Samsung LE32S71B no power/no backlight fixed

[piernov]

Hi,
Multiple issues finally solved on this LE32S71B.
First one was now power at all.
Obviously bulged caps on the secondary of the SMPS. I replaced the caps for the 24V line which looked ok too.
Still no power: a CBB was dead (slighty opened on the side), I think it was a 563J 650V.
After replacing it, with a 562J though, the TV turned on, but showed the infamous 2 seconds to black issue: backlight problem.

24V line for inverted was fine, caps on the inverter were fine. The issue was found to be the top-most transformer, with open-line secondary. (the others are around 1.1kohms). I couldn't find the transformer alone, so I bought the whole inverter on Aliexpress. Well… lucky as I am, it arrived dead: all fuses blown. No big deal, I desoldered one of the transformers (all tested OK on this "new" inverter board) and replaced it on the original inverter board.

So far so good, power ok and backlight ok. I'm only worried about a small buzzing noise coming from the inverter (not from the replaced transformer apparently), so we'll see for how long it will hold out.

[budm]

Re: Samsung LE32S71B no power/no backlight fixed

Hi,
Multiple issues finally solved on this LE32S71B.
First one was now power at all.
Obviously bulged caps on the secondary of the SMPS. I replaced the caps for the 24V line which looked ok too.
Still no power: a CBB was dead (slighty opened on the side), I think it was a 563J 650V.
After replacing it, with a 562J though, the TV turned on, but showed the infamous 2 seconds to black issue: backlight problem.

24V line for inverted was fine, caps on the inverter were fine. The issue was found to be the top-most transformer, with open-line secondary. (the others are around 1.1kohms). I couldn't find the transformer alone, so I bought the whole inverter on Aliexpress. Well… lucky as I am, it arrived dead: all fuses blown. No big deal, I desoldered one of the transformers (all tested OK on this "new" inverter board) and replaced it on the original inverter board.

So far so good, power ok and backlight ok. I'm only worried about a small buzzing noise coming from the inverter (not from the replaced transformer apparently), so we'll see for how long it will hold out.

So the replacement is smaller capacitance of 0.0056uF instead of 0.056uF? That is not going to work, it also has to be Polypropylene type for high-pulse withstanding type.

[piernov]

Re: Samsung LE32S71B no power/no backlight fixed

Yep, it has 10 times less capacitance. I honestly thought it would not work but it does so all good for now… This was the only cap I had with a sufficient voltage rating as it is on the PFC-boosted line. The replacement is rated for 2kV, otherwise I had only 250V ones, maybe one 400V but probably not with the same capacitance either.
I'm going to replace it as soon as I receive the proper part, but it's probably going to take a few more weeks.

[budm]

Re: Samsung LE32S71B no power/no backlight fixed

Can I see the board and the location of that cap?
Board P/N is BN44-??????

[piernov]

Re: Samsung LE32S71B no power/no backlight fixed

I think it was BN96-03775A because I downloaded its schematics (attached) to troubleshoot the PSU.
The capacitor is CM802, between the F9222L IC pin 23 and the transformer.
I'm going to attach a picture of the board and the cap later if I don't forget.

[budm]

Re: Samsung LE32S71B no power/no backlight fixed

CM802 is the DC Blocker/Coupling cap for the output of the half-bridge that drives the primary side of that 24V power supply section. It has to be the correct capacitance, the smaller capacitance will have higher Xc so the primary winding will not be getting the correct drive Voltage, I am surprised that 24V power supply will be able to run at full load, I will not run the TV with the cap 10 times less than the original.
BN96-03775A
Pictures provided by Shopjimmy

[piernov]

Re: Samsung LE32S71B no power/no backlight fixed

Ok, still not sure what this cap do (if you have some pointers on this topology…), but can it induce high-frequency ripple on the 24V line? That could explain the buzzing noise. Maybe it would still be filtered by the transformer and the filter caps on the secondary though.

[budm]

Re: Samsung LE32S71B no power/no backlight fixed

The 24V power supply will not be bale to deliver the full power due to lower capacitance, it is part of the output circuit that drive the primary winding of the transformer, it is like putting a resistor 10 time higher resistance than the original in place. Just put in the right one and fix it correctly. They put in 0.056uF for reason. I already explain what the function of that cap is and what type and value it should be replaced with.
See PDF page 10.

[piernov]

Re: Samsung LE32S71B no power/no backlight fixed

Cap replaced with a 563J 2K5V: the inverter is much quieter.
Here are some pictures.

[budm]

Re: Samsung LE32S71B no power/no backlight fixed

Did you high pulse withstanding Metalized Polypropylene (MPP) type? Polyester film will not last in that circuit.