BN44-00264A buzzing

[petehall347]

i have this samsung tv here with a buzzing BN44-00264A power / inverter board
it came to me dead . main fuse blown fs801s 6.3a
also a shorted fet qp801s qp802s ... not sure which was shorted but changed both anyway .
also changed up801s for good measure ..no other apparent faulty components in the area .
i haven't pulled any caps to test yet .i thought i would ask here first if anyone had any clues as to what is causing the buzzing .
noise seems to be coming from lp801s .
an important thing i have noticed when i plugged it into my mains power meter / conditioner i see it looking like the tv is pulling 3kv .. i dont think it can be as it would be bloody hot .
if i warm up that area with hair drier the noise almost disappears and the power meter drops a lot .if i freeze cp801s and cp802s the noise starts up again ...
also if i turn the back-lights down the noise lowers quite a bit . brightness affect it slightly too .
tv is le40b650t2w . not that it should matter right now .

[petehall347]

Re: BN44-00264A buzzing

well this has got stranger ... been working on it all day and getting no further ahead . mainly reflowing joints and subbing caps .
i decided to try it powered up with my variac as our line voltage is low . set variac to 240v powered the tv on .it seemed to fire up ok until the back-lights came on .then at that point my variac was sounding like a kettle boiling . at this point i switched the variac off quickly .
anyone have any ideas please ? this thing is driving me nuts

[CapLeaker]

Re: BN44-00264A buzzing

We need TV model number and high resolution pictures of the PSU and voltage measurements. Also parts locations like QP801 is as same as useful as your front left tire hasn't got any air, without the actual part number.
Check the DC voltages on the main filter cap (be careful 390 VDC when on!!!)with the TV off and on, check all the voltage rails on the secondary. Maybe you can explain the situation a bit better. If your TV needs 3KV , you wouldn't be sitting in your chair anymore and everything plugged in to your outlets would be toast. I doubt you are getting 3KV out of your wall socket.

[petehall347]

Re: BN44-00264A buzzing

first off i am not allowed to edit the posts ..its KW of course .
tv model number is in first post .
all voltages in spec .
part numbers in schematics
schematics here ....

[CapLeaker]

Re: BN44-00264A buzzing

Even then you would be drawing like 12.5A on 240V at a perfect PF of 1 which you wont have. No wonder your variac is whistling like a tea cattle. Actually the fuse should blow and maybe your main breaker too. The cord should be toasty warm too. The PSU shoudn't can't be drawing 3kw on 240V. So something is wrong with that measurement in any case.

[petehall347]

Re: BN44-00264A buzzing

Even then you would be drawing like 10A on 240V. No wonder your variac is whistling like a tea cattle. Actually the fuse should blow and maybe your main breaker too. The cord should be toasty warm too. The PSU shoudn't can't be drawing 3kw on 240V. So something is wrong with that measurement in any case.

yes i did say i dont believe it its more like switching noise feeding back into the mains supply .
think my next move will be to swap the main bridge rectifier bd801s for a new one .i think i have one in stock .
i dont have any 82uf caps for cp815 cp816 in stock so if needed i will order some in .
i did try a single 2.2uf in place of cp801s cp802s with no change in the buzzing noise or strange looking power draw .
the mains power meter is led.s in a 6 way protected power socket . i think its seeing the spiking rather than the actual current . as nothing hot except the thermistors nt812s nt811s . i expect those to warm up in normal use .

[CapLeaker]

Re: BN44-00264A buzzing

if anything is shorted in the primary section, the PSU won't turn on, so you can't have any voltages on the secondary, besides maybe 5V STBY.

[CapLeaker]

Re: BN44-00264A buzzing

And the other way... if there is a short on any of the secondary voltages on the PSU, the PSU could consume a lot of power until something blows, but then you don't have ALL the secondary voltages. Upload a high resolution picture of your PSU, so we can see whats going on.

[petehall347]

Re: BN44-00264A buzzing

sorry i cant get a pic unless i can borrow a flash camera ... there are plenty of pics of the same board on the net though as you are maybe aware .
as i said in the first post the set all seems to be working fine .its just this odd power supply problem it is having .

[CapLeaker]

Re: BN44-00264A buzzing

Well, that's why it is important to have a picture of YOUR PSU, and not someone else's. Maybe we can see / find something.

[petehall347]

Re: BN44-00264A buzzing

something i forgot to mention is if i unplug the plug that goes to the main board the back-lights come up to full brightness and the buzzing gets much worse .
i have looked over the board many times now with a magnifying glass and have tested values of suspect components .
one thing i found is the 82uf caps that are in parallel only measure 130uf .but are still on the board .

[dick_barton]

Re: BN44-00264A buzzing

Did you replace QP801S & QP802S with identical parts. Is there damage to CP810 or CP803.
Have you checked resistors RP805, RP821, RP822, RP806 and to diodes DP801 & DP802 in the event that they were damaged when the Mosfet blew.

[CapLeaker]

Re: BN44-00264A buzzing

can you identify where the buzzing is coming from? Did you check all the diodes and fets for shorts? Resistors for changed value (usually they go up).
dick was a bit faster. lol

[petehall347]

Re: BN44-00264A buzzing

Did you replace QP801S & QP802S with identical parts. Is there damage to CP810 or CP803.
Have you checked resistors RP805, RP821, RP822, RP806 and to diodes DP801 & DP802 in the event that they were damaged when the Mosfet blew.

i replaced with same part numbers yes from radio spares STF11NM60ND.. the controller chip up801s / fan7530 i bought from ebay so i am not sure if that chip is dubious or not . i know the fets will be genuine .
i may have checked those resistors but will check again shortly to be sure .

[petehall347]

Re: BN44-00264A buzzing

Did you replace QP801S & QP802S with identical parts. Is there damage to CP810 or CP803.
Have you checked resistors RP805, RP821, RP822, RP806 and to diodes DP801 & DP802 in the event that they were damaged when the Mosfet blew.

CP810 or CP803 i will have to remove as 1.8uf in circuit ..should be 220pf
rp806 is low in circuit 4.6k should be 10k its too small for me to remove .
DP801 & DP802 appear bad in circuit so will have to new ones before i remove them for further testing both .092 diode test and 98.3 ohms resistance in circuit both ways so arent dead short .
doh i just seen the resistors connected to them ..

[petehall347]

Re: BN44-00264A buzzing

on another note why is rp820 in this circuit ? it is just it appears to manage as good as short out a few components . dp807 for one ...

[petehall347]

Re: BN44-00264A buzzing

can you identify where the buzzing is coming from? Did you check all the diodes and fets for shorts? Resistors for changed value (usually they go up).
dick was a bit faster. lol

buzzing is around lp801s .. hard to actually pinpoint it with my hearing .

[CapLeaker]

Re: BN44-00264A buzzing

see rp820 like a fusible resistor. Resistors don't go lower in value when they are damaged, they go up in value or open. In my life I've seen only one resistor shorted. So if you find a resistor out of spec replace it. If the schematic sez different what is actually on board, it may be a undocumented change, or rev. change. Wouldn't be the first time. But you see a 10 ohm resistor measuring 15, 150, 1.5Kohm, 1.5Mohm or open, yes it is out of spec and needs to be replaced. Look what is printed on the resistor. Always check and mark suspects in circuit first, then take suspects out of circuit and measure again.
Did you check DP805?
What happens if you disconnect everything and just power up the PSU by itself?

[ReeceyBurger123]

Re: BN44-00264A buzzing

Change the pfc Ic whoch is a fan7529 I believe should sort the issue Ive done a few same issue. These sets power the inverter straight off the Pfc bus and the pfc Ic fails to turn on the pfc fets as much meaning the pfc caps voltage drops and becomes unstable causing noise on the inverter trannys.

[petehall347]

Re: BN44-00264A buzzing

see rp820 like a fusible resistor. Resistors don't go lower in value when they are damaged, they go up in value or open. In my life I've seen only one resistor shorted. So if you find a resistor out of spec replace it. If the schematic sez different what is actually on board, it may be a undocumented change, or rev. change. Wouldn't be the first time. But you see a 10 ohm resistor measuring 15, 150, 1.5Kohm, 1.5Mohm or open, yes it is out of spec and needs to be replaced. Look what is printed on the resistor. Always check and mark suspects in circuit first, then take suspects out of circuit and measure again.
Did you check DP805?
What happens if you disconnect everything and just power up the PSU by itself?

rp820 is fine .. i just cant figure out why it is where it is ..0.08 ohms 5w .almost a short circuit . look at the diagram as to what it is in parallel with ..i am trying to understand the logic in this design .