samsung t200hd won't power up

[jananas]

Hi,

I have a problem with my samsung syncmaster T200hd lcd monitor. It won't power up anymore.

I opened it to look at the capacitors, two were bulged and i replaced those. After putting it back together it still wont power up. I checked the output signals (the 5.1 V and 15 V) of the board and they give no output at all.

I also checked the voltage of the main filter capacitor (the big one 450 V 100 uF) and it gave a voltage of 600-700V. I have no idea if this is good or not but it seemed to high.

Can anyone give me some diagnostic advice on this? I'm new to this and google doesn't help me any further.


Kind regards,

Jan

[retiredcaps]

Re: samsung t200hd won't power up

I also checked the voltage of the main filter capacitor (the big one 450 V 100 uF) and it gave a voltage of 600-700V. I have no idea if this is good or not but it seemed to high.

1) Can you post a picture of your multimeter?

2) Are you measuring AC or DC across that main cap? You want to measure DC V across the legs of the cap.

If you are in North America, it should be around 165 V DC. If there is PFC involved, then it should be around 330 V DC.

If your mains are 220 V DC , it should be around 310 V DC without PFC.

3) What brand and series did you use for capacitor replacements? For example, Panasonic is the brand, and FR is the series.

4) What is the DC voltage across the startup cap (just above ZD101)? Is it steady or fluctuating?

[jananas]

Re: samsung t200hd won't power up

Ok so I decided to check the voltage again just to be sure and set it to DC, the output was zero now. I also thought I might try to measure it on AC just out of interest and it gave a negative value around -300.

Then I accidentally made a shortcut which created a bang and appeared to be a very convenient way to shut down all my electric appliances:P

Flipped over the switch and the power is back again.

Think the capacitor is broken now and I'm not sure if I can plug the board in again. Would this be a problem?

Im in Europe, the power out of the plug is 230V AC here.

[retiredcaps]

Re: samsung t200hd won't power up

I also thought I might try to measure it on AC just out of interest and it gave a negative value around -300.

If you are new to electronics, you will have to follow instructions exactly. This is high voltage stuff and you, your property, and/or your multimeter could suffer some serious damage.

When I say DC, I mean DC. You don't want to "try" things on the hot side of the power board.

The AC is converted into DC by a bridge rectifier (BD101).

So 230 V AC x 1.414 = 325 V DC

So at the large filter cap, you want to measure DC.

Think the capacitor is broken now and I'm not sure if I can plug the board in again. Would this be a problem?

Check BD101 for shorts.

1) Do this with power OFF and device unplugged from the wall.

2) Set your multimeter to 200 ohms.

3) Number the pins 1,2,3, and 4. Measure the resistance between 1-2, 1-3, 1-4, 2-3, 2-4, and 3-4. Any reading under 30 ohms suggests a short.

Do not confuse "1" of the left hand side of the multimeter with 1.0. The first means out of range and the second means 1.0 unit of your measurement.

PS. You should refrain from using this multimeter to measure the DC voltage across the main cap. It is only rated up to 250V max.

Get or borrow another multimeter that can measure up to 1000 V DC.

[retiredcaps]

Re: samsung t200hd won't power up

Also check the fuse by the AC plug. ALL resistance measurements are done with power OFF and device unplugged from the AC outlet.

A good fuse should measure less than 1.0 ohms.

[jananas]

Re: samsung t200hd won't power up

Thanks for the quick reaction.

When I say DC, I mean DC. You don't want to "try" things on the hot side of the power board.

Understood.

3) Number the pins 1,2,3, and 4. Measure the resistance between 1-2, 1-3, 1-4, 2-3, 2-4, and 3-4. Any reading under 30 ohms suggests a short.

all out of range, so I guess that's good.

PS. You should refrain from using this multimeter to measure the DC voltage across the main cap. It is only rated up to 250V max.

I have another one that goes up to 600V. I'll use that one.

A good fuse should measure less than 1.0 ohms.

It is out of range, don't know if it was before or if it burned due to the short I created.

The replacement capacitors are from KSC, I can't see the series. All the other information on it are the specifications (1000 uF, 16 V, -40 - 105 degrees Celsius).

I'll get a new fuse this week and do the Main filter cap measurement again then. I'll post an update on the results!

[retiredcaps]

Re: samsung t200hd won't power up

all out of range, so I guess that's good.

Your bridge rectifier is likely good.

It is out of range, don't know if it was before or if it burned due to the short I created.

That fuse is open and needs to be replaced.

The replacement capacitors are from KSC, I can't see the series. All the other information on it are the specifications (1000 uF, 16 V, -40 - 105 degrees Celsius).

I'm not familiar with KSC. For a SMPS application like this, you require low ESR caps like Panasonic FR/FM/FC series.

In addition, capacitors (C designation on the PCB board) die from age, heat, and shoddy build quality. Capacitors DO NOT have to be visibly bloated in order to bad. They can be out of tolerance uF (a 1000uF measures 20uF) and/or have high ESR (ohm). A multimeter will be insufficient to test for ESR. For that you need an ESR tester which costs between $50 and $300.

Most members here will recommend that you replace ALL capacitors with reputable brands from reputable sellers. Brands like Rubycon, Panasonic, Nichicon and United Chemicon are suggested. A list of recommended caps can be found at

https://www.badcaps.net/forum/showthread.php?t=2280

The one exception might be the largest capacitor on the power board which rarely, but not never, fails.

Here is an example of how to choose capacitors with respect to capacitance, voltage, diameter, height, ripple and ESR.

https://www.badcaps.net/forum/showpo...4&postcount=19

I'll get a new fuse this week and do the Main filter cap measurement again then. I'll post an update on the results!

After you replace the fuse, check for secondary DC voltages first. If you get 0.0 V DC, then go to the main filter cap.

If the main cap measures around 325 V DC, then measure that startup cap I mentioned earlier. BTW, that heatsink just below the startup cap is likely live. That means don't touch it with your hands. You should exercise caution with everything on the hot side.

Since it is likely you will have to flip the board over to measure the startup cap, use something like cardboard for separation so you don't short out something.

[jananas]

Re: samsung t200hd won't power up

Ok, so I finally got some time again to continue this project.

Here's the update:

I replaced the fuse and measured the secondary DC voltages (I interpreted this being the output voltages of 5.1 and 15 V). They were 0.0 V.

I continued with measuring the main filter cap. It gave a value of 317 VDC, a bit lower than the 325 VDC it should give. Would this be a problem?

I also measured the start-up capacitor (C103, just above ZD101). It gave 0.0 VDC. I replaced it and before I connected the whole thing to plug again in measured the resisctance over the start-up cap. This was 1.1 Ohms (multimeter on 200 Ohms). The soldering joints don't look like there connecting. Is this normal or is it shorted somewhere? (It's a 63V 47uF cap)


Kind regards,

Jan

[jananas]

Re: samsung t200hd won't power up

Update:

I connected it, voltage over the start-up cap is still 0.0 V.

Are there other components which I can check? I don't see other fuses that might be blown.

[retiredcaps]

Re: samsung t200hd won't power up

It gave a value of 317 VDC, a bit lower than the 325 VDC it should give.

317 V DC is fine. That tells us everything from the AC plug to the large capacitor is likely working.

I also measured the start-up capacitor (C103, just above ZD101). It gave 0.0 VDC. I replaced it and before I connected the whole thing to plug again in measured the resisctance over the start-up cap. This was 1.1 Ohms (multimeter on 200 Ohms). The soldering joints don't look like there connecting. Is this normal or is it shorted somewhere? (It's a 63V 47uF cap)

1) If you give the startup capacitor a gentle pull, it should be firmly connected to the pcb. If not, your soldering needs to be redone. If that cap is reading shorted, it may have been damaged during mishap. I suggest replacing it.

2) If you are getting 0.0 V DC across the legs of the startup capacitor, check (the pics are blurry and I can't quite read the pcb)

a) Q101. This is the power mosfet. Unplug the lcd and turn power off. Measure the resistance between pins 1-2, 1-3, and 2-3 "in-circuit". A reading under 30 ohms suggests this might be a shorted component.

b) Zd101, D181, and D183. These are all diodes. Use you diode checker on the multimeter. It should read between 0.2 and 0.7 V one way and open (0L or "1") the other way.

Report all your measurements.

[pedro]

Re: samsung t200hd won't power up

b) Zd101, D181, and D183. These are all diodes. Use you diode checker on the multimeter. It should read between 0.2 and 0.7 V one way and open (0L or "1") the other way.

Except ZD101, which is a zener diode ....

[PlainBill]

Re: samsung t200hd won't power up

Zero volts and zero ohms across the startup cap is either a shorted diode on the tertiary (run) winding or a shorted SMPS controller.

PlainBill

[jananas]

Re: samsung t200hd won't power up

The soldering of the start-up cap is solid. There are however a few scratches on the pcb. Could this create shorts?

Results of the measurements:

D101: 0.55 Ohms - Out of range
D103: 0.72 Ohms - Out of range
ZD101: 0,002 Ohms - 0,002 Ohms


Q101 with pins numbered like this, multimeter on 200 ohms:

1
2 3

1-2: out of range
1-3: out of range
2-3: 160 Ohms

I did some reading on the zenerdiodes and what I understand of it is that it can conduct in directions. However the conduction in the "reverse" direction should only take place when the potential over - and + is higher than the Zener voltage. When the multimeter is on diode testing it creates a potential over the poles. Could it be that this voltage is high enough to make it conduct in the reverse direction? Or is the resistance way to low anyway?

And how do I test a SMPS-controller?

I will make some new pictures tomorrow when I have some decent sunlight.

[retiredcaps]

Re: samsung t200hd won't power up

Except ZD101, which is a zener diode ....

Thanks for the correction.

[retiredcaps]

Re: samsung t200hd won't power up

Zero volts and zero ohms across the startup cap is either a shorted diode on the tertiary (run) winding or a shorted SMPS controller.

I marked up the pictures.

1) I believe the shorted diode that PlainBill is referring to is either the red, yellow, or green one. Without tracing it or having a schematic, I don't know. So measure all 3 with your multimeter on diode test and report the readings.

2) For the shorted SMPS controller, there is a 8 pin IC circled in blue on the backside. There will be a part number. Report it.

Look for its datasheet. Once you find the datasheet, measure the resistance between the GND pin and the VCC pin. Report the reading.

[retiredcaps]

Re: samsung t200hd won't power up

The soldering of the start-up cap is solid. There are however a few scratches on the pcb. Could this create shorts?

Post a picture of this area.

Results of the measurements:

D101: 0.55 Ohms - Out of range
D103: 0.72 Ohms - Out of range
ZD101: 0,002 Ohms - 0,002 Ohms

D101 and D103 are good. BTW, it is 0.55 V not ohms when using diode test.

ZD101 tests like it is shorted. For now, leave it in until the other tests are completed. It may be necessary to desolder ZD101 and retest it out of circuit to verify.

Q101 with pins numbered like this, multimeter on 200 ohms:

1-2: out of range
1-3: out of range
2-3: 160 Ohms

The pin 2-3 reading is a bit low, but it is likely okay.

[tom66]

Re: samsung t200hd won't power up

ZD101 is definitely shorted. It's a common failure in the startup section. It helps regulate the Vcc voltage. It's often built into the SMPS controller but not always.

A zener *will* test like a normal diode on any standard DMM as few zeners go below 2.4V reverse rating and most DMMs don't test >2V...

The device mounted to the heatsink on the secondary side is probably also a diode and should be tested.

Often a shorted diode in the secondary will cause a low Vcc voltage (a few volts) - too low to run but not zero Vcc... but I could be wrong here, as I don't have a schematic of the supply. (Unfortunately manufacturers don't tend to release them.)

The only problem with the zener is locating a replacement. Sometimes the voltage is written on them. Otherwise, the voltage could be inferred from the SMPS controller.

[retiredcaps]

Re: samsung t200hd won't power up

The device mounted to the heatsink on the secondary side is probably also a diode and should be tested.

Good catch. I missed that one due to the shadows in the photograph, but I clearly see the silver screwhead in the pic now that you pointed it out.

[jananas]

Re: samsung t200hd won't power up

Merry Christmas everybody,

Here are the results of the measurements.

The diodes that were circled:

D201C is the one near the heatsink

D201C: 0.178V out of range
D201B: 0.178V out of range
D201A: 0.178V out of range

D201: Strange, there's is a slot for it in the pcb but it is not there. Pretty sure it didn't fell off when I took the powerboard out of it's case. Soldering joints also dont look like something broke off.

The diode mounted on the heatsink:


D202, has three pins. Pins numbered like:

Near the edge of the board -1-2-3-inside of the board

polar signs indicate which lead of the multimeter I put on the pin:

1(+) - 2(-): 0,208V ..................... 1(-) - 2(+): out of range
2(+) - 3(-): out of range .............. 2(-) - 3(+): 0,207 V
1(+) - 3(-): 0,001V ......................1(-) - 3(+): 0,001V

SMPS-controller

Type No.: F PH9AD FAN7602 B
Can't find a datasheet, any advice on websites?

Further info:

There are some more diodes on the backside (near ZD101), dont know if they are part of the relevant circuit:

D102, pins numbered like this, polar signs indicate DMM lead on pin:
1
2 3

1(+) - 2(-): out of range..............1(-) - 2(+): 0,690V
1(+) - 3(-): 0,148V ....................1(-) - 3(+): 0,15V
2(+) - 3(-): 0,870.......................2(-) - 3(+): out of range

D104:0,674V - out of range

ZD102, pins numbered like this, polar signs indicate DMM lead on pin:
1
2 3
1(+) - 2(-): 0,990V..............1(-) - 2(+): 0,655V
1(+) - 3(-): 0,990V ....................1(-) - 3(+): 0,654V
2(+) - 3(-): 0,006V.......................2(-) - 3(+): 0,001V

I also added some extra photo's of the scratches, the soldering joint of the start-up capacitor etc.

Quick question about terminology: Do you mean by secondary side, the cold site?

Kind regards,

Jan

[retiredcaps]

Re: samsung t200hd won't power up

1) Before I comment on the latest findings, what is that replacement cap C103? It doesn't look at an aluminium electrolytic cap to me? Or is it axial lead?

2) Was the 0.0V DC readings taken with cap?