Emerson LF501EM6F has 21v and 13v but no vled+27v

[R_J]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

IC602 voltage is likely ok as that part of the circuit is working
On Q1607 the base voltage needs to be lower than the emitter, as it is now R1639 is keeping it higher, when ic1602 turns ON, the 17.8 will be near 0 volts and the R1638 will be dropping the voltage to the base of Q1607.

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

IC602 voltage is likely ok as that part of the circuit is working
On Q1607 the base voltage needs to be lower than the emitter, as it is now R1639 is keeping it higher, when ic1602 turns ON, the 17.8 will be near 0 volts and the R1638 will be dropping the voltage to the base of Q1607.

thank you for your help! i am starting to see how this works!
according to my measurements the base voltage IS lower than the emitter ...though only by about 0.1v. is this just not enough lower to do anything? how much lower does it need to be? is this discernable from the datasheet somehow?

i see how shorting pin 3 and 4 of IC1602 would drain the base of (most?) voltage and then it would allow Q1607 to pass the current its getting from D620A onto the capacitor C1618....i guess i should try that then?
is there any risk in it?

maybe i should just try to figure out why the LED isnt lighting up while i ponder that...

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

ok i traced out everything related to the LED side of IC1602 and labeled with voltages i found.
it seems like the ALL+3.3V rail might actually be used on this board?

it seems that when we jump P-ON_H2 with P-ON+21V that the 21v should go around turning on these transistors and allowing current to flow to the LED of the optocoupler, via the ALL+3.3V line but that is not happening. there is no voltage on that line.
on the P-ON_H2 line, i found two 10k resistors that are only reading 8.5k ... thats 15% tolerance...
maybe still ok? smds cant be mislabeled can they?

but what i notice about those 10k (8.5k) resistors though, is that they have 21v on one side and then only 0.7v on the other. how could they drop that much voltage?
I see that R701 and R702 make a voltage divider but with 8.5k as R1 and 47k as R2, according to the voltage divider calculator at https://ohmslawcalculator.com/voltag...der-calculator
the voltage i see at the point between the two resistors should be 17.784v
but im getting only 0.7v

strangely when i try to measure the resistance of that 47k resistor:
with black probe on the ground side of R702 it quickly climbs from 5k to 10k and then slowly climbs from 10k to 11.14k.
with red probe on ground side it quickly climbs from 5k to 10k and then slowly climbs from 10k to 12.15k.
substituting 12.15k into the equation instead of 47k gives me 12.35v which i am definitely not getting.

R693 and R669 form a similar voltage divider with the same marked resistors.
i get similar results trying to probe them. with the resistance topping out around 13k this time.

so what is happening here? it seems the 0.7v is not enough to turn on these transistors which are all marked ALG with a 1D on the side.
the voltage is there on P-ON_H2 but its getting lost somewhere...seemingly right after these 10k resistors.

the same thing is happening right after the diode D656A, there is a 10k resistor (R652) followed by a 47k resistor (R662) to ground. the same voltage divider in 3 places and each time, its taking the voltage thats there and turning it into 0.7v

ive tried pretty hard to turn up a datasheet on this part...but i cant seem to find the correct one.
if anyone has any tips on using these markings to search for datasheets id be all ears.
i tried on https://smd.yooneed.one/ to search for both ALG and 1D but cant seem to find a SOT-23 that matches what i think should be some PNP or NPN transistor.
i even searched for just LG cause maybe the A is some kind of brand mark?
i cant seem to come up with a datasheet that satisfies me though.
thanks again for all your help!!!!

its taken years but im finally just barely starting to understand SMPS

[vinceroger69]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

alg part maybe a KTC3875 npn

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

alg part maybe a KTC3875 npn

yea i had looked at that datasheet and couldnt see anything mentioning ALG so ...how can i know at all?

[lotas]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

yea i had looked at that datasheet and couldnt see anything mentioning alg so ...how can i know at all?

alg - KTC3875-GR

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

alg - KTC3875-GR

https://cdn.badcaps-static.com/pdfs/...8fd73c05da.pdf was the one i came up with.... how did u find the right one? thanks!

weird... when i put a space after the search i get different results from google. now i saw this page
https://www.alldatasheet.com/view.js...MaAvwSEALw_wcB
which i could have found...but would i have had to look at each datasheet in the list until i found the one that mentioned ALG or am i just supposed to know what brand it was from the marking on the package?

[lotas]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

And your datasheet also says ALG.

[R_J]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

thank you for your help! i am starting to see how this works!
according to my measurements the base voltage IS lower than the emitter ...though only by about 0.1v. is this just not enough lower to do anything? how much lower does it need to be? is this discernable from the datasheet somehow?

i see how shorting pin 3 and 4 of IC1602 would drain the base of (most?) voltage and then it would allow Q1607 to pass the current its getting from D620A onto the capacitor C1618....i guess i should try that then?
is there any risk in it?

maybe i should just try to figure out why the LED isnt lighting up while i ponder that...

The base needs to be at least .6v lower than the emitter.
Monitor the vcc+15v line and short pins 3&4 of ic1602, this will check that complete circuit, if it works then you can figure out why the opto is not turning on, like I said it may require the main board, which may supply the voltage and control of ic1602

One thing to check is C3214 on the main board, it is near the edge of the board close to L3201 coil

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

And your datasheet also says ALG.

man... i need to open my eyes bigger! thank you!

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

The base needs to be at least .6v lower than the emitter.
Monitor the vcc+15v line and short pins 3&4 of ic1602, this will check that complete circuit, if it works then you can figure out why the opto is not turning on, like I said it may require the main board, which may supply the voltage and control of ic1602

One thing to check is C3214 on the main board, it is near the edge of the board close to L3201 coil

requires the mainboard? that would be bad design. the previous PSU i worked on that was similar to this didnt require the MB to be able to be tricked into turning on the backlights... could this one be so different that it does?

i will try this test and report back results. also with the correct datasheet, i will update my drawings to include the transistors E, B, C's

i do have the mainboard. i have the whole tv. but according to what ive been told here in the past it wouldnt be a good idea to have the mainboard plugged in while you are doing "jumpering" of rails on the PSU to trick it into turning on...im sure this isnt strictly true...but i dont feel confident enough to know when it is and when it isnt!

[R_J]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

Why would that be a bad design? They just placed the 3.3v regulator on the main board instead of the power supply. What was the previous power supply? most of them are different, even between similar models.

It is always best to disconnect the main from the power supply when jumping inputs to supply voltages.

All you need to do is connect the main board and see if you get any voltage on pins 1&2 of ic1602.

It is obvious the ALL3.3v circuit is not on the power supply, and you don't have any voltage on ic1602 pins 1 or 2, so where does that voltage come from? likely the main board.

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

Why would that be a bad design? They just placed the 3.3v regulator on the main board instead of the power supply. What was the previous power supply? most of them are different, even between similar models.

It is always best to disconnect the main from the power supply when jumping inputs to supply voltages.

All you need to do is connect the main board and see if you get any voltage on pins 1&2 of ic1602.

It is obvious the ALL3.3v circuit is not on the power supply, and you don't have any voltage on ic1602 pins 1 or 2, so where does that voltage come from? likely the main board.

hmm it didnt seem obvious to me because it looks like there is a path from the diode D656A to the all+3.3v line but for some reason the voltage disappears after the 10k resistor... i will try all you said.

i say its a bad design because any time you have the mainboard being required to test the power supply then you have 2 boards that can either one be bad and hard to tell which one it is!
every power supply i have worked with can be tricked to turning on all their rails with no mainboard. this would be the first that didnt...

[R_J]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

hmm it didnt seem obvious to me because it looks like there is a path from the diode D656A to the all+3.3v line but for some reason the voltage disappears after the 10k resistor... i will try all you said.

i say its a bad design because any time you have the mainboard being required to test the power supply then you have 2 boards that can either one be bad and hard to tell which one it is!
every power supply i have worked with can be tricked to turning on all their rails with no mainboard. this would be the first that didnt...

So what voltage do you have on D656a? That diode seems to supply voltage for Q654 base, Q654 and Q650 supplies the RESET to the main baord

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

So what voltage do you have on D656a? That diode seems to supply voltage for Q654 base, Q654 and Q650 supplies the RESET to the main baord

what you say agrees with my drawing. did you see the picture i posted? i put down there that i saw 12.3v at the cathode of D656A but it drops to 8.6v after another diode, and then drops to 0.7v after a 10k resistor...there is another 47 k resistor after the 10k that goes to ground, making a voltage divider, but the math doesnt make sense to me. should be more than 0.7 ... but probably its just something else going on that i dont understand yet!

https://www.badcaps.net/forum/attach...7&d=1676794956

it looks to me like if Q654 gets turned on then there will be some voltage going thru resistor R653, which would put something on the ALL+3.3v line ... but am i looking at things wrong?

so i tried plugging the mainboard back in, and wouldnt you know it... the TV is working again like normal. i accidentally hooked it up first with the P-ON+21V and P-ON_H2 still connected together, and in that state, it was turning on, flashing the backlight and then rebooting. i removed the jumper clip, and was still getting the same thing. i unplugged the backlight and plugged it back in, and then the tv just starts up like normal. In my mind I have fixed nothing, changed nothing. yet it is fixed! but for how long? previously with MB connected i would get a solid red standby LED that would not respond to the remote. now it does respond to the remote. maybe the MB sitting for a while has reset it and got it working again. but maybe it will fail again soon.
i still really want to figure out how to get this power supply to power the backlights without having the mainboard connected!
for future diagnosing purposes. if and when the MB goes down again, how will i know that its really the MB or the PSU if i cant do this?

[R_J]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

I have a similar schematic, D657 is a 4.3v zener diode, 0.7v is on the base of Q654 which is turning Q654 on.
The AL+3.3V is supplied FROM the main board, so that is why ic1602 is not turning on

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

I have a similar schematic, D657 is a 4.3v zener diode, 0.7v is on the base of Q654 which is turning Q654 on.

it doesnt seem to be on though... i mean nothing is coming out of it...

so i did what you said and shorted pin 3 and 4 of IC1602 and sure enough, that is the secret ingredient to getting this PSU to power up without the mainboard attatched!

hooray! thats all i really wanted to know how to do initially. (though i DO want to completely understand all PSU topologies eventually and this was a great education towards that goal! thank you so much for your advice!)

so to get this PSU going by itself you have to apply 5v to both BL_ON and ADJ pins, Connect P-ON_H2 to P-ON+21V, AND connect pin 3and 4 of IC1602 to each other. 3 alligator clips are sufficient, thanks to the jumper wires. i pull them up a little to get some room to slip the teeth of the alligator clip under.

one additional observation: with the backlights disconnected, and the mainboard connected normally (and no PSU trickery) we do see the VLED 27v line come up to 28V and then after about 20 seconds it begins to die away (backlight not detected...why keep powering it, i guess?)

can i have a look at that schematic? i feel i was getting close to understanding some things and dont feel like stopping here.

[R_J]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

it doesnt seem to be on though... i mean nothing is coming out of it...

It makes perfect sense, The emitter is connected to ground, the transistor's base is turned ON so the collector is at ground potential

the schematics I am using is for a philips 55PFL4901/f8 https://www.electronica-pt.com/esque...ileinfo/36041/

[triplefour]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

It makes perfect sense, The emitter is connected to ground, the transistor's base is turned ON so the collector is at ground potential

the schematics I am using is for a philips 55PFL4901/f8 https://www.electronica-pt.com/esque...ileinfo/36041/

yea i guess that does make sense. im starting to see how these transistors work. whether they send current to another circuit or send it to ground is a powerful difference.
i get why we would send the reset line to ground (reset should be low, to turn on the tv right?)
what i dont understand is how Q650 receives any voltage on its base. the 0.7v goes to Q654 base but there is no direct connection to Q650's base. the collector of Q654 is connected to the base of Q650 but i dont see how that would provide any base voltage since its being sent to ground.

this whole area... quite confusing to me still

and i am reluctant to call the TV fixed, give it back to the customer only to have them call me in a few days ...weeks... would months be ok?

[R_J]

Re: Emerson LF501EM6F has 21v and 13v but no vled+27v

D656 charges C656 and before D657 starts to conduct, there is 0 volts on Q654 base, so the collector is being pulled high by R653 which is connected to AL3.3v so Q650 will be turned ON, and the reset pin will be LOW, as soon as D657 conducts and supplies voltage to Q654 base, Q654 conducts turning Q650 OFF and the reset pin is then HIGH.
The RESET is used to RESET the microprocessor

So is the tv working now?