troubleshooting TV SMPS sequence

[ilikesteel]

Re: troubleshooting TV SMPS sequence

just a BUMP.

[budwich]

Re: troubleshooting TV SMPS sequence

hmmm... not sure what to say. The "5v thing" looks like the scope wasn't triggering and hence the waveforms were all over the place. Might want to "line trigger".

On the others, not sure that you should be measuring with a load on the outputs but that just a comment... I would normally do both to see if there is a difference. Although you are thorough with your observations, I do believe you need to choose a time scale that allows one to see multiple "cycles" of the "DC" ... why... because you previous indicated that the bulbs flashed... but the wave form that you show doesn't really indicate that would be the case.... but maybe a "1.5" volt "swing" is enough to cause a bulb to "flash".

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Budwich,

remember, I spread those two signals out so I could capture on camera the fall and rise of the signal. It does not look like ripple. it's almost like a square wave or a degrading square wave signal that has a definite cycle of low to high. we're talking about a 2V swing from low to high. when I switched the time back the other way (to the left), you see two solid lines jumping up and down.



where could that kind of signal be generated from?

on the 5V vsb, that's a jump of about 4.75V to 5.36V. is that signal a being generated on the 1W small 5V board? both boards have separate signals that are jumping around. I'm really confused. what other measurements can make with the scope??

can we determine if the problem is on the HOT side or the COLD side of the PS, any guess? how can we determine if the problem is on the HOT or Cold Side? I'll be glad to make more types of measurements, just give me some guidance.

I can reconnect all cables and put TV back to normal. recheck the output voltages with the scope and see if they remain at +10-11v on the +12 and 20-22V on the +24 with the TV in normal operation.


what do think is the cause of the 5V VSB signal, cap 616 on the 1W PS board, 470uf at the output? I'd be glad to change that out and also the Main cap 180uf 400v if you think that may help?



Thanks very much,

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

I am a little bit late today, but there is something wrong with the 5v supply. It should 'the do that. Change the 5v output capacitor for now, retest.

Now for the other 2 rails: they really seem to go up and down in voltage. Question is: do they do that in a repetitive timely manner, or erratically?

[budwich]

Re: troubleshooting TV SMPS sequence

:-( again not sure what to say.

first, your investigation is much like a "search". You look at things at a particular point, in this case, the output of the supply going towards the panel / scaler. You look at what's there. It appears "funny" as in not regular. It is unlikely that the output is caused by the scaler since it ain't connected. That means that something further "upstream".... so you need to "paddle upstream"... :-) Look at the waveform coming into the diodes... what does it look like... is it similar or different than that of the output.... that perhaps tells you there is an issue with the diodes... maybe depending on what things look like. Does this make sense?

Anyways, before you go there, you need to determine any kind of "cycle" / "period" that you might be seeing. You are less concern with the value of "ripple" as oppose to period. That might tell you if something is wrong with any diodes or capacitors not doing their function maybe. NOTE: yes ripple is important if it is really bad... not sure if you are seeing that level of "ripple".

The usual purpose of a scope is to provide details about the waveform in terms of frequency, peak to peak values, and timing. These are the aspects that you need to compare and look at each point along the circuit. People use multiple trace / channels to do this more easily but a single channel can be used as well if "general waveforms" are being looked at as opposed to when closely timed digit type signal circuits are involved.

PS... as cap... just stated you need determine more about the output's "cycles"... if any repeatable trace patterns show up.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker BudWich,

ok I understand what you're saying. can I make scope measurements where the signal comes out of the secondary of the transformer for each rail safely? can the scope measure the voltage at an increased frequency rate before the diodes?

I guess since you haven't suggested it, I cannot make any safe measurements with scope on the primary side, right?

I'll look for a 470uf Cap with a similar voltage to replace on the 5V VSB board. for now I will leave the PS it as it is with NO LOAD on the rails. At least I'm getting reasonably close voltage outputs but with alot of junk within the signal.

I'll report back.

thanks again for taking the time to look at all the stuff I posted.

ilikesteel

[budwich]

Re: troubleshooting TV SMPS sequence

the frequency response of your scope is the limiting factor of what it can "see"... most basically scopes do 10-100 mhz (and more of course) which is way more than what you will be seeing.

Remember I was referring to your "bulb scope" test... all you are doing now is using a "big bulb" called a cathode ray tube "bulb" that has a little more capability to help you understand what the "bulb" is trying to tell you about what it is "seeing".

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Again, working on the secondary is no problem, even at the transformer on the secondary winding. The scope can handle that frequency with ease. That is adjusted with the time base on the scope. Now you be able to "see" the low voltage AC at 60khz or whatever. Now watch the AC and see if the voltage level is also bouncing. If you want you put one channel of the scope on lets say 12V DC, and the other probe on the secondary output transformer.

Here is a funny thing I noticed. It got mentioned now a few times that when the TV is back together and turned on, the 12V rail is showing 2V, while now it is showing 10 ror 11V unloaded?

Make another snapshot of the secondary DC, when then the voltage goes up and down, so I can see the almost square wave. 3 of them hops in one pic would be nice.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Budwich-CapLeaker,

I found a replacement for the 470uf on the 5 VSB board. same 470uf, just a higher voltage rating. afterwards, when I plugged the TV power cable into TV, I had the scope on the 5 vsb. It came right up to about 5.30 volts. now there was a little bit of noise on the signal, but it was very close to the max voltage . line nothing like the previous photo with all the ripple changes in it.

However, The +12 and +24 still have the voltage changing with about a 2V swing as shown on the scope photos previous.

I started to poke around DS3 and DS5.
I couldn't see a signal on the anodes of DS3 and DS5. like you had suggested, the scope may not be able to display the higher frequency signal.

I did manage to get the scope to view the common cathodes of DS3 and DS5. wow, this was a total shock to me. attached are photos of the +12 captured on the screen like yesterday and of the signal at the common cathodes of DS3 and DS5.

the scope was set to .5V per V/division, 20ms for time/division
the common cathode signal as you can clearly see goes from the GND line at the very bottom of the display, up to about 10V, with a huge spike or clocking/square wave riding on top going up to almost 30 volts. I don't get it.


what do you think is going on?

thanks
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

O.k. on the capacitor on the 5V rail and I've told you before that a bad cap will jump right out on a scope. Another thing we knew that was bad and it's off the check list. The early give away was the rising voltage when the set was cold and rising voltage.
I guess you learn as you go! Interesting things are happening inside a switching power supply!

The scope should not have any problem picking up the high frequency, low voltage from the transformer on the secondary winding. You just have to adjust the time base to make it scan faster.

With the first picture, I can't do nothing much. However in the second picture is some evidence I was hoping for.
The spacing in between... is that always the same, or is that erratic?
Do you have the scope set to AC or the DC coupling?

I am not sure what's going on yet, still trying to sort things out. I kinda can't believe that there is a 30V spike on top of 10VDC. That would knock the caps out on the secondary in a hurry. Unfortunately the picture is too blurry.
What I see is this: There is a square looking wave that turns sharply on and off. At 0.5V per division, it is 3V high (not 30) and it is roughly 70ms long followed by a 20ms break to the next one. I also see squiggly lines inside the square, but it's just too blurry.
Having fun yet?

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

with voltage/div set at .5V per division, using a 10X probe, that would make each division 5V, NO? second pic, clearly shows an approximate strong 10V level, with the big clocking/square wave looking pulse jumping up to almost 30V. that's what it looks like to me.

it's hard to hold the scope probe on the cathode of DS3 and fiddle with scope. the clip on the end of the scope probe cannot get enough of a bite on the center leg of the diode. I may have to improvise.

if I'm getting a funky signal there, where can it be coming from? do you think it's before the transformer? how do people troubleshoot SMPS's on the HOT side?

I cannot check the input side of the transformer, correct because of grounding the scope?

How can I check the main cap with the scope? wouldn't that be a helpful signal to look at? I will try and get more voltage samples with more definition.

If the funky signal is at the cathode of DS3, I would expect to be getting the same results at the output. I think we are because we know it's oscillating.

the primary side is being very mysterious. can I make DVM measurements somewhere and see if it's oscillating somewhere on the primary.

sorry, just blabbering right now. I feel we're so close to fining it, but seems like miles apart too.

Thanks,

ilikesteel

[budwich]

Re: troubleshooting TV SMPS sequence

you need to backup a bit. What / how exactly are you using your scope.... ie. are using a single channel or multiple channel (one probe or two?). next, where are you attaching your ground for your measurements? Where are you attaching your probe (each one?).

Why am I asking things like that? Your statement "I couldn't see a signal on the anodes of DS3 and DS5" doesn't make any sense... first because there are TWO "anode" points involved in each "diode package". It is highly unlikely that you would get no signal at both those points. The reason that you are not getting any signal on one of the points maybe TRUE is you have set the ground connection on the GROUND as shown in the circuit diagram in your thread.... this is because that point IS ground all the way back to the "anode point" in question. So you need to be clear on your test points.

I too am having a hard time viewing the pictures clearly (partly causing I am viewing these on a tablet) so take my comments with some guard... but the second photo almost appears to be some form of two channel trace, one showing a "higher frequency AC wave" along with a second, low frequency, DC like wave. High frequency one, I would say was coming from the input into the diodes while the low frequency on is somewhere on the other side of the rectifiers.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

O.k. on the 10x probe. That would indeed set 0.5V to 5V, unless the scope is set to x1, x10 or x100. Some scopes do that even automatically.
When you look at a (working) PSU lets say 5V and you set the scope to DC coupling, and 0.5V per division, you should see 1 division, correct?
If something is hard to fiddle with, always solder on a piece of wire to make a secure connection. Not worth slipping off etc.
You cannot check the primary side of the PSU because of grounding, yes. You don't want to short anything to GND of your scope.
The main filter cap is in the primary, so no.
The funky signal what you see I have no idea yet, but it is (what I could see of the blurry pic) at the same time there than the square.
I know there is something called Back EMF, but that doesn't fit this case, as the squiggles are within that square and not after. Also the PSU doesn't turn off and on.

Logic tells me that something would have to fiddle around with Pin 14 of the TDA16888 (PWM output voltage sensing input) in order to make the PSU raise or lower voltage.
The PSU actually handles some current, but it would be interesting to see what the same rail actually does under load (lamp trick).
Man, this is getting to be a long post. I printed out the Schematic and I put down all the voltages you measured on the print out from this thread page 5, post #87.
The problem I am having right now on the TDA16888 is why isn't there any voltage on pin 14 and pin 2 also pin 6? Can you check R67, THR1 and measure pin 4 of that PFC combi IC?

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CpLeaker-BdWich,

I can recheck the IC tda 16888 on the pins you requested. I did it with the DVM before running a pigtail wire from the NEG of MainCap as the gnd.

Can I check that with the scope, using the scope GND connected to the pigtail wire from the main cap NEG?

I'll try and provide more accurate data in reference to the scope setup and where I'm testing.

just for info, all measurements on the secondary side of the PS I used the chassis GND, or the 3 GND pins on the PS connector conn3, pins 2-4.

also, using one x10 scope probe, only one, connected to CH #1.

get back to me on the main cap scope possibility. I'll try and do the measurements tomorrow.

thanks,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Don't hook up that negative lead of the probe to the negative of the main filter cap, last warning. You will short it to EARTH GND through the scope! Now if you measure with your DMM between the negative of the main filter cap and GND of the board it will measure open. So again, don't do it! I know, looking at the schematic, it seems like everything on the primary has the same GND as the secondary, well it doesn't!

To measure the main cap: you either need: switch it out with a new one, ESR meter, a oscilloscope and a signal generator, a Isolation Transformer that separates the PSU from the mains, or with a differential probe (measurement). Look up youtube : differential probe
There are some that have a specific differential probe and others that use 2 channels at the same time, not hooking up the probe GND, instead using one channel as positive and the other as negative.

You did just fine using the GND points in the secondry of the PSU.

BTW, also check that Pin 9 on the TDA.

"I can recheck the IC tda 16888 on the pins you requested. I did it with the DVM before running a pigtail wire from the NEG of main cap as the gnd."

Correct!

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker-BudWich,

OK, made some measurements on IC TDA 16888. I did not make any PS output voltages with the scope this time.

main Cap = 366v-367v.

THR1 = 6.6 ohms measured in circuit, no power.

R67 marked with 221, 220 ohm resistor measured .215 on 2K scale. (215 ohms)? measured in circuit no power.

now for the TDA 16888. many of the voltages are oscillating similar to the secondary readings. how can that be, they're on two different sides of the PS, Hot and Cold.

Everything in RED is oscillating just like the secondary output voltages for the +12 and +24.

Pin 1 1.5v PFC-Iac ac line voltage sensing

Pin 2 0-.6v Vref 7.5v reference

Pin 3 .2-.7 PFC CC PFC current loop compensation

Pin 4 0v PFC CS PFC current sense

Pin 5 GND

Pin 6 0-.14v PFC CL sensing input for PFC current limitation

Pin 7 GND

Pin 8 0-1.2v PFC OUT PFC Driver output

Pin 9 6.6-8.3v Vcc supply voltage

Pin 10 .01-.2v PWM OUT PWM driver output

Pin 11 0v PWM CS current sense

Pin 12 0v SYNC Oscillator synchronization input

Pin 13 0-.24v PWM SS PWM soft start

Pin 14 0-.2v PWM IN PWM output voltage sensing input

Pin 15 0-.06v PWM RMP PWM voltage Ramp

Pin 16 0-.4v ROSC Oscillator frequency setup

Pin 17 4.73v PFC FB PFC voltage loop feedback

Pin 18 0-.24v PFC VC PFC voltage loop compensation

Pin 19 .01v PFC VS PFC output voltage sensing input

Pin 20 6.4-6.5v AUX VS auxillary PS voltage sense.


why are the voltages oscillating on the chip, where can it be coming from.


Thanks guys, your special for sticking with this so long. I find it a learning curve!

thanks,
ilikesteel

[budwich]

Re: troubleshooting TV SMPS sequence

If I understand things right, the vcc is too low and thus the ic is not turned on, but is in standby.... maybe

[ilikesteel]

Re: troubleshooting TV SMPS sequence

BudWich,

noted BudWich. let's see what CapLeaker has to say. the problem must be on the hot side.... I think. I'm not too familiar with PFC circuits and their components. If somebody says "you have to have xxV at this point, I can check it and report bck what I measured. it's unclear to me what voltages are right and wrong. it's interesting but a little over my head.

thanks
ilikesteel

[budwich]

Re: troubleshooting TV SMPS sequence

no problem... I am just going by the application sheets.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

PFC circuits can drive even a seasoned electronics engineer mad. However, I was able to find another datasheet from Infineon on the TDA16888 that explains a little what voltages to expect. Page 17. See attached.

I am pretty sure that the problem is in the primary section of the PSU.

Pin 9 has to have at least 14V (max 17.5V) for the IC to go from internal standby mode into operation mode. If it falls below 11V, it goes back into standby mode.

Next interesting thing:
If necessary both outputs, PFC OUT (pin 8) and PWM OUT (pin 10), can be shutdown on external request. This is accomplished by shorting the external reference voltage at pin 2 (VREF) to ground. To protect the external reference, it is equipped with a foldback characteristic, which will cut down the output current when VREF (pin 2) is shorted.
(Well we only got 0.6V here tops!)

Finally, In order to reduce the overall current consumption under low load conditions, the oscillator frequency itself is halved as long as the voltage at pin 13 (PWM SS) is less
than 0.4 V (disabled PWM section).
(Got only 0.24 here)

Pin12 SYNC isn't hooked up in our case.

So it looks like the PSU is protecting itself for whatever reason.

The only thing that looks good to me is Pin 17 PFC FB and Pin 20 AUX VS.