troubleshooting TV SMPS sequence

[ilikesteel]

Re: troubleshooting TV SMPS sequence

BudWich, CapLeaker,

poking around in my stock of old boards, I found (3) marked 474 resistors exactly the same size. that's a 470,000 ohm resistor, very close to the 510,000 I need. I destroyed all three trying to install them!! they are a pain in the A__! they're so tiny it's ridiculous. trying to solder them in is almost impossible.

I guess I'll try the 1M ohm and see what happens. I'm sick thinking about maybe having to replace the 510,000 R36 resistor if that's what it is. I'll order a dozen!!!

give me a day or so to try the 1M..

Thnaks a bunch,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

little trick during install with the iron: clean both pads off clean. Add a little solder on one pad and put on the smd chip. Solder it on until it is flat and in its proper place. Then solder the other pad. Don't touch it anymore. lol
BTW: No problem with a hot air station! ;-)

[vinceroger69]

Re: troubleshooting TV SMPS sequence

i was trying to solder a small resistor earlier i found this video good and informative
https://www.youtube.com/watch?v=66GV4OuShzI

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker, BudWich,

Well .... Good news and Bad news.

good news: I kept looking through my old boards and I found 3 more 514 resistors, however, these were the larger size. I jury rigged it. soldered one end to the outside pad vertically and ran a jumper from Pin 18 to the other pad suspended in the air. checked for reliability, OK. I get a reading of 510,000K across resistor from pin 18 to outside pad on the board.

bad news: reassembled all cables, just as it was before taking back off TV. had the same symptoms as I first did. powers on, led goes from red to blinking green.

+12 fluctuates between .8V to about 2.4V, meter oscillates between those figures.

+24 fluctuates between 19V-21.9v meter oscillates between those figures.

Main Cap is 359V-360V. Im back where I started. I'll listen to what ever you have to tell me. I don't have a good feeling about this one. what's next?

Thanks again,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

This doesn't make any sense to me. If I just look at the 12 and 24V rails I would say that something is dragging the 12V rail down or something is fubar in the feedback circuit / TDA16888 IC.
Check around the optocoupler and IC10. Force the PSU on by itself, remeasure 12 an 24V again.
I am going to run out of ideas soon.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

ok I'll check components around opto isolater. I'll also dbl check that the 510K resistor I installed at R36 is still intact and not OPEN. Perhaps, I'll check voltages on TDA 16888 too and the PS power on with the PS board by itself, no connections to scaler, using 1K resistor between 5VSM and Pin & of PS, Pon/Poff.

Thanks,
ilikesteel

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker, Vinceroger69,

Vinceroger69, great video of soldering techniques.

CapLeaker,

Checked components around IC10, none of the readings jumped off the board as being questionable. I rechecked the resistor I installed for R36, 510K, it's still intact and gives a reading of .500 on 1M scale.

I jumped PS to turn on from 5VSB to Pin 7 PO/POFF, no cables attached.

+12 and +24 were both very low. like 1-2v on each. however, that was with no load.

I did manage to check voltages on TDA 16888. I'm not sure how to interpret the values I received. here they are. no load on PS.

PIN
____

1. PFC -IAC 1.5v

2. Vref .00-.60v (oscillating)

3. PFC CC .20-.50v (oscillating)

6. PFC CL .01v

8. Vcc 8.2-9.9 (oscillating)

Pins 11-16 0v

17. PFC FB 4.67v

18. PFC VC .04-.18v

19. PFC VS 1.2-1.5

20. AUX VS .30v



Thanks again,

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

how many volts do you have on the main filter cap now?

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

main cap 360V. ran a pig tail from neg side of main cap and clipped to NEG lead of meter to make the voltage measurements on TDA 16888. do you think I would get a different reading on TDA16888 if everything was connected as normal?

Thanks!!

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Well, something has changed. Now you report that both (12V and 24V) rails are low, like 1 and 2V. If you measure between the main filter cap negative and pin 7 on the TDA16888, you should see a short. There should be no difference measuring having the negative probe of the DMM on either place for measuring around in the primary.

I don't get it: You get 360V on the main cap and now almost nothing is coming out the other end on both rails on the secondary. I wonder if the transformer has a thermal fuse that blew?

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

I'm more confused than you can expect. perhaps the low readings were because there was no load on the +12 and +24. do the voltages on TDA 16888 show anything?

I did check pin 7 to the neg of the main cap and yes, there was continuity (short). I chose to add the pig tail to make it a little more safe to do the measurements. the pins on TDA 16888 are very close and I did not want to slip off a pin or touch another adjacent pin at the same time. I did the measurements from the bottom of the board.

are you confident the TDA 16888 is working? tell me where to look and poke and I'll do it. thermal fuse? I'll check the schematic again. I'm open for suggestions!

thanks,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

I do remember a situation quite a while back here on badcaps. Something wasn't adding up in the secondary, because the PSU wasn't installed in the TV. Apparently the PSU needed a GND bridge, which was made by installing the PSU back into the TV.

The TDA16888 is working, otherwise you don't have 360V on the main filter capacitor. I thought with that kind of low voltage almost next to zero on the secondary, the PFC should be off, but no it is still running.

It is just really weird. There is juice in the primary, but nothing coming out in the secondary. The only thing between is the transformer, inductors, diodes and the capacitors.
So if the secondary isn't shorted but the voltage is that low or nothing, I wonder if the thermal fuse went open inside the transformer. If that is open, no power will go through.

But still, it just doesn't make any sense having before 20V on one rail and 2V on the other, no it is just 1 or 2 V on both rails. When you measure on the secondary, you can't use the main filter cap GND. You would have to use the GND on the secondary.

By any chance... what is the ESR of the primary capacitor (if you have an ESR meter)?

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

I don't have an ESR meter. wish I did have one. remember, when the PS is reading the low voltage out on both rails there is no load on the PS and it's not installed in chassis. when I do get the +21V and the +2, everything is hooked up and connected as normal, except it's not mounted in the chassis.

I'm not familiar with a fuse in the transformer. I tried measuring the voltage at the transformer secondary, Budm mentioned that my meter probably would not read the voltage because of the higher frequency. where can I read the secondary voltage, after the DS3 and DS5 diodes? what should the voltage be at that point?

on the schematic there is a 101 1KV cap C5 across the primary winding. also, there is a component marked B4 connected to one end of that cap. what is B4?

can I measure voltage at the cathodes of DS3 and DS5??



Thanks,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

The voltage measured at the common cathode on DS3 and DS5 is DC and you should get close to the voltage what the rail is supposed to be. On some PSU's they are exact bang on, on others they may be little off.
Matter effect, if you look at the schematic, they are paralleled to each other. You could take one diode out, and it still would work, with diminished current capability of course. Your DMM may not be able to look at the AC as the frequency is too high. It is like 60khz or 120khz, vs a 60Hz regular 120V AC.

Got no idea what B4 is. That C5 with the R29 in series is a "RC snubber circuit".

O.k. I am getting really curious now. I'd like to have a high resolution picture of each board, straight shot and the whole thing together. Also again the model number of the TV.

[budm]

Re: troubleshooting TV SMPS sequence

B4 is just the Ferrite Bead inductor, same for B1.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

That's what I thought, without seeing it... must be some sort of a filter.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker, Budm,

attached are pics of the tv boards. I did sectional pics of PS.

TV is RCA L37WD12. the very small board at the top right is the 1W 5V STNDBY voltage board.

I reattached the PS to the TV, connected all cables. I did not mount PS to chassis.

The main cap still shows 359V-360V with set fully powered on.

I checked the voltage at the common cathodes of DS3 and DS5 for +12 rail, and also at DS2 and DS4 for the +24V rail.

The readings were the same as when I first checked the TV.

+24V rail showed +19.0 - 21.9V

+12V rail showed +1.5 - 2.1V.

any help with readings right at the diode junctions?



Thank you all again.

ilikesteel

[budm]

Re: troubleshooting TV SMPS sequence

The whole thing does not make sense to me yet, since the 24V and the 12V are generated by the same transformer, the outputs are regulated by monitoring the output Voltage of both 12V and the 24V as you can see in the diagram, the resistor RS14 for the 24V and RS19 for the 12V.
Can you remove inductor LS6 from the board and see if you will get 12Vdc between the two legs of CS32 (1000uF 16V)? if still no Voltage, then remove CS55. We have already verified that the inductor LS4A, LS1B, DS3, DS5 are OK. I am beginning to think there is something bad in the winding between pin 10, and pin 11 of T1.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Budm, CapLeaker.

ok, I'll try to get to a s a p.

thanks,
ilikesteel

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Budm, CapLeaker,

Do want me to power on and check voltage after removing CS55. Not quite sure what you want me to do. I will remove L6 and then fire it up and check across + and - of cap CS32. if I have to remove CS55 what do you need to me check with CS55out of the circuit?

sorry... I'm a little confused.

Thanks
ilikesteel