LG TV wipeout

**Skepticist** · 31-07-17, 03:55 PM

After thinking about it a bit more - the reason the reference voltages don't line up could be due to the 12V rail voltage being so low that there's no conduction either through the led in IC102 or IC252 so there's no voltage drop across R289.
So the subnormal voltage is being determined by something else on the primary side around IC101?

**loopyloo** · 31-07-17, 05:14 PM

Originally Posted by Skepticist

At least there are a few parts that have proven to be ok

Got me suspicious of C115, C116 and ZD102 on the HV side - what sort of voltage do you measure across ZD102 (actually pins 3-4 of IC102)?
If any of those were a bit leaky it would throw things out of whack and there's still IC252 to think about - maybe alter vref by placing a 1k resistor in parallel with R293 or R294 to ensure the reference change affects the 12V supply
I'd expect to see a noticeable change in the 12V supply output (but not enough to be problem)
The cathode voltage on IC252 is determined by (1+(R291+R292)/(R293+R294)) * Vref and the numbers you measured don't seem to agree with that
Also try disconnecting R258 from Vref just to see what affect it has on the 12V rail, if any
All regulation is done on the 12V rail and the other supplies can't run away on you if the 12V is being regulated (currently it seems over-regulated)

Are we past the point of no return yet? lol
The operation will be a success even if the patient doesn't make it

Nah nah nah can't stop now ha ha.
Getting too close. I'll go do some more measuring. lol

This must be good, we've had 6,400 views lol.

**loopyloo** · 31-07-17, 05:27 PM

Guess what .....IC101 pin 4 and ZD102 cathode side have a huge voltage on them .... 60mV
The other 2 pins are at ground

**Skepticist** · 31-07-17, 05:59 PM

According to the datasheet IC101 pin 4 (RFmin) provides a reference voltage of 2V - must have a huge overload to drag it down that low
Another short lurking in the shadows?

The HV spike that blew it up originally must have distributed some shrapnel around the board

The fault must be on the RFmin side of R114 8.2k which exonerates the opto and its smoothing components so could there be a whisker of solder under IC101?
With everything dead and discharged, RFmin to ground should measure 4k or more. Even if Q103 (off pin 1 CSS) was punched through, RFmin to GND should be 2k or more.

**tw2005** · 31-07-17, 07:27 PM

Originally Posted by loopyloo

Nah nah nah can't stop now ha ha.
Getting too close. I'll go do some more measuring. lol

This must be good, we've had 6,400 views lol.

well i hear the internet is filling up and I'm running out of popcorn.

Now if you can work it out, maybe It'd be worth me starting a thread a Panasonic LED PSU that's dead. I have no idea with since you both have a clue. Got schematics but no data on some of the chips, typical .

**loopyloo** · 31-07-17, 07:37 PM

Originally Posted by Skepticist

According to the datasheet IC101 pin 4 (RFmin) provides a reference voltage of 2V - must have a huge overload to drag it down that low
Another short lurking in the shadows?

The HV spike that blew it up originally must have distributed some shrapnel around the board

The fault must be on the RFmin side of R114 8.2k which exonerates the opto and its smoothing components so could there be a whisker of solder under IC101?
With everything dead and discharged, RFmin to ground should measure 4k or more. Even if Q103 (off pin 1 CSS) was punched through, RFmin to GND should be 2k or more.

RFmin is 8.2k to gnd. IC101 should be ok, I cleaned the pads well before inserting the new chip.
Trying to measure SMD components is terrible, not like it's easy to remove them for checking then put them back.
Solder joints on the board look ok but just for the sake of it I might go over some of them while I wait for my next light bulb moment..

**tw2005** · 31-07-17, 07:58 PM

Originally Posted by loopyloo

RFmin is 8.2k to gnd. IC101 should be ok, I cleaned the pads well before inserting the new chip.
Trying to measure SMD components is terrible, not like it's easy to remove them for checking then put them back.
Solder joints on the board look ok but just for the sake of it I might go over some of them while I wait for my next light bulb moment..

Here it is, now back to work

**loopyloo** · 31-07-17, 08:05 PM

Originally Posted by tw2005

well i hear the internet is filling up and I'm running out of popcorn.

Now if you can work it out, maybe It'd be worth me starting a thread a Panasonic LED PSU that's dead. I have no idea with since you both have a clue. Got schematics but no data on some of the chips, typical .

We're charging entry fees after 8,000. lol
Panasonic loves to keep everything hidden, even to the point of using components with their own identity on them.

**tw2005** · 31-07-17, 08:33 PM

Originally Posted by loopyloo

We're charging entry fees after 8,000. lol
Panasonic loves to keep everything hidden, even to the point of using components with their own identity on them.

Right, I better not leave then.

**Skepticist** · 31-07-17, 10:26 PM

After combing through the datasheet I see that the RFmin 2V reference is rated for approx 2mA max so the overload is not really disastrous but enough to kill the voltage. My calcs show the load on that 2V should be barely 0.5mA and the only thing connected to it apart from 3 resistors is the STBY pin 5 which is a comparator input so expect that to amount to microamps. So where is the missing (if it is in fact lost) 1.5mA going is the question.

It appears that the chip is stuck in 'soft start' mode (my guess anyway) because Css never charges to 2V which is the point where the feedback loop should take over.

**loopyloo** · 31-07-17, 11:04 PM

Maybe I should lift ZD102's cathode from the board and try another measurement at IC102 pin4 ?
I already replace a 15v zener elsewhere. never know.

**Skepticist** · 31-07-17, 11:53 PM

Have you measured the voltage drops across R108, R113 and R114? might give a hint as to where that current is going, although I'm thinking it's not through any of those resistors. ISEN is next to STBY which is directly connected to RFmin and anything bridging pins 5 & 6 would definitely kill the 2V reference via R116 (43 ohm).
Just thoughts - have you done a pin to pin continuity test on pins 1 - 8? (de-energised of course) and each pin to ground as well might be telling

Also you mentioned earlier that IC601 VCC was re-connected - is the PFC now running with the short removed (and 340V connected to Q601/602 drains?
Not that it's significant to the fault on IC101, just wondered

If you keep eliminating all the good parts all that'll be left is the crook one

There's no voltage on ZD102 and even if it was short cct there'd only be about 0.25mA flowing courtesy of R114 which is not enough to affect the 2V reference

The pic of the copper side of the PCB is not sharp enough for me to make out much detail of the SMD layout but those tracks are mighty close together and it would only take a microscopic whisker to short out a 2V 2mA supply (worked out that a conductor 0.05mm diameter would do it without fusing, even <0.01mm if it's Cu).

**loopyloo** · 01-08-17, 02:41 PM

No actually I never thought of doing voltage drops at all, well I have been reading across components but not with that in mind anyway I'll have a look at them.
Yes, have checked all chips for shorts and to their gnd.
Q601 and 602 not looking good, 340v on drains but gates and sources have zilch.

I'll do more looking this arvo and get some re-readings as well

**Skepticist** · 01-08-17, 03:34 PM

Our 1000s of readers must be the edge of their seats

**loopyloo** · 01-08-17, 03:51 PM

Originally Posted by Skepticist

Our 1000s of readers must be the edge of their seats

Yeah don't want to fix this too quick, entertainment value. ha ha

**tw2005** · 01-08-17, 06:17 PM

Fixed it yet?

Just fixed my Panasonic PSU, Couple of IC s and in Business.

C'MON

**loopyloo** · 01-08-17, 07:33 PM

Yeah well you'll keep !

**loopyloo** · 01-08-17, 07:37 PM

Something to ponder over :
R108, R113 and R114 voltage drops. They ramp up and down between the figures below so showing the IC is trying to start.
R108 60-70mV
R113 80-90mV
R114 0-500uV

**Skepticist** · 01-08-17, 08:30 PM

Most of the current is in R108 but still next to nothing at about 17uA and the others are less.
If there's no short between tracks then the only possibility left looks like an internal fault in IC101 unfortunately

The 2V reference is internally generated and, far as I can tell, is dependant only on Vcc being applied
so it would be tempting just to be certain before throwing out yet another IC out to lift the RFmin pin off the track and apply 12V DC to Vcc from an external power supply (no mains connected to the board) just to be sure that the 2V reference is actually gone.

I'm getting interested enough in this chip to get a few for myself to play around with now

Quite a change from the PWM flyback types I've made in the past.

**loopyloo** · 01-08-17, 09:04 PM

Originally Posted by Skepticist

Most of the current is in R108 but still next to nothing at about 17uA and the others are less.
If there's no short between tracks then the only possibility left looks like an internal fault in IC101 unfortunately

Don't tell me that, it was a $7 chip. Is RS gonna believe me when I tell em they sent me a faulty chip ?
So I checked out IC101 pin voltages with respect to it's gnd while I was in there.
Pin1 = 22mV
Pin2 and pin8 = 0V
Pin3 = 95mV
Pin4 and pin5 = 85mV
pin6 = 0V
Pin7 = 1.5V

Pin9 = unused
Pin10 = 0V
Pin11 = 150mV
Pin12 = 9.7V
Pin13 = Unused
Pin14 = Cycling 40-44V
Pin15 = Cycling 40-44V
Pin16 = Cycling 44-48V

Maybe I could put the old chip back in ? Not sure It was faulty or not but I found it on the shed floor where I'd trodden on it a few times.