troubleshooting TV SMPS sequence

[budm]

Re: troubleshooting TV SMPS sequence

The set up in this RCA is not typical, it has the 1W external standby power supply that supplies the STBY to run the main board and for driving the power replay that will supply 120Vac to run the main power supply board, they could have done without that 1Wpower supply.
T2 is the standby power transformer but in this case it not being used as STBY because it is not always on, that transformer is the one that supplies the power to run the TDA chip as shown in the diagram.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

BudM,

Thanks Budm, great job you and everyone. the first time I checked the 5vsb with the scope on the small1W board, there was alot of signal distortion sitting on top of the 5.36V going just below 5V. I did change the cap C616 on the 1W board and that cleared up the distortion.

what you're saying is there are two 5V stndby voltages, one on 1W board and another on the PS itself?

anyway, glad I made it through the storm. your tireless efforts on so many posts is unbelievable. glad you kept an eye out on mine!

ilikesteel

[budm]

Re: troubleshooting TV SMPS sequence

because the LD7550B IC12 is the PWM for the other transformer where Vcc for the PFC comes from and it also does the power friendly 5V rail. That C44 is the starting cap, r77 is the starting resistor.
See it as 2 PSU's. One that is switched on and off (12/24V) and the other for 5V that runs continuously and is also some ecofriendly version PWM.

I cannot explain why there was 360V on the main filter cap while the PFC is in the off condition. I didn't read the whole TDA16888 datasheet through.

I did not read from the beginning to see if he has unstable 5VSTBY or not, but the clue was when he reported the fluctuation for the TDA VCC, if he looks at Q1 switched transistor on the feeding side he would probably see the fluctuation there too, and I believe the the voltage fluctuation was higher than 14V but his meter was not fast enough to show that but it was enough to get the TDA IC be running on and off so the main cap was charging and discharging, if he looks at the VCC with scope he will probably see what the actual Voltage really is.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Your welcome!

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CApLeaker,

Thanks again. it was a great learning experience for me. I think I developed some extra troubleshooting skills because of it.

thanks again.

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

because the LD7550B IC12 is the PWM for the other transformer where Vcc for the PFC comes from and it also does the power friendly 5V rail. That C44 is the starting cap, r77 is the starting resistor.
See it as 2 PSU's. One that is switched on and off (12/24V) and the other for 5V that runs continuously and is also some ecofriendly version PWM.

I cannot explain why there was 360V on the main filter cap while the PFC is in the off condition. I didn't read the whole TDA16888 datasheet through.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

"bump"

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker-Budm-BudWich-EVERYONE who contributed to this thread,



It Works!!!! The TV came right up, MENU screen came right up, looks beautiful.

could not have even thought to repair this TV without this forum and all the great people who contribute their time and effort to repair a TV they have never laid eyes on. Just amazing brilliance by all on this forum.

Special thanks to CapLeaker, Budm and BudWich who seemed to have offered much of their time and have educated me a little more on SMPS troubleshooting. I'm more confident to checkout problems a little more in depth before having to post a help call.

The TV is an older model and was probably not worth buying a PS or circuit boards to repair, that's why I took the challenge to see if I could spend a few bucks and MANY hours to get it working.

fortunately I had some spare boards to get components from and I didn't have to spend any money except for the 6 caps in the two secondary supply's.

Again many thanks to everyone.

How can that one cap C44 10uf, 50V, cause all this chaos in a power supply? Yes, I did replace a SMD resistor 510K R36 which was OPEN, that hung off of pin 18 on the TDA 16888.


ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

What this thread made it interesting, was the effect that quite a few things just don't make any sense. The pulsating secondary, the pulsating PFC, 360V on the main cap besides the PFC being in off state? There were quite a few "interesting parts" that shouldn't work like that, but yet it did somehow. The weirdest of them all is the 360V on the main filter cap, despite the PFC being in an off state.
Anyhow... I am glad it worked out. After all, we did spend quite a few hours on distance troubleshooting and this thread has gotten loooooooooooooooooooooooong.

"That is the startup/running cap for the 5VSTBY, so the 5V STBY was fluctuating too then?"
That is what I was banking on.

ilikesteel:
Hope you had fun troubleshooting your power supply. I bet it feels good having repaired something and return it into service besides putting it into the landfill.
The TV should turn on (unless there is trouble with the backligts) and you should be able to play with the menu's etc. However, you should find something like a DVD player and check it out. Not that there is another problem hidden at the panel, Tcon board, or main board. Also remember to put that 12V pin back into the plug and reverse the forced on mode on the PSU.

[vinceroger69]

Re: troubleshooting TV SMPS sequence

Just want to thank you guys for sticking with this one. I always learn something new, and as a viewer of the thread, appreciate the time and effort everyone puts in.

+1 when your learning electronics there some good tips on voltage testing etc and how to follow the schematic etc, i do think though if a full recap was done on this board or a esr meter was used if you have one of course it may of found the fault a lot quicker, but reading this thread again there was also a faulty resistor so it wasnt just a capacitor issue well done for getting this supply running your like me like a challenge and dont give up easily.

[attainteddragon]

Re: troubleshooting TV SMPS sequence

Just want to thank you guys for sticking with this one. I always learn something new, and as a viewer of the thread, appreciate the time and effort everyone puts in.

[budm]

Re: troubleshooting TV SMPS sequence

That is the startup/running cap for the 5VSTBY, so the 5V STBY was fluctuating too then?

[budwich]

Re: troubleshooting TV SMPS sequence

great... glad it worked out. Might have been the "long way"... but any way is the good way when things work.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker-BudM,

Hell, I couldn't wait for tomorrow to check it!!!!


I found a replacement for C44 10uf 50V. changed it. I believe the ld7550b is actually IC#12. I followed trace and made sure we're talking the right component.

are you ready for this???

Main Cap now has 384V STEADY

checked voltage on + side of C44 which goes to Pin 5 Vcc on ld7550b.
measured 12.54V STEADY!!

measured Pin 2, feedback comp, 2.09V STEADY.


Changed GND connections and Measured the +12 and +24V. they measured 11.85V and 24.0V STEADY.

I think we (all of YOU) did it!! I was just the shlep being guided by great minds. tomorrow I will reconnect all cables and stuff and then fire it up. I don't have any input for it, but I should get the black and white confetti screen, also menu boxes should display, right?

Thanks isn't enough, amazing grit by all of you to stick with me.

Thanks,
ilikesteel

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

I will check that tomorrow. yes, I replaced the cap C36 right away, before doing any measurements resistance or voltage.

By the way, I could not find F2 anywhere on the board, bottom or top side, I just don't see it. I checked real close, but did not find it.

Neg of main cap is my GND. also, I do have two probes for the 314 oscilloscope, just say'n.

Thanks..

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

ilikesteel:
just to understand correctly, you changed out the C36 first before you made that measurement there. If it is, that trashes the bad capacitors idea on the line flagged Q, which feeds Vcc to the PFC combi IC. That means the problem has to be before that again.

IC2 is a LD7550B PWM. There is C44 on the pin5 (Vcc) of IC2. Check that. The turn on voltage is 16V. The Feed Back (COMP) is pin 2. I wonder if that is pulsing too?

[ilikesteel]

Re: troubleshooting TV SMPS sequence

BudM and CapLeaker.

you guys are something else!

ok made some resistance measurements and voltage measurements.

first, I found a direct replacement for Cap C36 47uf 50V. replaced it.

R73 marked 220 22ohm resistor, measured 21.6 ohms

R17 and R19 were marked 472, 4700 resistor, measured 4600 ohms.

The voltages measurements were made with the main cap NEG lead as my GND lead to the DVM.

attached are two photos of schematic with voltage readings. I can do this all over again if needed, if there is something that may not be clear. just tell what you need.

getting closer??

thanks guys!!!
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

No problem... I miss stuff too once in a while... nobody is perfect. Your input is appreciated as always budm.

I figured that out with Pin 9, once he came up with all the measurements and I downloaded a different pdf about the TDA16888 from Infineon. It gave a ballpark what the voltages should be.

[budm]

Re: troubleshooting TV SMPS sequence

he used the negative of the primary cap. I said the same thing earlier, that the PFC combi IC is actually in standby mode at this voltage. It needs at least 14v to switch into operational mode.

Ah, I missed that info due to long threads, looks like you are on the right path, he needs to check those DCV of the filter caps for the VCC1, VCC.
It still does not make sense that he has 360V on the main cap if the IC is in the shutdown mode, something is not right.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

he used the negative of the primary cap. I said the same thing earlier, that the PFC combi IC is actually in standby mode at this voltage. It needs at least 14v to switch into operational mode.