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

[triplefour]

SO i learned some things about these emerson power supplies from this thread: https://www.badcaps.net/forum/showthread.php?t=65611
where i was able to successfully get the power supply to power up and turn on the backlights, proving that i only had a main board issue. the customer was happy with the result, so he referred me to his neighbor who happens to have a very similar TV but the bigger model.

his issue is different though. when i opened up the set i noticed right away that there was a rather large gecko who had found his way in there and died on the PSU.
looks like he touched PFC VIN and probably the GND jumper as well.
the PFC VIN jumpers are bad looking (rusty) but still have continuity.

so i tried to "trick" the power supply on via the method i learned in the other post. i connected P-ON_H2 to P-ON+21V and i applied 5v to BL_SW and ADJ jumpers.
i get the AMP+13v but i do not get any voltage at VLED1, 2 or 3
and it seems there is a rail that feeds all three of them: VLED+27V
that is also 0V
it seems it should get this voltage from the cathodes of Diodes D710, D711, D712 via the secondary of the bigger transformer but those are all 0 as well.

i see 158v at the Primary Filter Cap and the other big caps around that area, and at the primary of the bigger transformer, but it seems like it should be boosted higher than this. since there is a PFC according to the markings on the board.

poking around some more i noticed a jumper wire marked VCC+15V that is at 0v.
this goes to one of the pins of IC1601 which i assume is the switching control IC.
VCC is probably pretty important for this IC to do its job, so i went looking for where this 15v should come from and from what i can tell, it should come from the smaller of the 2 transformers, thru some diodes. i see 18v coming from the cathode of diode D620A. i trace it via J1617 and thru R1656 to a 3 pin diode D1604 where 18v appears on one side but not the other. on that other side is where the VCC+15V jumper wire would take that voltage via a few other jumpers to the IC1601.

i think i may have found the bad component, but i cant find a datasheet for it.
maybe someone can help?
it is marked as far as i can tell BKD and it also has a 2H on the side.
the voltage enters on the side with only one pin, but doesnt appear on either of the pins on the other side. testing in diode mode with red probe on this pin i see open circuit from this pin to either of the others.

please have a look at the pictures and let me know if my reasoning is correct
but more importantly. How to find the replacement component?
THANK YOU!

[vinceroger69]

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

BKD is possibly a KTC8050S-D

[CapLeaker]

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

You are on a good path. You figured out that the PFC a isn’t working and you don’t have back lights and 27VDC. These are all related to each other and you got to figure out why the PFC a isn’t boosting.
Start at the Mosfet, and the gate drive circuit. Read the Emerson threads, they give very good clues on where to look. I fixed one years ago and the problem started in the secondary with one of two larger diodes being shorted, causing a whole whack of components to blow up. I had fixed it with parts out of my junk pile and it worked until recently where the TV fell.

[triplefour]

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

You are on a good path. You figured out that the PFC a isn’t working and you don’t have back lights and 27VDC. These are all related to each other and you got to figure out why the PFC a isn’t boosting.
Start at the Mosfet, and the gate drive circuit. Read the Emerson threads, they give very good clues on where to look. I fixed one years ago and the problem started in the secondary with one of two larger diodes being shorted, causing a whole whack of components to blow up. I had fixed it with parts out of my junk pile and it worked until recently where the TV fell.

thanks. i think its something to do with the diode i pointed out that voltage comes to but does not leave from....this should power the VCC line for the main switching IC ... im tempted just to jump the diode with any other diode and see what happens....but i would like to replace it with the proper one if i could! gotta look thru my parts and see if i have one of these...'


edit: its not a diode. i read the marking wrong. its not D1604 its Q1604

so maybe the voltage is present at the collector, but since nothing is coming to the base, nothing is going to the emitter?

[triplefour]

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

BKD is possibly a KTC8050S-D

upon closer inspection, the marking on the board does not say D1604.
it says Q1604

so... its probably a transistor. what makes you think its that one though? just want some insight as to how u identify these things? thank you!

[lotas]

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

upon closer inspection, the marking on the board does not say D1604.
it says Q1604

so... its probably a transistor. what makes you think its that one though? just want some insight as to how u identify these things? thank you!

The notation "Qxxx" is the transistor.

https://aliexpress.com/item/1005004820952020.html

[triplefour]

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

The notation "Qxxx" is the transistor.

https://aliexpress.com/item/1005004820952020.html

i have already figured out my mistake thinking the Q was a D.
what i want to know, is how you can tell from those markings on the chip itself
(BKD) that it is this particular transistor you are linking me to?

i mean i can see now from reading the datasheet that it shows the marking BK and tells what the D would be for (Rank)
but if i didnt already know this, how would i have found this datasheet?
im trying to figure out how to do as much of this as i can without asking for help!

[CapLeaker]

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

Look if there are any shorts in the secondary. Fix the secondary first if there are shorts, then the PFC circuit in the primary.

[triplefour]

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

so if im using that datasheet, i can see that pin 2 is the base, which has 0v on it.
i can see that this base should get some voltage from C1618 thru 47 ohm resistor R1634.
there is no voltage on this capacitor. it gets confusing in this area but it looks like that capacitor C1618 should be fed from Q1607 (pin3, if pinout is same as others) thru 180ohm resistor R1637
Q1607 shows voltage 18.25v on pin1, 18.08v on pin2 and 0v on pin3, so nothing going to that cap.

need help identifying this part. Q1607
markings appear to show an 8, an E, a o and an S but i really have no idea what order to read in...
or if those digits are correct. this is the best pic i could get of it.

been looking all over for any shorted component. find none.

[R_J]

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

Check if D1608 is shorted? that circuit might be controlled by IC1602, What are the voltages on the two pins on the primary side?
On the primary side of ic1602 does pin 3 or 4 go to hot ground and the other to J620?

[triplefour]

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

Check if D1608 is shorted? that circuit might be controlled by IC1602, What are the voltages on the two pins on the primary side?
On the primary side of ic1602 does pin 3 or 4 go to hot ground and the other to J620?

D1608 is not shorted. reads 923 one way and around 1800 and climbing the other way. (diode mode)
pin 3 of IC1602 is hot ground (negative leg of primary filter cap)
voltage at pin 4 is 17.8
im pretty sure the voltage comes from D620A
i have marked how i think the current is flowing and the voltages at certain places. is this correct?
the pink line is where i think the current would go but i have 0v there

i found this seemingly useful site for decoding SMD markings but there are so many things 8E could be
https://smd.yooneed.one/code3845.html

it is smaller than the other one (Q1604) which is SOT-23
so this must be SOT-323
that narrows it down to 3 things in the list. 2 of of which are npn transistors and one of which is a voltage detector.
probably its the NPN transistor?
MUN5215 (Motorola)
or
UNR5215 (Panasonic)

sadly the pinout of these 2 devices is not the same. collector and emitter are switched around. so still gotta do some work to figure out which it is
[edit] i was reading things wrong. they have the same pinout according to the 2 datasheets. that is assuming 8E is the marking...which it may not be according to the post below!

[R_J]

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

8E is likely the date code, "So" is likely the part number. ic1602 has 18 volts on it, this likely needs to be low to turn on the VCC+15 circuit. You will need to check the secondary side of the optocoupler to see what is needed to turn IT on.

[triplefour]

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

8E is likely the date code, "So" is likely the part number. ic1602 has 18 volts on it, this likely needs to be low to turn on the VCC+15 circuit. You will need to check the secondary side of the optocoupler to see what is needed to turn IT on.

yes i think it is SO that is the code to be used. it turns up only one result in SOT-323 and its a PNP transistor whos pinout matches what i was expecting, according to how i have drawn the current flowing.

2SA1586 - toshiba

see also, updated picture of what i think is happening

so the secondary side of the optocoupler is the LED which needs 1.2v to turn on (according to my memory of optocouplers)
i see 0v on that side of the opto. ill do some tracing and figure where it should come from.

i can see right away that pin one of the opto goes thru a resistor (331ohms) and ends up on pin 15 of the mainboard connector which connects to jumper wire ALL+3.3V
the other TV i worked on that had this similar power supply had this ALL+3.3V as well marked on the board but it turned out not to be actually there. what about this time?
seems like it would make sense for the all+3.3v to be present on this board. if that were there, the opto would turn on, allowing as you say, the voltage on the other side to go low. not sure how this voltage going low would enable the 15v VCC but im kinda taking your word for it at this point


i notice something strange right away though. the other opto thats right there has 24v on one leg of the LED and 23 on the other....isnt that much too high for a led that only wants 1v? or is that why there is a1v difference there? the diode is dropping that volt?

the 24v comes from the cathode of diode D650 which gets it from the secondary side of the smaller transformer.
measuring in diode mode, between pin 1 and 2 i see 1600 one way and infinite the other way. not sure if that number is high but that checks out as diode behavior to me.

[lotas]

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

So - kta2014 (SOT323)

[triplefour]

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

im confused about Q1607 though. we have voltage at the emitter, and at the base....shouldnt we have current flowing out thru the collector? doesnt this mean this component is bad?
OR...if this is a PNP transistor, doesnt the base have to be negative for current to flow? i cant see how we would be able to get a negative voltage there. the base voltage comes almost directly from the Diode D620A, it just goes thru a 10k resistor

[R_J]

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

1 volt should be enough to turn on the optocoupler, so it might be that the optocoupler is bad.
I don't believe this version of the power supply uses ALL+3.3V, That part of the board does not have any of the components populated.
What is the voltage ACROSS the emitter and base of Q1607?

Short the output side of ic1602 and see if you get the VCC+15V

[triplefour]

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

1 volt should be enough to turn on the optocoupler, so it might be that the optocoupler is bad.
I don't believe this version of the power supply uses ALL+3.3V, That part of the board does not have any of the components populated.
What is the voltage ACROSS the emitter and base of Q1607?

Short the output side of ic1602 and see if you get the VCC+15V

i think i may have confused you by talking about the OTHER optocoupler.
the one you told me to look at has 0v at its LED side
but the OTHER one has the 1v drop over it.

Q1607 measuring voltage across E and B (according to how i have it marked in my picture, the 2 legs right next to each other) i see 0v
which is odd because i see 18.3 on the emitter and 18.2 on on the base...so i was expecting to see 0.1v or something close to that for a voltage across...
huh?

[triplefour]

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

i may have confused myself... about these two opto couplers. the solder mask is very confusing. there are spaces for 4 of them but only 2 are populated. and they are called
IC1602 and
IC602 !!!!!

poor choice of marking! they look almost the same.....
sorry for the confusion.

IC1602 is the one closer to the edge of the board
IC1602 has 17.8v on pin 4 and 0v on pin 3 (phototransistor side)
IC1602 has 0v on both pin 1 and 2 (LED side)


IC602 is the one just above that, further from the edge of the board.
IC602 has 0.46v on pin 4 and 0v on pin 3
IC602 is the one that has 20.8 on pin1 and 19.8 on pin 2

[R_J]

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

IC602 is used the feedback to regulate the voltage of that power supply section.
IC1602 is used to turn on the VCC+15V that supplies the rest of the primary circuits including the one that supplies the 27V for led.
When I ask to check the voltage ACROSS the emitter and the base, You need to place one probe on the emitter and one probe on the base
If IC1602 has no voltage on pins 1 or 2, then that is likely why you are not getting the VCC+15V. in other words the power supply is just in standby.

It is possible that the main board supplies the voltage for Ic1602 since this board does not have the ALL+3.3v
I suspect the main board needs to be connected for those voltages on ic1602 to be present.

[triplefour]

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

IC602 is used the feedback to regulate the voltage of that power supply section.
IC1602 is used to turn on the VCC+15V that supplies the rest of the primary circuits including the one that supplies the 27V for led.
When I ask to check the voltage ACROSS the emitter and the base, You need to place one probe on the emitter and one probe on the base
If IC1602 has no voltage on pins 1 or 2, then that is likely why you are not getting the VCC+15V. in other words the power supply is just in standby.

that is how i measured the voltage across Q1607. with one probe on E and one probe on B, and i got 0v...even tho i expected to see like 0.1v

updated the picture to include optocoupler voltages

so should i still try shorting pins 3 and 4 of IC1602?
is that 0.46v on IC602 pin 4 normal? for a feedback?