Finding A Power Supply Schematic

[This followup was posted to sci.electronics.repair 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 questions...

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?

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Michael Karas 
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Michael Karas
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the cited author.]

Is there a "bleed-through" route for start-up?

Reply to
N_Cook

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 normal.

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Michael Karas 
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Michael Karas

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

Reply to
N_Cook

Michael Karas schrieb:

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.

HTH

Reinhard

Reply to
Reinhard Zwirner

Its common practice for the SMPSU control chip Vcc to come from an extra winding on the chopper transformer. Before the PSU starts up - there has to be some source of Vcc to get it kick started.

Very often there's a high resistance resistor (or 2 in series) from the +ve of the mains bridge to provide the initial start up supply for the control chip - these resistors are very prone to going high/OC.

Reply to
Ian Field

cited author.]

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,badcaps.net/forum. Pictures help.

PlainBill

Reply to
PlainBill

the cited author.]

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

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(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).

formatting link

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Michael Karas 
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Michael Karas

the cited author.]

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:

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Which leads to:

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And the datasheet: (watych out for line breaks)

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IF you are in the USA, they are available from Digi-Key (ww.digikey.com) 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.

PlainBill

Reply to
PlainBill

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 transistor.

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,

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Michael Karas 
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Reply to
Michael Karas

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 componentry?

Reply to
N_Cook

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

PlainBill

Reply to
PlainBill

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 monitors.

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Michael Karas 
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Michael Karas

I see that even after all these years, you still haven't got your priorities right 8^}

Isaac

Reply to
isw

:^)

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

Michael Karas

Reply to
mkaras

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.

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Michael Karas 
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Michael Karas

--snippage --

A while back, I acquired a very nice LCD monitor with both VGA and DVI-D interfaces. At the time, the only interface adapter I had was VGA (this is on a Mac which uses an external adapter). Later, I got a DVI-D adapter and cable, and was able to compare the two. Despite my most careful comparisons, using both text and images, I was unable to see any difference in the image quality between the two.

VGA is not all that bad.

Isaac

Reply to
isw

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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Michael Karas 
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Michael Karas

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