1/4 watt so we used 3 resistors to make that value :)
     
  12. lynx

    lynx Member

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    all the voltages around the board seem to me they are fine! other than the voltage that goes to the lm317t which is 23.20V and makes it hot and maybe the 32v output which when i measure it's 30v and if i keep the multimeter on the rail any longer it falls up to 28V the transistor the schematic shows at that point it's a C337 maybe it supplies too little current at that voltage and even a multimeter it's a load for it??
     
  13. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Wow that's great news. So the only problem now is the 15v rail is too high in voltage?
     
  14. lynx

    lynx Member

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    yes it's too high... only way for the 317t to remain cool is if i don't connect the dvd power connector.
     
  15. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Hi again

    The only thing that makes sense now is a "dry joint" or a hairline crack on the PCB that is causing havoc.

    I would do this:

    1. Unplug the PSU from Mains.
    2. Take a 560 Ohm to 1K Ohm resistor (5W) and hold it across the main smoothing cap to discharge the circuit completely.
    3. Closely examine the PCB for any signs of bad (dry) joints. If in doubt, resolder it/them.
    4. Next, take your Meter and thoroughly check that there is continuity on all tracks that are connected together.

    Regards,
    tvtech
     
    Last edited: Apr 21, 2014
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  16. lynx

    lynx Member

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    that's what i'm doing all the time when i take off the board... also most of the joints have been resoldered...but i'll check again
    since you say it doesn't make sense.

    something irrelevant i want to ask, do you think it could be powered from a square wave inverter?
    and if not what issues it could cause to the specific PSU?
     
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    What I'd like to see is the transformer pins:
    pin, should be value
    (18) 23
    (15) -22
    (12) +8
    (11) +8

    I found both ends of FL1, but not FL2 on the schematic.
     
  18. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Normally SMPS do not have issues running from Inverters. Be that pure Sine Wave or modified Sine Wave......why do you ask??
     
  19. lynx

    lynx Member

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    OK..here's what! the lm317 input goes to 23.20v WHEN i have connected the large connector (the one most of the voltages go to)
    when it is disconnected then the input of the lm317 is 13.55v exactly and steady!


    because i have an inverter but i'm only using it for resistive loads...and now viewing the
    input of this device i thought if it could be possible to damage the smps ic if i use it.
     
    Last edited: Apr 21, 2014
  20. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Seeing this thing has been plugged in and out so many times...is there at all a possibility that maybe a wire broke off somewhere and you did not solder it back in the right place.....

    Trying to reason logically here and not complicate things :eek:

    Regards,
    tvtech
     
  21. lynx

    lynx Member

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    no wires seem broken, everything it's in place... having the PSU outside the dvd-r unit if i connect it to the mains without any connectors on it the voltage at the input of the lm317 it's 13.55v, now if i connect the large connector where all the output voltages go to the processing board
    then the input of the lm317 becomes 23.20v and if i try to connect the 12v/5V connector of the dvd to the PSU, so it can spin then the lm317 becomes extremely hot and no motor spins.
     

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