troubleshooting TV SMPS sequence

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

OK, I made some measurements this morning. some made sense and some where a little off, but I did get readings on questionable measurements. however, I did find one major issue. connected to pin 18 of the TDA 16888 is a resistor R36, according to schematic it's suppose to be a 100K ohm resistor.

It's not a 100K resistor, it's marked as a 514 smd resistor (510K). it's TINY. this resistor measured OPEN in circuit at all settings of the meter. I removed it from the circuit and checked it again, and yes, it was open. I almost lost it, it was so small.

could this one little tiny resistor be the cause? pin 18 is the PFC VC, Voltage Loop Compensation.

I did not measure any voltages due to the fact I found this resistor to be bad and it was in the feed back section of TDA 16888 just as you suggested.

anxiously waiting for a reply.

Thanks,
ilikesteel

[vinceroger69]

Re: troubleshooting TV SMPS sequence

I have heard of the ebook written by Jestine Yong, but I don't have it to read.

Same as ive been looking for this book too there is a torrent of the lcd monitor book online but not the smps power supply one.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

I have heard of the ebook written by Jestine Yong, but I don't have it to read.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

OK, I'll forge ahead! give me a day or two and then I'll update the post. I think I'll reconnect the cables and connections back to normal, this way I won't have all the extra wire dangling in the way and the TV will be in its natural state. Ill also check the main cap for voltage again under normal load/connections.

Have you heard of the eBook "troubleshooting and repairing SMPS power supplies" by Jestine Pong?

http://powersupplyrepairguide.com/

Thanks again for sticking with me, and also to the other forumites that have commented on the post.
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

If I work on a PSU, it will be by itself and I force it on. As for the light bulb load, you can do either or. Probably I'd be having no load on it first, because of all the wire jazz hanging around, but it may be harder to troubleshoot. Here is another piece of advice: When resistors go bad they go higher in resistance. You find one out of spec, replace it. Zener diodes can be hard to check, as you need to take them out and actually find the "knee" with a PSU or use the oscilloscope with an component test circuit.

A few days ago I had a dual voltage PSU here on the bench. It only had half of the rated voltage on both rails. Turned out to be a bad capacitor next to a heat sink that was in the feedback circuit. Exchanged that cap and the starter cap (for good measure) and PSU is ticking like on day 1.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

Thanks for the explanation. I'll first do a resistance check on the components you mentioned in the pwm/pfc circuit. when I do the power on test, do I leave the rigs that I have with the lamps connected, or do I reassemble all connections and check the pfc voltages just as it was when I first looked at it?

give me a day or two to study everything. I've learned alot in the last week or so about power supplies. too bad this supply is small and compact and tightly fitted with components. it makes it harder to troubleshoot.

thanks,

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Your are right... now you are getting deeper into the works of a PSU. Look at the spec sheet from the PWM/PFC combi. It will tell you the pin out. Some things you can't measure with your DMM as you need an oscilloscope. But not that you grab now the next best oscilloscope from a buddy, because you could damage it and the PSU if not used correctly.

Again, look at the spec sheet of the combi IC. The pin out will tell you what is what. For example pin 7 is GND, pin 9 is Vcc. A lot of answers to your questions are in the schematic and the data sheet. The ground reference point in the primary section is always the negative leg on the primary filter capacitor, but you can use pin 7 around IC1.

Some measurements you can do a safe way with power off. For example measuring the resistance of the resistors in series that supply the IC1 PWM combi and circuits around it (F,J,G, or sensing circuit P). There are always a few resistors in series. Usually a few 1MOhm followed by another one or a resistor with a different value.

How the primary works is: The bridge rectifier converts 120V AC to 170VDC. The Power Factor Correction circuit boosts the 170VDC to 360VDC or more. No 360V (only 170V) on the main filter cap means the PFC isn't working. Also no PFC boosted voltage means no 12/24VDC on your power supply. So the circuit does kind of work, so I don't really think it is the IC1 itself, rater a voltage supply problem to the IC or something going on with the feedback, telling it to fold back. The rectified voltage goes up and down a bit, same as the boosted voltage, as the incoming AC is not always steady either. More voltage going in = more voltage coming out.

BTW, if something measures bad or is questionable in circuit, either take it out or lift a leg and check the component again.

Budm may have a few more tricks up his sleeve, he is really good.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

OK CapLeaker, I'll give it a shot. this is by far the furthest I've been in troubleshooting the any PS. are the voltages that are on the chip all DC voltages? can I use pin 7, the GND pin as the common GND point. also, could I use the NEG leg of the bridge rectifier as a GND reference? I'll check only the voltages on the IC pins.

getting back to the main cap. shouldn't the main cap have a steady voltage on it. that shouldn't oscillate, right. what produces the main cap voltage? isn't the main cap before the IC?

let me read through the pdf and setup some logical plan of attack, then I'll report back.

please answer the above questions before I start.

Thanks again,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Not sure what the problem is at this point... is it something in the feedback that changed (resistor, capacitor, diode, transistor), not enough power to power up the TDA16888 PWM/PFC combination IC reliable, or that this combi IC is bad.
Whatever you measure in the primary section, be extra careful and don't slip. You are on mains potential.

I would start with measuring around that PWM/PFC combi IC.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

I'm open for suggestions! what's the cause of the low main cap voltage, main cap questionable, not charging all the way? is it the main cap that boosts the voltage up to 360V and greater? I don't what normal is for this set.

thanks
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Umm... blinking bulbs you say? They shouldn't be doing that. They should be full brightness on both rails. Yeah... the 24V rail and the 12V rail are on the same transformer. So what one rail does, the other rail will do the same thing. 346V on the main cap isn't high enough for my liking, as it should have like 360V or more.
So you had only 2.6V on the 12V rail and 7.9V on the 24V rail? Seems like the 12V and 24V rails can't handle any load. Hmm... maybe we are barking up the wrong tree on troubleshooting the 12/24V secondary... maybe there is something wrong with feedback circuit to the primary.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

made the measurements you wanted.

here goes:

main cap checked while under load test. voltage was oscillating between 346V and 359V. again, meter reading was oscillating up and down between those two voltages.

+12 rail, oscillated between .3Vto 2.6V. you can actually watch the meter voltage reading change up and down. the lamp was blinking at a steady off/on rate.

+24V rail, oscillated also from .8V to 7.9V, same results as the +12V rail. meter oscillating between voltage readings and the two lamps also blinked simultanious on/off.

hope this proves something and leads me to the fix.

Thanks CapLeaker
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Ok. Here is how this bulb trick as a load works. One bulb 12V lets say 1A. Two bulbs in series are 24V, 1A. Two bulbs in parallel are 12V, 2A.
So you put the single bulb on the 12v tail and the 2 bulbs in series on the 24V rail. Fire this thing up and measure the voltage on both rails.
A half an amp is a bit thin, but the idea is to see what the PSU does when there is a little load on it. When under load, also check the main filter cap again.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

Before attempting the experiment of the voltage measurement, here's what I have rigged so far. I'm ready to do it, just want to make sure what I have is correct. I don't want to blow up the PS or me!

enclosed are 3 pics, one each of the wiring rigs I made and the other for the 5VSB through a 1K resistor to Pin 7 of the PS.

I checked the wiring rigs for operation by connecting them to my car battery. they worked perfectly. the single 12V was nice and bright. the two in series using the 12V were dimmer. I took one of the two in series and connected it directly to the 12V and it was bright.

The lamps I'm using are #89 automobile bulbs, rated for 7.5W, .5amps, 12.8V

the cables from the PS conn3 to the scaler board bp103 are completely removed.

what do you think?

Thanks again,
ilikesteel

[MLOO55]

Re: troubleshooting TV SMPS sequence

Un short, the proble sm arises un los voltage at the output,really, as I can read even withouth load so the problem should be un the hoy si de por the feedback, I think so. It would be good idea to check the voltage at the ic un spite the voltage at the main capa are ok.Mayor the resistir change its value.
Rgds
Mloo55

[CapLeaker]

Re: troubleshooting TV SMPS sequence

My post said 12Ohm. It should read 12 Ohm. The space went missing. Doesn't matter... you got the idea anyway and that is what counts. Like I said, you can load the rails up either with light bulbs or with a resistor, depending what you have on hand. The chance of having a large resistor is lower that using automotive lights. Personally I have a few homebrew "rigs" that I use. Of course a programmable DC load would be better, but I can't justify the price for one. Get yourself some automotive light bulbs. 2 in series, you can test 24V. It doesn't have to be 1A exactly, just within PSU specs for now.

As for me, I would look that the PSU gets forced on without any other boards / backlights connected. Then I'd put one light bulb on the 12V rail and put two 12V bulbs in series on the 24V rail. Turn the PSU on and measure both voltage rails while under load.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

"bump"

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

I want to do this correctly to be sure it's the right thing to do. should I try and find a spot on the PS to jump the 5VSB to pin 7, or take it from the small board that supplies the 5VSB to the on/off switch? I never checked for the 5VSB on the main PS board.

your post said 1 AMP draw with a 120 ohm resistor

on the +12, a 120 ohm resistor with give me .1 amp @.12watts.
A 12 ohm resistor will give me 1 AMP @12 watts.

on the +24, a 24 ohm resistor will give me 1 AMP @ 24 watts (big)
a 240 ohm resistor will give me .1 AMP @ 2.4 watts.

will the smaller amp draw be ok for both?


which do you prefer?

Thanks for your patience
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Right now in order for the PSU to turn on, you have the scalar board connected. At least that is what you said in one of your posts. I want to get rid of that, so the PSU gets forced on without the scalar board connected to it. Pin 7 on Connector 3 (scalar connector) is On/OFF cording to the schematic. So put a 1k resistor between it and STBY voltage.

As for a load to test the voltage rails, use some light bulbs, or calculate the load resistor.
V=IR; for 12V you need like a big 12Ohm resistor. They get hot in a hurry, so you need a large one.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Capleaker,

you want me to connect a resistor between the +12 pin and the GND pin on the power supply as a dummy load drawing 1AMP, right?

should I also do that for the +24V supply, or leave that on the scaler board?

I'm confused about your following statement:

"I also would like to be able to get the PSU running without that scalar board attached. So try to unhook that and also jump pin 7 on connector 3 with your PSon together." sorry, please make that more understandable for me.

Thanks CapLeaker
ilikesteel