troubleshooting TV SMPS sequence

[budm]

Re: troubleshooting TV SMPS sequence

I just download and read the spec sheet of the TDA16888, that VCC on pin 9 of the TDA :Pin 9 6.6-8.3v Vcc supply voltage per your reading is way below the under voltage shutdown point. What did you use for the GND point for the meter.

Page 20:
Undervoltage Lockout
Vs,up is 14V Typ.
Vs,dwn is 11V Typ.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

If you would have an ESR meter, you could have ruled out capacitors a long time ago.

With pin 9 being so low, the PFC is in internal standby mode. It could very well be another capacitor problem, causing the PFC to change when the VCC juice runs out.

[budm]

Re: troubleshooting TV SMPS sequence

That TDA chip does the job of running PFC and SMPS circuits.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

BudM,

WOW. Great deducing!! I will try and follow you directions and make measurements you requested.

I think what you're saying is with the Pin 9 Vcc pin oscillating between 6.6-8.3v, the rest of the outputs on TDA 16888 are being affected by the same oscillation causing problems.

Manny Thanks
ilikesteel

[budm]

Re: troubleshooting TV SMPS sequence

'Pin 9 6.6-8.3v Vcc supply voltage' I think the clue is that the VCC should not be fluctuating like that.
The VCC is generated by STBY power transformer T2 winding pin 5 and 6, the AC is rectified by D30 and filter by cap C36 (should be checked for Capacitance and ESR, or just replace it), the dc is then fed to R73, output at 'Q' VCC1 is then fed to switched transistor Q1 (2SB1132, PNP) EMITTER (check the DCV to see what you get at this point), Q1 is turned on by OPTO IC2A/B, the OPTO is turned on when the ON/OFF (PS-ON) is present. The Output (VCC is clamped by 15V ZENER Z2)) is at the Collector pin which is fed to F2 (you need to find out what the F2 device is) and filter by cap C27 (check or replace this cap) and fed to IC1 VCC PIN 9.
That VCC is also used for turning on the SOFTSTART POWER MOSFET Q6.

[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.

[budwich]

Re: troubleshooting TV SMPS sequence

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

[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

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

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

[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

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

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?

[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.

[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

[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

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

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.

[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".

[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