BN44-00274a dead

[ajshoe]

Have a Samsung PN50B550T2FXZA plasma set thats dead - no led lights, no TCON led lights, no standby voltage. Have re-soldered several bad spots on PSB board. looking at schematic - not getting 120 VAC at BD801S (HS802). followed circuit and don't see how 120 AC gets to this rectifier circuit. Replaced NT801S power thermistor SCK206. coil LE802 seems good. anyone have an insight into this ? checked all diodes and caps. thanks

[ajshoe]

pics attached

[lotas]

AC voltage is supplied to the diode bridge through a relay.

[R_J]

Have you checked FB801S? You will not have A/C at BD801S until RL802S is energized, and this will not happen until you have the standby circuit working. If the fuse is ok, check the solder connections on ICB801S

[dskall]

At what point are you losing voltage?

[ajshoe]

will check ICB801S tomorrow. all 4 fuses are good. I see now - relay has to be enegerized to get 120 VAC across bridge. plug power in - absolutely dead. no +5 VDC standby, no clicks.

[ajshoe]

Checked all solder connections. No 5 vdc on standby.

[dskall]

What is voltage on pin 4 CB8015?

[ajshoe]

before I get voltage on CB8015 - is this done with all connections to PSB ?
can the PSB be tested for standby voltage on bench (nothing connected but 120 AC) ?
thanks

[R_J]

Originally Posted by ajshoe

before I get voltage on CB8015 - is this done with all connections to PSB ?
can the PSB be tested for standby voltage on bench (nothing connected but 120 AC) ?
thanks

Yes it can be tested on the bench, You should get stby 5.2v like this.
To power on the rest of the supply, this power supply requires you install about a 1k resistor between stby 5.2v and PS_ON to turn on the other voltages.
Check the DC voltage ACROSS CB816

[ajshoe]

dskall - you want voltage on pin 4 of HS802 (ICB801S) ? don't see a cb8015 anywhere on board or schematic

[dskall]

Looks to be controller for standby transformer. Mounted on heat sink.

[ajshoe]

voltage across CB816 is 135 VDC.
voltage on pin 4 of ICB801S (HS802) is 0 VDC -when first plugged in goes to 0.35 VDC and then down to 0 VDC
saw bad solder on ICB801S (HS802) joints - touched them up. still 0 VDC on Standby and above checkpoints

[R_J]

That voltage is fine, it is the voltage that supplies icb801s and should also be on pins 1 & 2 of TB801s transformer. Keep the negative lead on CB816 (-) to check voltages in the primary circuit.
With a/c disconnected, check if RB807 is ok, it should read 0.4Ω (short) If it is open, the ic is defective.

Also, was NT801S bad? ie burnt? if it was then the RL801S relay contacts are bad and the relay should be replaced or at a minimum the contacts may be cleaned.

[dskall]

Since I am trying to learn here. What supplies start up voltage to looks like CB8015 or CB801s. To start controller it needs >15v if I read datasheet right.

[ajshoe]

NT801S ohmed out at 20 ohms - it was not bad or burnt. I replaced it with new one anyway. don't believe relay or relay contacts are bad.
I'll check RB807 tomorrow. thinking the ICis bad - had 3 cold solder joints on it. The solder quality on this board is bad. found dozens of cold solder joints.

[R_J]

Many times if the ic had poor solder connections, the ic becomes defective, sometimes they survive but not always. If RB807 is ok, the ic is likely still bad. I would also replace CB803 and CB815, they can dry out over time but still look and sometimes check ok. CX823 (standby output cap) could be weak as well on this age of a tv.

[R_J]

Originally Posted by dskall

Since I am trying to learn here. What supplies start up voltage to looks like CB8015 or CB801s. To start controller it needs >15v if I read datasheet right.

What schematic are you using? I don't see CB8015 or CB801s
In the datasheet you will see it has an internal startup cel that takes the voltage from the drain (pin1) and internally supplies startup vcc, and charges cb803 to about 18v. once it starts, DB802 keeps CB803 charged.

[dskall]

The above pdf from LOTAS and o/p picture of board. How is transformer getting voltage. Looks like it has to go through relay. Or does it get from NT801S?

[R_J]

Originally Posted by dskall

The above pdf from LOTAS and o/p picture of board. How is transformer getting voltage. Looks like it has to go through relay. Or does it get from NT801S?

A/C is supplied from live, to one end of NT801s, the other end of the ntc connects to DB810~DB812, This Dc connects to FC803 (fuse) and charges CB816 to approx. 150vdc. This supplies the transformer Tb801s
Once the power supply is turned ON, RL801s shorts out the ntc to supply a bit more current for the standby circuit when it is operating at full capacity and supplying VCC & VCC_S to the other circuits in this supply.