Samsung Syncmaster 914v

[retiredcaps]

Re: Samsung Syncmaster 914v

The capacitor seemed to be getting up to around 10-10.5 volts (taking a few seconds) before discharging almost instantly and repeating.

Since your voltage is fluctuating across the startup cap, it could be the following:

1) the startup cap is bad. These caps never bloat when they go bad.

2) the SMPS IC is bad.

3) there is something shorted on the output side like a diode causing the SMPS to start, stop, start, stop

Changing the startup can and testing for a shorted diode are the 2 easiest things to do.

[momaka]

Re: Samsung Syncmaster 914v

For U205, it's about 10.7M ohms between pins 1 and 2, out of range between pins 1 and 3, ditto for pins 2 and 3. Are there more pairs I might test, or does this seem to indicate there's no short in either?

When you say out-of-range, do you mean you get a "OL"/"1" on the left side of the screen of your multimeter or do you get 0.00? I just want to clarify, since a lot of new people here with manual multimeters confuse the "1" on the left side of the screen with 1 Ohm. A "1" on the left side of the screen with no other numbers means out-of-range.
If you do indeed get an out-of-range reading, then those transistors should be okay.

Proceeded to test the transformers.
For both T301 and T302, in the row of 6 pins, designating the pin closest to the edge of the board as Pin 1, I measured the following:
Pins 1+2: <1 ohm
Pins 1+3: Out of range
Pins 1+4: Out of range
Pins 1+5: <1 ohm
Pins 1+6: <1 ohm
Pins 2+3: Out of range
Pins 2+4: Out of range
Pins 2+5: <1 ohm
Pins 2+6: <1 ohm
Pins 3+4: <1 ohm
Pins 3+5: Out of range
Pins 3+6: Out of range
Pins 4+5: Out of range
Pins 4+6: Out of range
Pins 5+6: <1 ohm

Looks good. Pins 1 and 2 are a pair that is connected to the same pad, so it's good that they are showing <1 Ohm. Ditto for pins 5 and 6. Now as far as getting <1 Ohm for pins 1/2 to pins 5/6 is also normal in this case, because those inverter transformers have very few turns of wire and that's why they appear shorted. So those readings are good again.

For the 3 pins on the other side of each, none showed a resistance within range between themselves and any in the row of 6. The resistance between the outer two pins in each group of 3 was 1146 for T301, 1141 for T302. Definitely within a 3% tolerance.

This looks good too, so the inverter transformers seem to be fine.

The capacitor seemed to be getting up to around 10-10.5 volts (taking a few seconds) before discharging almost instantly and repeating.

Interesting.
That contradicts the fact that the monitor power LED stays ON with the inverter disconnected. If the monitor is indeed staying ON with the inverter disconnected, then that cap should have a stable voltage.

So I would say double check the measurement across that cap. If it still fluctuates like that, then the next step would be to replace that cap. After that, check that the monitor still stays ON. If it does, disconnect the CCFLs from the inverter and reconnect back the inverter jumper J308. Judging by the readings you got for the inverter transistors U204 and U205, they should be okay. Once jumper J308 is reconnected, try powering the monitor again. Does the power LED stay ON solid or does it repeatedly turn ON/OFF like it did as you described in your first post?
- If it repeatedly turns ON/OFF, then that rules out the CCFLs as being a problem and there's probably still a fault in the inverter circuitry.
- If it stays ON (unlikely), try connecting and disconnecting different CCFLs in different numbers and see which one(s) make the inverter turn OFF.
NOTE: NEVER connect or disconnect CCFLs when the monitor is plugged in. High voltage from the inverter is DANGEROUS!

Another thing you can test is the group of those ceramic caps above U204 (C214, C215, etc.). See if you get a short circuit (or less than 10 Ohms) across that group.

[KevinB550]

Re: Samsung Syncmaster 914v

When you say out-of-range, do you mean you get a "OL"/"1" on the left side of the screen of your multimeter or do you get 0.00? I just want to clarify, since a lot of new people here with manual multimeters confuse the "1" on the left side of the screen with 1 Ohm. A "1" on the left side of the screen with no other numbers means out-of-range.
If you do indeed get an out-of-range reading, then those transistors should be okay.

It is indeed a 1 on the left side, glad to hear the transistors/transformers are okay. If they were bad, I might just elect to replace the whole board altogether as I'm not very confident in my ability replacing components with large quantities of leads and/or leads grouped very closely together.


Also, I checked the diodes ZD101 and ZD102 using the diode test setting on my meter. ZD101 read 1865 with red on positive and black on negative lead, 533 when reversed. ZD102 reads 1867 and 660, respectively. I admittedly do not know what the displayed number represents, the meter's manual says the approximate forward voltage drop will be displayed (test current is ~1mA). At any rate, when I try to measure resistance in either direction, for either diode, it simply climbs steadily out of range.

For the ceramic capacitors, all measured out of range for resistance. Should the thermistor measure at ~5 ohms at room temperature?

I apologize for taking a while on replacing the other cap(s). I'll be moving out in under a week and wanted to get settled in on the other end before doing any more soldering.

Thank you all for the continued help, I've been drawing it out for a while, but I do promise to keep returning!

[momaka]

Re: Samsung Syncmaster 914v

Also, I checked the diodes ZD101 and ZD102 using the diode test setting on my meter. ZD101 read 1865 with red on positive and black on negative lead, 533 when reversed. ZD102 reads 1867 and 660, respectively. I admittedly do not know what the displayed number represents, the meter's manual says the approximate forward voltage drop will be displayed (test current is ~1mA).

As long as you don't get a clear short across a diode (I consider less than 50mV very suspicious), then the diode is probably okay. A typical diode will normally read 300 to 800 mV out of circuit in one direction and open-circuit (out-of-range) in the other. In circuit, it can be different, but you will probably still be able to tell if it's shorted or not.

For the ceramic capacitors, all measured out of range for resistance. Should the thermistor measure at ~5 ohms at room temperature?

Yes, most thermistors have several Ohms of resistance at room temp and that drops as they warm up. Ceramic caps measuring out-of-range is a good thing.