Samsung LE40B620R3W dead!

[lesnajair]

Hi All

As above, no red led nothing! Seems quite a common problem with bad mosfets and snubbing capacitors, but with the exception of FS801S keeping on blowing (rather than FS802S in almost all other posts!).

Having researched the power/inverter board BN44-00264A, and reading through posts on badcaps, I have replaced the following and their location;

CI890 220pF 1KV
CI891 220pF 1KV
Q1820/Q1821 7N50U x 2
FS801S fuse

After replacing these and applying power the same FS801S fuse blew again!
I removed the remnants of the fuse and had a look over the board and wondered whether one of the transformers could be at fault? The transformer at TI804S is highlighted and I took measurements across the pins in the red box. Pin 1 is at the bottom with pin 4 at the top, meter set to auto range;

1-2, 1-3, 2-4, 3-4= all 1.51 kOhm
1-4 = 3.03 kOhm
2-3 = Short (000.1 kOhm)

Comparing wit the other transformers, there are only measurements showing between pins 1-2, with no other readings on any other pair of pinsI hope this makes sense?

After the FS801S blew I took readings across the 2 x 220pF capacitors which were CI890 = 6.9 MOhm CI891 = 8.9 MOhm

I hope there is enough information to go on! I've taken the pictures in sections to try to show in enough detail.

Thanks in advance.

[budm]

Re: Samsung LE40B620R3W dead!

Per your picture;https://www.badcaps.net/forum/attach...1&d=1443485303
Pin 2 and 3 are tied together, you can see the trace between the two pins.
Were Q1820, 1821 tested as shorted?

[Hamie]

Re: Samsung LE40B620R3W dead!

UM8015

Have you tested it?

[ReeceyBurger123]

Re: Samsung LE40B620R3W dead!

Also check the diodes near the fets they tend to fail causing no Stby.

[lesnajair]

Re: Samsung LE40B620R3W dead!

Hi. Thanks for your reply. My skill level is not professional but keen hobbyist and pretty competent with repairs but fault finding and diagnostics are still an art I'm learning!!

The mosfets didn't test as shorted but appeared to show faulty. I removed them completely. Meter set to diode mode, neg to source pos to drain no reading. Momentarily touch pos to gate then back to drain still no reading. Both tested the same.
Regarding the transformer, I was aware that pins 2 and 3 were connected but wanted to give a full description! Is there any significance to pins 1 2 3 and 4 all being 'connected' in some way as the other 3 transformers only give meter readings between pins 1 and 2?
Having removed the pair of 220pF capacitors and testing them they appear good as I tested them in my esr device. Just to be safe I replaced the capacitors and mosfets anyway.

When I acquired the TV the only evidence I had about the fault was that the owner said it failed whilst watching TV and on inspection the fuse at FS801S had blown.

Thanks again for your help. Les

[lesnajair]

Re: Samsung LE40B620R3W dead!

Thanks Reecy. Diodes are all ok. I'm just stumped by the FS801S fuse that keeps blowing. Other posts regarding this board all have the fets, capacitors (that are linked) and the diodes that tend to be the problem but mine are ok. The FS802S fuse is still ok too!

[Hamie]

Re: Samsung LE40B620R3W dead!

Um8015.

Needs to be tested.

What's the part number?

[lesnajair]

Re: Samsung LE40B620R3W dead!

Hi Hamie
Thanks for your advice. I haven't checked that one but will do later on after work. Will post results. Thanks

[vinceroger69]

Re: Samsung LE40B620R3W dead!

this is what in shopjimmy repair kit so maybe test these parts on your board
Fuses 2 FS801S, FM802S
Transistors 4 QI820, 821, QP801S, 802S
Integrated Circuits 4 UB802, UI801, UM801S, UP801S
Resistors 1 RM805
Capacitors 1 CI892

[saddle_au]

Re: Samsung LE40B620R3W dead!

I've had this board in where the main fuse goes, but the next one is ok. Be careful, you are working in around the primary mains area in the circuit.

First, remove the bridge BD801S. This separates the mains filter section from the switcher section. Measure resistance between active and neutral. If it's low, remove parts in the filter section until it isn't. If it's not low....

Leave the bridge out, put another fuse in and plug it in.

a) If it blows, look at the MOV VX801S first. I'd not normally suspect CY801S, CY802S, CX801S or CX802S (they're supposed to be self healing for punch throughs) but they can fail rarely. The trick here is to work your way from the mains input back to the removed bridge. You only have MOVs, Caps and some common mode transformers here. All the caps and MOVs are big, chunky and easily removed temporarily. They are not essential for the test and if they're out and the fuse still blows, you're down to the common mode transformers.

b) If it doesn't blow, check the diode bridge carefully. Use a meter in resistance and look for shorts between any of the pins (had several where that has happened, especially when the owner wraps the fuse in alufoil and tries to cure it using vaporization). Then check it using diode test. If you're not sure about the bridge, bend the +POS leg out horizontally and put it back in so the other three are there, but no positive volts go to the circuit. If another fuse blows now, the bridge is the culprit. If not put it back in and check the fets QP801S and QP802S and the main storage caps CP815 and CP816.


======= Stay alive 101 ============

BE CAREFUL in all of these tests, this is mains voltage and it's not forgiving. In a former life I worked on voltages above 6kv and that's another level of grief, but it makes no difference whether an accident is massively fatal or just fatal (I always find it amusing when people quantify it that way).

-Do not do this on your own (or with a couple of beers first).
-Do not move the set while it's energized.
-Never grab things from behind the screen if you are infront. Always see where your fingers are.
-If its face down, do not lean over the set any more than half way for any reason while it's live. Anytime you have to lean over the set is a chance of contacting it in the wrong way.
-Do not lean over the set more than half way when the cable is plugged in energized or not. That's what accidents are.
-Have a switched socket (not in the wall, but inline with your lead) where you can easily plug it in, stand back and turn it on.
-Always turn the power of first away from the set, do not rely on pulling out the cord to turn it off.
-When you remove the plug, then let any caps discharge, then work on the next bit.

Sometimes I make sure the only mains socket available for this is shared with my radio, it the radio is off, then I'm way more cautious dealing with the set. If it's on, then it's a matter of discharge time.

===================

[lesnajair]

Re: Samsung LE40B620R3W dead!

Great advice saddle au. I'm always very careful! Will be sure to work through your reply later.

Thanks for all help everyone, it's really appreciated!

[lesnajair]

Re: Samsung LE40B620R3W dead!

****SOLVED****

Hamie - Thank you Sir! Tested the component UM801S (KA431SLMF2) and it was fine.

All of the items in the repair kit seemed to test ok apart from the QP801S FET (11N60). It had shorted. The corresponding 220pF 1Kv capacitor was also good, so my only failed components turned out to be that FET and fuse FS801S. Replaced and tv working fine!

Thanks to all of you geniuses who prevented me from going down the wrong route and stick to the basics!

Regards

[budm]

Re: Samsung LE40B620R3W dead!

Thanks for the follow-up, that QP801S is for the PFC Voltage booster circuit, luckily the PFC IC controller was not damaged otherwise it would have taken out the new MOSFET.