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Finding A Power Supply Schematic

Discussion in 'Electronic Repair' started by Michael Karas, May 24, 2013.

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  1. [This followup was posted to and a copy was sent to the cited author.]

    I am repairing an LCD monitor that has the very common turn on problems.
    This is an I-INC IF281D unit with model number showing as HSD1040. I am
    in the process of replacing all the capacitors on the PhiHong power
    supply board (model PSM217-404-H-R) and have noticed that one transistor
    looks like it got excessively hot. I would like to replace that
    transistor but have to first ID the part number. To this end I have two

    Does anyone know where it may be possible to get a schematic for the
    PSM217-404-H-R power supply board?

    The overheated transistor is an SMT device with a code DFRG marked on
    top. Any idea as to what that part number may be?
  2. N_Cook

    N_Cook Guest

    Is there a "bleed-through" route for start-up?
  3. [This followup was posted to and a copy was sent
    to the cited author.]

    I do not really know what you are asking about here. This unit came to
    me as a freeby already taken apart and the report that "It does not turn
    on". So I just dived in with the power supply capacitor replacement
    since that has worked many many times on other monitors (~ 15 so far). I
    noticed that the particular transistor in question has discolored leads
    and solder joints and the PC board has turned to a darker color than
  4. N_Cook

    N_Cook Guest

    You need to identify the supervisor IC and get its pinout hopefully.
  5. This


    is a typical schematic for a SMPS. The supply voltage for the
    controlling IC is provided via R1 for the first moment - I think this
    is N_Cook's "bleed-through" route. When the circuit is working the
    supply voltage is generated by W3/D1/C6.

    If R1 is open and/or C6 has a short the SMPS won't start.


  6. Guest

    Google 'marking code DFRG' and see if anything makes sense. Also, SMT
    is a very generic term. If we knew the specific package, it would
    help us help you.

    Another resource, especially whre power supplies for LCD monitors are
    concerned is www, Pictures help.

  7. Hmm, can't you see my posting as of May 24 with the link to that


  8. N_Cook

    N_Cook Guest

    But MLCCs then have their own problems, excess heat from PbF fixing, lead to
    cracks and then metal migration into the cracks due to the tiny geometries
    I don't know the relative frequency of each failure process though
  9. [This followup was posted to and a copy was sent
    to the cited author.]

    Here is a link to a picture of the transistor in question.

    (Note link above may be split by a line break).

    The transistor package body is 2.5mm x 4mm. Lead spacing is 1.5mm.
    According to a document I checked here at the next link this package is
    a SOT-89 (also apparently referred to as a T0-243 or an SC-62).
  10. Here is it:


    Still wondering

  11. Guest

    Excellent pictures, thanks. I believe the actual marking code is DF;
    the R indicates the gain and the G is a prouction lot or date code.

    My favorite source for arcane marking info:

    Which leads to:

    And the datasheet: (watych out for line breaks),_2sd1768s,_2sd1863,_2sd1898_rohm.pdf
    IF you are in the USA, they are available from Digi-Key
    ( for $.53 each. (Choose USPS First Class Mail for
    lowest shipping cost.)

    As a general rule, I dissaprove of replacing parts because they 'look
    bad'. A few minutes making measurements with a DVM can give a wealth
    of information.

  12. [This followup was posted to and a copy was sent
    to the cited author.]

    THanks for the info. I had also done some research on some other sites
    and find that there are multiple possibilities for this device marked
    with a DF.

    Sanyo NPN 2SD1623
    Philips PNP BF621
    Siemens PNP BNF21
    Rohm NPN 2SD1898 **

    ** The part that you located.

    I agree that this part should not be replaced if it is working. I had
    done some preliminary checking with the meter and it did not seem to
    respond with any forward biased junction readings and so I needed to get
    information as to whether this was some type of FET or a blown BJT

    When I get a power cord setup to be able fire up the board I will do
    some checking. I may also need to rig up a temoprary heatsink for a
    large diode bridge that gets screwed to part of the case that had to be
    removed to get at the board. Initial tests can probably avoid that if I
    just put small loads on the output rails for testing,
  13. It is Michael Karas who should try to repair his power supply by
    replacing the capacitor corresponding to C6 (or the resistor
    corresponding to R1) - not me! _I_ just tried to explain N_Cook's
    "bleed-through" route by a typical SMPS schematic as an example.


  14. N_Cook

    N_Cook Guest

    Probably just incidental and irrelevant to the main problem, as it can only
    get stressed while the SMPS is operational. I've still not seen you state
    the identity of the supervisor/controller IC/s ? or is it discrete
  15. Guest

    Note that the marking codes seem to be chosen by the manufacturer.
    Always make sure you are dealing with the same package type.

  16. [This followup was posted to and a copy was sent
    to the cited author.]

    I will post back here when more work on this power supply has happened.
    Normal workweek obligations force one to set aside tasks like repairing
  17. mkaras

    mkaras Guest


    Priorities are just that . . . something that somehow gets set by what we choose to do.

    Michael Karas
  18. [This followup was posted to and a copy was sent
    to the cited author.]

    Well, I received the new capacitors for the monitor power supply and got
    those installed. The power supply fired right up and produced its 5V
    STBY output. I then tested the other outputs by connecting the PSON
    signal on one of the connectors over to the 5V STBY and all of the +5, +
    12 and +24 volt rails all came up. I tested the thing on the bench with
    some load on each output and it works nicely.

    Unfortunately when the PSU is mated up with the video controller board
    in the monitor it fails to come up and act like a monitor. The only
    thing that does appear to work is that the EDID device does provide
    valid data back out through the D-Sub HD15 connector to a host computer.
    It shows the proper native monitor ID strings and the correct native
    resolution. But no other functioning is in operation.

    Since this thing was given to me for free and the fact that it only
    supports analog VGA signalling (no DVI connector) I will probably not
    spend any more time futzing around with it.
  19. [This followup was posted to and a copy was sent
    to the cited author.]

    I am inclined to agree that analog VGA can be pretty good. A lot depends
    upon the quality of the cables, its lengths and how well the video
    controller outputs are terminated.

    Since LCD type panels will digitize the analog signal anyway there is
    also a lot that can be done on a decent monitor regarding how the signal
    is sampled.

    In my personal experience with quite a variety of different LCD monitors
    attached to an almost equally diverse array of different video driver
    implementations I have both situations. In some instances where an
    analog VGA to DVI direct comparison was done there was virtually no
    discernable difference. On the other hand there were examples where
    there was a distinct difference in the displayed image - even on a
    monitor that had both DVI and analog VGA connections fed the same
    content from the same video controller.
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