RCA L32WD22 No 15v No 24v no backlight

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

R921 8ohms tested with monitor dc'd from AC outlet Resistance is not futile
Pin 8 .2v dc Tested with monitor plugged in and turned on. Appears to be getting incorrect voltage...
Pin 6&7 0L tested with monitor dc'd from AC outlet Not sure here on these fairchild semiconductors to be honest this IC do you need to have correct power for the gate to open or how does it all work what should be a proper reading and why?

[budm]

Re: RCA L32WD22 No 15v No 24v no backlight

R921 8ohms tested with monitor dc'd from AC outlet Resistance is not futile
Pin 8 .2v dc Tested with monitor plugged in and turned on. Appears to be getting incorrect voltage...
Pin 6&7 0L tested with monitor dc'd from AC outlet Not sure here on these fairchild semiconductors to be honest this IC do you need to have correct power for the gate to open or how does it all work what should be a proper reading and why?

1) I do not understand the reading of the resistor "921 8ohms tested with monitor dc'd from AC outlet Resistance is not futile" what is the"dc'd from AC outlet Resistance" You have to do is to measure the two legs of the resistor, no power to the TV.

2) "Pin 8 .2v dc Tested with monitor plugged in and turned on. Appears to be getting incorrect voltage..." I thought you get 14VDC on the post 19.

Errr sorry did not see your post in time was trying to number things out!

Let me give them a look.

UPDATE this one matches the service manual as well.

IC901 056T 368 12 IC FAN7529MX SOP-8

Pin 8 vcc it appears

VCC = 14V and TA = -40°C~125°C unless otherwise specified

UPDATE:

D901 and D902 pass diode test.

Trying to devise a way to have everything hooked up and upside down and check the voltage to that pin when powered on.

Start voltages is 11-13v it seems though according to the sheet.

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

Hrmmm I was pasting it from the datasheet on what to look for more or less for a note so you didn't have to dig up the datasheet.

Resistance as usual checked with 2 legs in circuit was more than 1 ohm on the radial resistor marked R921 (the resistor coming off the leg of the 3pin mosfet I believe.) I also checked right next to it and it was fine as well to be honest.

I am not getting the proper voltage on vcc pin 8 it read as high as 1.3v DC but seems to bottom out around .2v DC.

So in summary I am not getting the correct voltage to the fairchild pfc ic on that pin.

What gives it that voltage and when I guess would be my next question is it the 15v which I have stated has no reading?

[budm]

Re: RCA L32WD22 No 15v No 24v no backlight

The VCC pin 8 gets its Voltage (from AUX winding of the standby power supply transformer) through switched Transistor that is turned on by the PS-ON signal. So 1.3V is not going to work, you need to trace out pin 8 to see where it goes and also need to check the resistance between pin 8 (vcc) and the GND pin of the PFC IC to see if it shows low resistance. Since we do not have the diagram of the board, tracing out the connection will be required to draw up the diagram of the board, it will be time consuming, I hope you will have time follow through.
So you are sure that the Source resistor is measured as 8 Ohms, that is high, typical value for the Source resistor is <1 Ohm.
What resistance do you get when you touch the two probes of the meter together?

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

Things that will be helpful to the process on my end:

I have 2 DMM's
I will take readings with all 2 meters to verify.
I also own a scope but I am not the most knowledgeable scope owner and I have a small fear of ruining it or the probes it is a very expensive items normally I attempt to check power to see of the power signal is dirty or clean on dc or ac lines where I am certain that I will not short anything out by slipping off a pin or making contact with another pin these things are not cheap but I am willing to use it if need be. Techtronix TDS 540 500mhz 1GS/s so it is available to put to use as a last resort.

Readings I will retake:

-NO POWER source resistor resistance.
-NO POWER pin 8 vcc and pin 6 gnd resistance.
-WITH POWER AND TURNED ON EVERYTHING CONNECTED vcc pin 8 and gnd pin 6 DC voltage.

Things that would be helpful from your end:

-A possible modification to the picture listing the parts that you are aware of which transformer is which or maybe even different circuit areas a label them so I can become a bit more educated on the SMPS operations within the TV I am sure most all of them follow a similar pattern.

- Don't be afraid to go say 10 steps out and say if this happens then check this if that happens check that I am not too dull to attempt to follow along it will give me a lot to go by and it will allow me to grasp an understanding of the train of thought when it comes to repairing. so far everything has been Very Helpful in furthering my education on a known good approach to tackle problems that crop up.

Any other things you would like to say or is their anything listed that is too much of a burden for you to take on?

It is my goal to keep things as straight fwd and simple for you from here on out I will do my best to do this so their is no question as to what I say I will try and be more attentive and thorough so we can arrive at a resolution. And I will attempt to stick it out.

Thanks BudM

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

Reading Name 1:
-NO POWER source resistor resistance.
Value:
.5 ohms

Reading Name 2:
-NO POWER pin 8 vcc and pin 6 gnd resistance.
Value:
1.8 Mohms

Reading Name 3:
-WITH POWER AND TURNED ON EVERYTHING CONNECTED vcc pin 8 and gnd pin 6 DC voltage.
Value:
.8 Volts DC

both meters read the same for all three readings I would say they are likely proper and accurate.

So there is a start.

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

As far as a schematic ... I believe I posted a schematic of everything a full service manual ... But to make it a bit easier to follow I took the liberty to snap a high resolution picture of the power board schematic hope this helps! Connection 901 three pin is the ac in from the outlet Maybe this will help tremendously on your end now I can see what stuff is a bit easier too! I'm going to follow the schematic a bit.

Before that looking at the board I seen jumpers that basically took pin 8 to an emitter pin on a transistor on the other side of the board checked that transistor and checked out good infinity one way and .577 the other on collector and emitter when checking against the base if I remember correctly a diode test with .600 or there about is a good baseline for testing transistors in circuit well I am going to look and see if I traced it out right by eye although I cant really see how it would be wrong but I guess stranger things have happened with trying to find this stuff.

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

According to the schematic it is correct Q905 would be the transistor

hrmmm d916 checks out as well coming off the T901 Pin 5 so I assume I should check pin 5 and pin 7 and see what voltage I get off the secondary winding there possibly not sure on what I am supposed to get there actually?

80TL-24T-8-L
LI TAI DASH 2B-90 0740

These numbers do the designate anything worth mention when it comes to the operation of the transformer or number of turns in the windings or size of the wire or what have you? Just curious if someone knows this.

[budm]

Re: RCA L32WD22 No 15v No 24v no backlight

OK, sorry I completely missed the service manual you proided.
Do not try to use the scope to take any measurement in primary (hot side) unless you have isolation transformer to power the TV.
This is how the PFC IC get its 'PFC_VCC' on pin 8 of the PFC IC:
When PS-ON is applied, the PS-ON will turn on Q904 which in turn will turn on the LED inside the OPTO IC904, the PHOTOTRANSISTOR inside the OPTO is then turned on, this will provide the bias current through R944 (1K Ohms) to bias on the linear regulator transistor Q905, the 15V Zener diode ZD931 is to maintain 14.4VDC output at the Emitter of Q905, the Input Voltage feeding the Collector is from the AUX winding of the standby power transformer T901 which is rectified and filter by D916, C915.
So verify that the resistor R977 feedng the D916 is OK.
Verify that you have around 20VDC between the two legs of C915.
Verify that 15V Zener diode ZD931 does not show conduction in both direction.
Verify the Voltage between the two leg of Zener ZD931 is around 15VDC.
Verify that Q805 is good.

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

Having a heck of a time with Q805 I think you mean Q905 the transistor

Q905 is good .577 in both direction for collector and emitter against the base as mentioned above the last posting.

R977 is a resistor with very little resistance I would assume it has a zero on it indicating 0ohms 1/4 watt resistor it appears to be fine.

ZD931 is conducting only one direction

Need to check C915 and ZD931 voltages will test and report back as soon as possible.

I am almost wondering if it isn't possible that the opto ic is not faulty I have actually wondered this all along to be honest because it reminded me of the optocouplers in projectors you have to fake out to use another light source. I had succeeded in doing this on a viewsonic projector I managed to scoop up cheap but I then learned a couple of lessons do not use halogen bulbs as they get super hot and started to melt the projector then I learned that if the color wheel is bad you are SOL either way :P yey DLP.

Anyhow thanks again!

Glad to see I'm not the only one who posts wrong numbers and overlooks stuff from time to time BudM is human after all :P

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

C915 20.5v DC

Zenner as I said tests good only one way but only .03 vdc so something is wrong there!

R944 is .993 Kohms it is a 1megaohm resistor so I would say this should suffice. (I think you meant this instead of R977 as well)

Opto ic's are they a common fault ? I assume an led inside them could burn out as often as you see christmas lights burnt out thus not allowing the PSON signal to follow through right?

[budm]

Re: RCA L32WD22 No 15v No 24v no backlight

"R944 is .993 Kohms it is a 1megaohm resistor so I would say this should suffice. (I think you meant this instead of R977 as well)" Nope, it is R944 which is 1K, please look at the diagram again, so 0.993K is = 993 Ohms.

"Zenner as I said tests good only one way but only .03 vdc so something is wrong there!" so the DCV across the Zenner is only shows as .03VDC or the diode test is shown as .03?
If the DC reading is only .03V, then what you can try is to short pin 3 and pin 4 of the OPTO, then you should have VCC at pin 8 of PFC IC. If you do get the VCC then it is either bad Opto or bad transistor driving the OPTO.

R977 is the ZERO Ohm jumper.
"Verify that Q805 is good." You are correct, it is Q905, sorry.

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

Volts DC is .03 I'm afraid for the zenner diode...

so I jumped pins 3 and 4 and checked pin 8 of the PFC it was a no go .9 volts dc on pin 8

hrmmm very odd I cannot find a datasheet on this opto ic or would this one be the same as

Transistor Q905 is good.

Capacitor C915 is 20.5v

So something is giving me troubles here...

[budm]

Re: RCA L32WD22 No 15v No 24v no backlight

If you only getting .03V on the Zener with pin 3 and 4 of the opto shorted together which means the bias resistor R944 (one end is end is connected to 20V source , the other end is connected to Zenner)is proving the current to feed the 15V Zenner and bias on the transistor, if you only getting .03V on the Zener, then you either have bad Zener or shorted circuit around the Zener since there is nothing there to make Voltage goes that low.
Are you 100% sure that Zener tested good? the resistor tested good, so there is nothing else.
"Zenner as I said tests good only one way but only .03 vdc so something is wrong there!"

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

hrmmm interesting the zenner is reading 0.00 both ways on the meter on a diode test I would say it is likely toast IDK.

Tried diode test and low ohms setting... Sheesh I don't know I checked and double checked the dang thing with two meters dunno how that got mixed up unless I just checked one way both times IDK what happened originally...

Well I have another one on a parts board that checks out good with the same color bands it is from an insignia 26" tv should be the right zd I would say got to go to bed though its like 4AM here running on fumes. Tomorrow I wake up and do some smd work although it is killing me I'd like to do it now lol I am aware that it could be possibly the wrong ZD dunno what that would do but I'm going to try it it is free. It is the same color as these http://www.ebay.com/itm/RLZ15B-15V-4...item43ce9e0b88

15bux for a few diodes get out of here I say! I'll just try it if I blow up the board I'll buy a new one lol gotta take a risk unless there is some way to test it bench power supply 0-30v available?

I will orientate it the same way and hope that it works correctly and if it does well hopefully that solves the problem! Amazing though you must have really dug in deep on a lot of LCD power boards to pick all this stuff out so quickly.

Thanks I will test it out tomorrow!

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

zenner diode replaced

Test for conduction , meter set to diode test, tv unplugged.
Result 0L one way and .718 the other way tested with two meters

Test for voltage when TV is powered on putting probes on each side of the zenner diode red on the positive side black on the negative side tested with both meters a few times now just to be certain.
.4 v dc one way other way I have the same negative value. -.4 v dc

Tried it with the opto ic shorted no change.

I am likely missing something you mentioned at this point I'd better go back and do some more testing would be nice to just possibly follow the voltage from the cap to the zenner this should tell me where my problem is I suppose.

This is how the PFC IC get its 'PFC_VCC' on pin 8 of the PFC IC:
When PS-ON is applied, the PS-ON will turn on Q904 which in turn will turn on the LED inside the OPTO IC904, the PHOTOTRANSISTOR inside the OPTO is then turned on,

this will provide the bias current through R944 (1K Ohms) to bias on the linear regulator transistor Q905,

the 15V Zener diode ZD931 is to maintain 14.4VDC output at the Emitter of Q905,

the Input Voltage feeding the Collector is from the AUX winding of the standby power transformer T901

which is rectified and filter by D916, C915.

Not 100% positive on the diode if it is correct but the color bands are the same and it came off of a smps that uses nearly the same electronics an insignia 26" lcd tv position R390 I am having trouble locating the service manual for that one.

PSON Pin 11 is 3.5v when switch is turned on to green status light. Double checked that.

This thing is a bit of a whale to tackle but I am not giving up quite yet.

[budm]

Re: RCA L32WD22 No 15v No 24v no backlight

Just use variable power supply and 1K resistor is series with the Zener, you know how Zener works, right? look it up.
Like I said and as shown in the diagram, you have 20V source feeding the 1K resistor and another end of the resistor is fed through one leg of the opto, another leg of the opto is then feed the 15V Zener diode.
using unknown diode will cause more confusionnsine you have to verify if it is Diode or Zener diode with correct Voltage.
There is not much in that 15V regulator circuit and you had already verify that the transistor and the PFC IC are not bad/shorted out per your testing which I am going by based on your reports.
May be at this point you should by new board instead.

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

I guess I am having confusion matching the schematic to the board I am still not seeing how the circuit works entirely so I need to upload the pictures of what I have traced out maybe you can help me here I am curious on how it is traced out I suppose it is possible that the schematic could be different if the model power supply does not match I am not entirely sure but just to be sure maybe you could help me trace this out further?

Also is there somewhere I can test previous to the zener diode to be sure I am getting correct voltage like possible at the resistor for instance? Where would I put my probes as I assume there is supply voltage like you say of 20v on the bottom side of the R944 resistor however when I test for it I am not seeing that so it must be where I am putting my probes I suppose might be a dumb question at this point I am dumbfounded on how the 20v gets to that point.

[budm]

Re: RCA L32WD22 No 15v No 24v no backlight

C915 Negative leg will be where you put your black probe GND ref for you meter for Voltage measurement of the Zener, the Collector, the Base, the Emitter of the Q905.
Just trace out to see if the positive leg of the C915 goes to the Collector and if you do have 20V at the Collector.
It is just a matter of looking at the schematic and look at the trace and use Ohm meter to see what goes between point A and point B.

[infringer]

Re: RCA L32WD22 No 15v No 24v no backlight

odd I get all low voltages through the whole thing putting the black probe to the negative

But I will note when I put the positive lead on the positive end of the cap and check the zener diode it is correct (looking at the photo I uploaded before) 20v on the right side and 14v on the left side so I think it is safe to say that the diode is an exact match and I checked pin 8 like this as well and it displayed 14v dc as well so the zenner has to be good it is somewhere somehow the voltage from the cap is not making it beyond the capacitor I don't understand what the heck would do this but I have learned a bit from this board thanks to you do you have a dogecoin wallet I'd gladly tip you for the help!

Still working at it but I have a full time job to attend I actually work with electronics for a living normally I can figure stuff out fairly well but getting down to the dirt of it all would be nice beyond board or part swapper.



Just got to figure out why there is no voltage beyond the cap really just all really low readings less than 1v... on everything you mentioned the zenner the collector the base and the emitter like .4v - .7v dc while I still do have 20v on the cap beyond the capacitor there is nothing it is just odd to me this is why I have been thinking of a way to test beyond it.

Hrmmm possibly a dry solder joint on the cap itself I think I will run a continuity check tomorrow and see what I can find gotta wake up in a few hours!

Thanks again it is a work in progress at least the zener is correct I know that much.