Samsung PN42C450 B1DXZA Doesn't Power On

[Eaglecrest]

I have a Samsung PN42C450 B1DXZA where the red standby LED lights, but will not power on. When plugged in it will do a double click within a sec and then a couple seconds later you get a quieter single click. This sequence happens twice.

When you hit the power button this click sequence repeats, PS_ON has 3.25 V at stand by and drops to .06 V then rises to 3.25 between the two click series and then again after the second series. This process can be repeated a few time before the "standby" voltage stays at .06. My understanding is that this TV has a high standby voltage and energizes when PS_ON is pulled low. If so, then it is trying to energize and fails; it tries twice then stops. Then after multiple attempts the system shuts down and will not try further without pulling the plug to reset. Does this seem correct?

Attached are photos of the TV and boards as well as voltage readings. I only have 3.25 V PS_ON and STD 5.21 V. Since no other voltages are generated I am also assuming that the power board is bad. Does this seem correct?

Also where I can look for the schematics for this TV?

The Problem Trace photo is more of a questions. The traces are dark in spots under around the CM 820-2 area of the board. Does this indicate a problem?

[vinceroger69]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

Start by reflowing/adding a bit of fresh solder too all the devices attached to the heatsinks also to any of the larger parts like transformers etc as we have seen in the past as bad soldered joints (cant always see with naked eye) will cause issues like this.

[Eaglecrest]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

I was looking at some of the rework stations for sale that have an adjustable soldering station and a hot air station. Would the hot air be a good tool for reflowing? Also, are you suggesting the power board only to reflow?

[tvtimmy]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

Just heat up each joint with a solder iron and put some fresh leaded solder on.

[Eaglecrest]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

Thanks TV Timmy. Any thoughts about the trace lines in the Trace problems picture? Are these ok or do they show a problem?

[tvtimmy]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

When in doubt, whip it out! Your multimeter that is. Put it in continuity mode and put one probe on a solder spot and go past the suspected bad trace to another solder spot on it and listen for the beep. Beep is good.

[Eaglecrest]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

I looked at the power board to determine which pins to resolder. As I looked at the board, a number of the pins are very close to each other and adding more solder seems like it will end up making a blob of solder across the pins (see attached photo of the pitch between the pins).

Your thoughts and suggestions would be appreciated.

[R_J]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

I would suspect a fault on the Y-sus or Z-sus board, before the power supply. Have you tried monitoring the Vs voltage when you first turn on the tv and see if it comes up for a second.

with all the plugs connected check CN804 VS. If you are checking the VS without a load it may not come up due to ovp circuit.

With power off, you could also check the resistance across VS and ground for a short, or check the fets on the Y-sus and Z-sus to see if any are shorted

[Eaglecrest]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

I would suspect a fault on the Y-sus or Z-sus board, before the power supply.

So would you expect the voltages to be present if these boards were disconnected?

Have you tried monitoring the Vs voltage when you first turn on the tv and see if it comes up for a second.

I will test this.

check the fets on the Y-sus and Z-sus to see if any are shorted

.

How do I identify the field effect transistors and how do I test them?

[R_J]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

So would you expect the voltages to be present if these boards were disconnected?

Likely not, unless you use a load on the power supply VS. You can use two 60-100 watt incandescent lamps in series from ground to VS as a load, the lamps will light if the VS is there.

How do I identify the field effect transistors and how do I test them?

they are usually black plastic with three legs and mounted to the heatsink, thats the large black piece of aluminum. Using diode test, check that there are no shorts between the three legs, Or use google

[Andrew F. Ali]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

The 2 household light bulbs in series as a load for the VS, with the VS connector disconnected from the Y and X Bds is a good way to tell if your PSU is producing VS voltage. The bulbs MUST light up bright and with the bulbs lighting, measure the other voltages that are produced by the PSU like 15,17,5V for example.

[Eaglecrest]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

With all connectors plugged in I checked VS when the set is plugged in and when the power button is pressed, but no reading. I also checked VS_ON, connector 802, pin 6, under the same conditions and it gets around 3 VDC for a second or so then goes to 0. The LED on the logic board also comes on at the same time.

Resistance between VS and ground is 1 ohm.

[R_J]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

You have a short on the VS line, very likely shorted fets on y 0r z(x) board

[Eaglecrest]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

There are 5 transistors on the Y sustain and they read in the Mega Ohm range so I assume they are OK. On the Z Sustain (X Main) there are 5 transistors and the top and bottom ones are shorted (Q4001 and Q4007). Both are labeled:

G
RJH30E2
071B

The bottom one shows 0 ohms in all combinations the top one shows 36 ohms between bottom and middle and bottom and top legs. Middle to top legs show 0 ohms.

What would cause these two transistors to blow out?

[R_J]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

It's hard to say exactly, but there is a firmware update that needs to be done after so that could be part of the reason. If it is a BN96-13067A, Shopjimmy, https://www.shopjimmy.com/samsung-bn...ain-board.htm# has the board (Price reduced) for $31.16, might be easier and cheaper to go that route.
sometimes people that have ordered the fets ended up with so called fakes, and the shorted again. Its also a good idea to check the gate resistors, if they are open the drive ic could also be defective.

[Eaglecrest]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

Quick checking seems like there are stories of Z and Y sustain boards causing each other to blow. What is chance of replacing this board and seeing it blow again? Any other checks worth doing first?

Also why would need for firmware update cause transistors to blow?

[R_J]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

What are the chances? I have know idea, many times it is just the x board that goes, sometimes the panel can have an issue and cause either board to fail. Others may have damaged the y board when they attempted the repairs to the x using fake parts, who really knows.

I suspect the firmware adjusts the timing pulses of the control board that drives the y & x boards, other mfg like Panasonc also had this issue.

[Andrew F. Ali]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

In my many years of repairing Plasma TV I have never come across a X-Main Bd to go bad and cause the Y-Main Bd to go bad as well or vise-versa. Most tomes it is the Y-Main Bd that fails. I would recommend after testing the PSU with the light bulbs and verifying this is good I would try disconnecting the X-Main Bd and check if the Vs still does not remain steady. This will make look at the Y-Main now. With the cables disconnected do a resistance check across the Vs and GND pins on the Y-Main and X-Main Bds. to determine which board is faulty and pulling down the Vs.

[Eaglecrest]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

Thanks Andrew and RJ.

I will do the follow up testing suggested; however, if I understand the fast track (attached) document correctly it will not run with either X or Y main disconnected.

I am assuming that all VS's are the same and I can connect the light bulbs attached to any of them. There is a larger VS pin on the top of the power board (test point?) that would be easier to connect and I am assuming i could ground it to a screw or heat sink. Am I correct here?

At any rate it seems like the two transistors are bad and need to be replaced. I agree it would be easiest to replace the board and they aren't too expensive on ebay, but the transistors aren't too expensive either on ebay and I would love to give it a try to fix the board.

The less expensive transistors are pulls, any reason not to try this route?

[R_J]

Re: Samsung PN42C450 B1DXZA Doesn't Power On

DO NOT buy the cheapest parts, buy them from someone reputable, there are lots of poor quality parts out there and replacing them with "fakes", and having them just fail could lead you to suspect something else being at fault when it is really just the fake parts.
Search the other threads, for this model or the board number, someone may have posted a quality source