Samsung LE32A437 power problems

[manoftheyear]

Got this TV for free. Problem is most of the time it won't turn on. Normally the red front led blinks about five times and the tv turns on, in some cases it blinks five times and then stays in standby. And sometimes the whole TV is dead with no standby led.

I noticed when the VCC voltage of controller IC STR-A6159 is below 20V the tv works, if it goes above that it has start up problems, might blink five times then hangs with the standby led being red and no response to remote or the power button. If the voltage goes to 21V and above the whole TV is dead with no standby led.

Are these voltages normal? The datasheet of the IC claims the maximum start up voltage should be 19.2V. I have above that when tv is in standby?

I was told it has been tried with two different mainboards with results being the same, 431 shunt regulators, opto-couplers and the controller IC has been changed with no difference in behavior.

Almost all the caps have been changed to quality ones and the ones that are original have been tested fine with an ESR meter.

Don't have a camera here right now to take pics, but attached the schematics for the PSU BN44-00213a.

Any suggestions??

[retiredcaps]

Re: Samsung LE32A437 power problems

Are these voltages normal? The datasheet of the IC claims the maximum start up voltage should be 19.2V. I have above that when tv is in standby?

I'm still learning all the nuances when reading a datasheet, but I use the "typical" voltage as normal. In this case, 17.5V. When something is running close to minimum or maximum or slightly past, I worry.

At 20V, it is 14% over the "typical" voltage.

Almost all the caps have been changed to quality ones and the ones that are original have been tested fine with an ESR meter.

When I see "almost all", I wonder why not all? If the small startup cap as not been changed, that could be your problem? That could be CB802 (22uF 50V) in the schematic. The little caps rarely bloat or look distressed, but still could be bad.

Did you try tracing back the source of the 21V? See if the DC voltage across CB802 is 21V.

[manoftheyear]

Re: Samsung LE32A437 power problems

When I see "almost all", I wonder why not all? If the small startup cap as not been changed, that could be your problem? That could be CB802 (22uF 50V) in the schematic. The little caps rarely bloat or look distressed, but still could be bad.

Did you try tracing back the source of the 21V? See if the DC voltage across CB802 is 21V.

I did not have the exact sizes in well known brand and did not want to double the capacitance value. The voltage across the CB802 is the same. It's about 19.8 when lowest, and about 21.2v when highest. It seems like with time the voltage increases.

I don't think this a cap problem though.

[retiredcaps]

Re: Samsung LE32A437 power problems

I don't think this a cap problem though.

Humor me and replace it even if the replacement cap is 47uF.

[retiredcaps]

Re: Samsung LE32A437 power problems

What is the voltage on the OCP pin (pin 1)?

[manoftheyear]

Re: Samsung LE32A437 power problems

Changed the cap to 47uF Nichicon PJ. No change. The voltage on OCP pin is 7.4mV? What does this tell?

[tom66]

Re: Samsung LE32A437 power problems

Is there a zener across Vcc? It may be open...

[retiredcaps]

Re: Samsung LE32A437 power problems

Is there a zener across Vcc? It may be open...

Possibly DZB801 from schematic?

[budm]

Re: Samsung LE32A437 power problems

That Zener is 28~32V rating;
http://www.datasheetcatalog.com/data...BZX84C30.shtml
How is CB804 2.2/50V, part of the regulation feed back?

[manoftheyear]

Re: Samsung LE32A437 power problems

How is CB804 2.2/50V, part of the regulation feed back?

The cap is changed and it's OK.

What about the two zeners DZB802 and DZB803 that are part of the feedback, could they cause this if faulty?

[tom66]

Re: Samsung LE32A437 power problems

If so, that zener seems too high (design error?) Maybe try substituting in a 15-16V zener and seeing if it returns to normal operation?

[popeye]

Re: Samsung LE32A437 power problems

That Zener is 28~32V rating;
http://www.datasheetcatalog.com/data...BZX84C30.shtml
How is CB804 2.2/50V, part of the regulation feed back?

Please check 30v is what this zener should be it is that for a reason.

[popeye]

Re: Samsung LE32A437 power problems

there is a common fault with these sets with the glue used to hold things in place it goes conductive always remove all this gunk most of the time it is just 2 electralitics that cause this cb804 50v-2.2U & cb802 50v-22u.the only other thing I have had is the little blue caps.

hope this helps.

[high-tech]

Re: Samsung LE32A437 power problems

Try testing QB802, I had start problems on 1 of these Power supplys and that tranny was bad. its an NPN Spatial Planer

[budm]

Re: Samsung LE32A437 power problems

OK, I download the spec sheet of the IC STR-A6159.
The VCC MAX is 35VDC, so 30V Zener to clamp the VCC line is correct.
The startup voltage of 19.2V is that when the START-UP pin gets the main B+ applied to pin 5, that voltage is routed through the switch inside the IC to the VCC pin to charge up the starting cap, when that voltage reach 17.5V (typical), the IC will start running and the drive voltage will drive the Gate of the MOSFET, then the VCC will taken by the external Voltage supplied by the AUX winding of the transformer, the internal switch will open so the VCC will not getting the voltage from START-UP pin 5 any longer.
If the VCC drops down to 10V, then the operation will stop. If the VCC pin goes up to 31.2V the IC will also stop the operation.
This STANDBY 5V power supply is running 24/7 the minute you have the TV plugged into the outlet. so if the TV is plugged in, do you have steady 5V?

[manoftheyear]

Re: Samsung LE32A437 power problems

budm, thanks for making things more clear, seems like I was a bit lost with the STR-A6159 IC and how it works huh?

Anyway the TV has been working today like it should, the QB802 was all over that glue popeye mentioned, I removed the glue, resoldered again that transistor and now the TV has been running without any problems. The VCC voltage of the IC is now steady at 20.1V when the TV is running while earlier it would sail somewhere between 19.7v and 21.3v. The voltage increased the longer the tv was on.

If I remember right I had already resoldered earlier this same transistor while resoldering many other components too so could it really be the glue that made the TV work like it did?

Thanks to everyone who participated in this thread!

[Foppel]

Re: Samsung LE32A437 power problems

Thank you guys. I fixed mine also with recapping and resoldering QB802. I found this interesting video on youtube: http://www.youtube.com/watch?v=opwR9BMyR7Q

best regards - Foppel

[vdab.sam]

Re: Samsung LE32A437 power problems

I have same problem with my TV same type of power supply. If I switch on voltage between Vcc and GND (on STR A6159) drop (from 18.5) to 12.5V. Is anybody can help me.