Grundig / Beko LCD TV power supply problem

[Khron]

Ok, last night got a 26" Grundig LCD TV from outside the dumpster downstairs.

Chassis / inside are made by Beko. Power supply's basically two flyback converters on the same board. Silkscreened on the board it says the outputs are 24v @ 5A and 12v @ 5A. Controllers are Infineon ICE1QS01. Caps are / were 3x G-Luxon LU 680u/35v and an LZ 220u/35v on the 24v output, and 4x Daewoo RF 1000u/25v on the 12v output.

Now... The 24v output is around 3,9v and the 12v sits around 10,6v. That's with or without a load (two car bulbs in series on the 24v and a 12v fan on the 12v output).

The voltage on the filter caps for the ICE's are 14.895v for the 12v one and "bouncing around" between 12 and 15v on the 24v controller.

I've replaced, albeit temporarily, the 3 big Luxons with two Panasonic FK 1000u/35v, and the small one with a pair of Rubycon YXF 47u/35v. Same Ruby's on the ICE Vcc's. Also replaced the 150u/400v G-Luxon primary cap with a 150u/400v Elna. Even went as far as replacing the two rectifiers on the 24v output with STPS20H100's (originals are Fairchild "YM3010N", on which i've yet to find a datasheet).

Any clue what i should be looking for? My hunch says "feedback", but i've yet to trace the circuit...

[Khron]

Re: Grundig / Beko LCD TV power supply problem

With slight embarrassment, i offer a small update:

Except the two 24v, three GND and one 12v pins on the output connector, the last pin was as-of-yet unknown. Upon bridging that to the 12v output with a 15k resistor, the psu came alive! Load bulbs lit up, 23.67v, and the fan i had on the 12v line spins a bit faster than before (at 10.6v).

Guess i'll now have to open a thread in the TV section, since the main board doesn't seem to turn on the psu...

[Longbow]

Re: Grundig / Beko LCD TV power supply problem

Maybe not. The supply voltages still aren't correct.

[Khron]

Re: Grundig / Beko LCD TV power supply problem

Well, i tried out the tv last night, it turned on, got into the OSD, but... i found out why i found it where i did: smashed screen, 75% unusable

Ah well... Parts to scavenge, i suppose

[senndogg]

Re: Grundig / Beko LCD TV power supply problem

hi khron.
i have a similar psu in my hands.
can you help me with sth?
the 12V is ok, but the 24 not. the main transistor is gone (literally exploded) with the ic circuit and some other components (i found an opened diode and a resistor). i read in your post that the main ic is an Infineon ICE1QS01. but i wanted to know, if possible, what is the main transistor? from my experience it might be a 11N50C.
thank you.

[ben7]

Re: Grundig / Beko LCD TV power supply problem

hi khron.
i have a similar psu in my hands.
can you help me with sth?
the 12V is ok, but the 24 not. the main transistor is gone (literally exploded) with the ic circuit and some other components (i found an opened diode and a resistor). i read in your post that the main ic is an Infineon ICE1QS01. but i wanted to know, if possible, what is the main transistor? from my experience it might be a 11N50C.
thank you.

You should check the secondary rectifiers. Be sure to check all the parts on the mains side, because when a controller IC blows it can take a bunch of parts with it.

Also check R964 (0.22 ohm) and R909 (68K ohm), and D910.

[selldoor]

Re: Grundig / Beko LCD TV power supply problem

@senndog This may interest you - I dont think model numbers have been mentioned anywhere in this thread but this is a repair kit for a 26" grundig bush beko etc
http://www.chsinteractive.co.uk/elec...cd26tv07hd.htm
Includes the item you mention

[senndogg]

Re: Grundig / Beko LCD TV power supply problem

first of all, thank you for the interest shown.
Ben7: i have checked them, everything ok.
selldoor: yes, 20N60C3 (as shown as in the picture) could do the job, although 20Ampere mosfet might be a little exagerated for a 140W LCD tv.

well, i think i'll discuss the problem from the start in this thread.
TV had been serviced elsewhere but previous technician did not succeed.

i found:
IC900 totally destroyed. ICE1QS01.
T900 power mosfet shorted.
R964 0,1Ohms ((-) to the Source pin of transistor) opened.
D904 SMD (from oscillator to the gate pin of transistor) opened.
R963 150 Ohms resistor (from the source pin of transistor to "Overvoltage Fault Comparator" pin of controller) opened-burned.

other components including fuses are ok.

with a little observation:
the 12Volts section works properly. interesting, it has the same controller, the ICE1QS01 with a-maybe-less-powerful mosfet and much less other components. surprisingly, the "Overvoltage Fault Comparator" pin of controller is grounded and not connected with a 150ohms resisitor to the Source pin. the source pin too is connected directly to (-) and not with a >1ohm resistor as in the 24v section.
i checked the ICE1QS01 datasheet and "Overvoltage Fault Comparator" was an option which you could delete by grounding that pin.
my electronic knowledge does not go that far. yesterday, i read all day long about SMPS controllers and learned many things, but i still dont know why in this case the manufacturer enriched with components the 24V section and reduced them in the less powered 12V rail. it has got to be with power, for sure, but how it would behave if i followed the 12VDC schematic too to the 24V?
i mean, if i ground the OFC pin, would the PSU be reliable? maybe i have gone into deeps but i really want to learn how to build a simple and cheap SMPS myself. maybe the ICE1QS01 isnt the most reasonable controller, but i find it simple. do you agree? or sth better can be done if i remove all components and follow another schematic of an another good controller?

thank you for your time! your feedback means a lot to me!

[senndogg]

Re: Grundig / Beko LCD TV power supply problem

now i see, that the original .22 ohms resistor has been swapped from the previous technician for a 0.1 one. does this affect anything?

[ben7]

Re: Grundig / Beko LCD TV power supply problem

now i see, that the original .22 ohms resistor has been swapped from the previous technician for a 0.1 one. does this affect anything?

Actually yes, the resistor is probably for the current sense, so maybe this is why it blew up.

[selldoor]

Re: Grundig / Beko LCD TV power supply problem

My electronics knowledge is far less than yours - you probably need input from more knowledgeable members.
Just one thing in pic5 R903 (which I dont think has been mentioned) looks badly soldered on the right hand leg - (may just be shadow on the pic)

[senndogg]

Re: Grundig / Beko LCD TV power supply problem

yes ben7. in the ICE1QS01 schematic, the corresponding pin had to do with the current sense. but i cant see how a lower value resistor can raise the voltage in an application like that. although now im blaming directly R964 for the failure.
selldoor you are right too, it is badly soldered but not so bad, it works. maybe because of the low current which goes through it.
anyway i think i should start collecting the components for the repair.

[budm]

Re: Grundig / Beko LCD TV power supply problem

With smaller value (lower Ohm) Source pin current sensing resistor, the MOSFET will have to conduct more current through that resistor to develop enough voltage to trigger the shut down, so basically when 0.1 Ohm is used the over current trip point (more than twice as high as it was original design) will be higher, in this case, it is higher than the MOSFET can handle so the shutdown function become useless. They pick the 0.22 Ohm for reason. It is like putting in 20A fuse in place of 10A fuse. The resistor is also precision, I think it may be 1% resistor. This is one part of the circuits that must use the same value component.
If you look at page 17 of the spec sheet, the OFC is 1V for the trip point.
So the MOSFET will have 10A flowing through it before the circuit will shutdown when 0.1 Ohm is use for sensing current.

[ben7]

Re: Grundig / Beko LCD TV power supply problem

With smaller value (lower Ohm) Source pin current sensing resistor, the MOSFET will have to conduct more current through that resistor to develop enough voltage to trigger the shut down, so basically when 0.1 Ohm is used the over current trip point (more than twice as high as it was original design) will be higher, in this case, it is higher than the MOSFET can handle so the shutdown function become useless. They pick the 0.22 Ohm for reason. It is like putting in 20A fuse in place of 10A fuse. The resistor is also precision, I think it may be 1% resistor. This is one part of the circuits that must use the same value component.
If you look at page 17 of the spec sheet, the OFC is 1V for the trip point.
So the MOSFET will have 10A flowing through it before the circuit will shutdown when 0.1 Ohm is use for sensing current.

Yeah, the repair shop did a bad job on trying to repair that.

[budm]

Re: Grundig / Beko LCD TV power supply problem

Not understanding how it works can cause a lot more damages than what it was started with.

[senndogg]

Re: Grundig / Beko LCD TV power supply problem

i imagine this scenario, the mosfet had failed (bad quality, overstress, overheat) and shorted. at this point the 0,22 resistor has blown. maybe it has burned in such a way that has became unreadable. so, the previous technician has guessed by putting a 0,1 one. this has deteriorated things by applying high current to the OFC pin, as witnessed from the blown R963-150 Ohms and the destroyed IC.
now i have a 12V similar IC section, which i could have followed, but this does not help me that much, because the constructors have gone the easy way. pin OFC is shorted to ground.
i want to order only the:
IC ICE1QS01
the 20N60C3.
it seems to me much easier shorting the OFC and bypassing the R964. but i also hate explosions :P

[senndogg]

Re: Grundig / Beko LCD TV power supply problem

hi (again)
thanks to khron i am collecting the components, and the power transistor of the 24V section is a Toshiba 2SK3562. specs: 600V drain-source, and 6A/24A continous/pulsed current.
question, can i swap this with a IRFS630, specs: 200V drain-sourc, 6.5A/36A cntns/pulsed current?
i have a bunch of IRFS630 taken from some old CRT displays.

[Khron]

Re: Grundig / Beko LCD TV power supply problem

Sounds ok, but you might wanna check that the gate charge (Qg) isn't (much) greater than the original

Going by the pics, looks quite identical to the one i have, and the model number's identical. Mine's got one extra common-mode choke, an X-cap and two Y-caps near the fuse.

"Just to make sure", you might wanna replace the small 47/68u caps near the two PWM IC's.

[senndogg]

Re: Grundig / Beko LCD TV power supply problem

it is: typical 28 in the toshiba, and typ 22 and max 29 in the IRF.

[Khron]

Re: Grundig / Beko LCD TV power supply problem

Sounds like the perfect "tool" for the job, then. Good luck!