Vestel 17pw06-2 made some smoke

[jesterace]

Hi,
I made the unfortunate mistake of buying a couple of TV's that had these crappy vestel parts inside. I tried to fix this one and gave up, but now another TV with the same board has also blown so i bought a new board for that. I thought it would be sensible to compare the new and old blown board to see if it helps me figure out whats wrong, but so far i'm struggling.

The board is a 17PW06-2, the schematic i am using is:



When the board initially died, it only appeared to have one shorted output diode. I replaced the diode and the TV ran for about 2 minutes before making an almightly bang and leaving a skidmark on the TV chassis

A good inspection found that all of the current sense resistors (R301+R304 in my case) were exploded. The PWM controller ICE3BR1765J was burned on it's CS/Current Sense leg and tested short. Nothing else appeared to be burned or test shorted that i could see.

I replaced all of the output diodes, the PWM IC and the CS resistors with 2R2's. Oh and naturally the fuse was popped as well, so i changed that.

I took the precaution of powering up the board in the TV with a light bulb. The lightbulb stayed off thankfully, but i noticed that the red led on the set just kept flashing. The transformer(s) also kept ticking in time with the flashing led. My only understanding of this symptom from past experience has been from short circuits making the set shutdown and restart. I powered up the TV again without the lightbulb incase it was the bulb stopping the TV entering standby, but it still kept flashing and ticking.

I've been slowly comparing the new board i purchased with this one, making resistance comparisons and diode mode comparisons. I haven't come across anything that is way off. All the caps have been tested with an ESR meter and the values match up.

After studying the schematic some more i got the datasheet for the IC301 PWM controller and tested it's pins while turned on. I've found that the CS pin jumps about and hits up to 9v in DC mode. Apparently the max threshold is about 1v before the chip turns itself off and restarts? So it would seem there is still an over current situation, which originally killed the board in the first place.

I really don't know where to go from here, any ideas i can try?

P.S. one thing is niggling me and that is the output diodes of TR300. The original diodes were SR5100, but when i purchased replacements i was told that SB5100 is an equivalent. However, when i test the diodes in diode mode, i get a drop of .200 with the SR's and .300 with the SB's. Would this difference cause any problems?

Help, much appreciated.
James.

[tom66]

Re: Vestel 17pw06-2 made some smoke

I don't know if it applies to your set, but on the 17PW26 I've found differences between boards where some are built with the 5VSTBY and 3V3STBY on the power connector shorted (so the power board outputs only 5V) and others have separate power supplies though there still is a 3V3STBY... It's odd... But in any case, check all the little 0R jumpers on your two boards and compare them. Removing the appropriate jumper removes the short and the set works fine (despite receiving a bit too much juice on the 3V3)

[R_J]

Re: Vestel 17pw06-2 made some smoke

Are you sure you mesured the CS voltage correctly? 9 volts across four 2.2 ohm resistors that are in parralel can't be right, thats close to 18 amps. are you sure they are not open again?
What is the voltage on pin 7 Vcc (vcc_main_12)
Also check D314, you could have bad optocouplers, ic300 or ic301
The diodes should be fine, specs are virtually the same

[jesterace]

Re: Vestel 17pw06-2 made some smoke

Hi thanks for coming back to me, i actually did make a mistake with my reading of the 9v. When i released the ground tag i was using wasn't a true chassis ground.

Anyway after playing around with this all day this is how far i've got.

I found a zender diode d314 that was short circuit, although i seriously don't recall it being shorted when i compared the boards. I replaced the zener and now the TV does it's green/red flashing routine and goes into standby. HOWEVER... after about a minute, you get a click or maybe what sounds like a tiny spark and then the set turns off and does it's green/red routine again, goes into standby and the process repeats.

It's a bit of progress at least. Does this new symptom give you any more clues? After seeing the way this vestel model of boards seems to fail, with a nice explosion doing plenty of damage, i'm reluctant to keep turning it on again now until I've got a bit more knowledge. At least there is a bit of progress with it today.

I'm not an electrical mastermind, but was that zener something to do with voltage regulation for the FB line?

[R_J]

Re: Vestel 17pw06-2 made some smoke

The d314 zener seems to be protection for the opto. since the zener was shorted, the opto could be bad, it provides the feedback to regulate the circuit.
What voltage do you get at L301, it should be 12 volts.

[jesterace]

Re: Vestel 17pw06-2 made some smoke

Hi again,

I think i've gone back a step now. I scavenged an optocoupler from another dead board. I chose one that runs a different circuit (IC201) to be sure i wasn't picking out a dead part. It's listed as the same spec on the diagram that is used for IC301. The markings on all of the opto's on the donor board were different though, i guess a just different brand?

Anyway after installing the opto. the board was dead and started clicking. So i put the old opto. back in and there is life again, it cycles between red and green... but this time it doesn't sit in standby for even 1 minute like before. Now after the cycle, the led goes red and fades out quickly before cycling again.

The voltage on L301 bumps about between 4.6v and almost 5.8v. I checked the voltage on the PWM chips VCC pin too and that jumps around between 10.80v...12.XXv and even up to 16v at one point.

Weird thing about the optocoupler?

[jesterace]

Re: Vestel 17pw06-2 made some smoke

After trying it again, i've compared all the voltages on the PWM with the good board. Everything checks out the same except for the FB pin which is about 2.15v on the bad board, and upwards of 3v on the good one. The L301 voltage is hovering around 5-6v so this must be why.

[jesterace]

Re: Vestel 17pw06-2 made some smoke

For some reason i cannot edit my last post. Anyway, more info. Probed a few more voltages and although the 12v side of things does fluctuate, the thing that stuck out for me is the 5v line going from L401. It's pretty static at 3.4v. Going a bit drastic i decided to remove IC401 and inject 5V from a current limiting PSU. Even with the PSU turned on to 5.3v, L401 is still measuring 3.4v... so at least i know the 5v generating circuit might be ok? Something else along the line is bringing the voltage down

[tom66]

Re: Vestel 17pw06-2 made some smoke

L401 is part of a buck converter. It will convert the 12V to 5V by rapidly switching pulses through IC401 (Dual MOSFET) and L401. Is that step working?

[jesterace]

Re: Vestel 17pw06-2 made some smoke

I'm pretty sure that circuit is working, but it only outputs about 3.4v or so. hence why i decided to try injecting 5v into the output myself, but it still only registers 3.4v ish when i do that. If the TV is off, my injected 5v reads 5v at L401, but if the TV is on then it gets pulled down to 3.4v and jumps around a little bit.

[jesterace]

Re: Vestel 17pw06-2 made some smoke

Sorry i know i am probably sounding confusing and jumping back and forth. It's turning into a bit of a rabbit hole and of course i'm learning things too. Another little update though. It seems the zener D314 that i replaced may have been a little bad. I've got a Peak DCA tester that works with zeners and it was saying that the zener had a zener voltage of about 4v?!? Yet the one in the good board was more like 8-9v. I picked out another zener and replaced. It tests more consistently with the zener in the good board now. Except that the TV is in tick tock mode again.

Probing the BA pin on the PWM ic gives me 0volts or there abouts. This is the OVP protection line in the diagram. I might be understanding it wrong but it seems that they have used the OVP line not to check for overvolt, but to check for the presence of all of the voltage rails instead?

Oh dear this is a big learning curve.

[R_J]

Re: Vestel 17pw06-2 made some smoke

The OVP line comes from IC300, do you have a working 24 volt supply?
does the relay RL100 click in? I suspect it does not if the +24 is not there.
If the relay does'nt close the power supply is running with a reduce +400 volts which I suspect is lowering the other voltages

[tom66]

Re: Vestel 17pw06-2 made some smoke

So you used a current limited supply right when you injected 5V, so how much current was being drawn? How did you measure the 5V?

[jesterace]

Re: Vestel 17pw06-2 made some smoke

Hello again i have been spending a bit more time on this today learning.

None of the voltage rails are stable and the more i read the more people suggest to look at feedback, feedback, feedback... So I've made some attempt to try and test it.

I got the board on the bench again (no AC attached) and this time tried injecting 12v into the VCC of the PWM IC, and 12V into L301 which is where the the PWM 12V supply should arrive. I thought of doing this as it might help me to see what the feedback is doing when the 12V line is stable from my own regulated PSU.

This is interesting. When i supplied a bang on 12v supply to L301 and the PWM IC, the FB/Feedback pin of the PWM IC was about 4.05v. The datasheet says that anything over 4v will shutdown the IC (Overload/Open loop). At 4.05v, it wasn't quite enough to trigger a shutdown though for some reason, so I tested this further by increasing the injected voltage to about 12.5v and then the IC pulled the FB line down to 0.09v (shutdown). This seems to be correct with that the datasheet says. What baffles me though is why the FB line is reading over 4v when the 12v i am injecting to L301 is stable and correct? I'm assuming this is getting closer to the fault? I know when i probed the FB line on the good board, it was always well under 4v (3.2v or so).

So my theory at the moment is maybe there is something screwed with the feedback causing it to indicate to the PWM ic that the voltage is too high when it isn't?

It would be great if someone could confirm if my suspicion is right. I.e. Stable injected 12v at L301 should NOT result in 4v+ on the FB line...

Thanks,
James.

[tom66]

Re: Vestel 17pw06-2 made some smoke

The feedback may be set at a threshold higher than 12V. For instance could be designed to regulate at 12.5V.

The way these feedback circuits work is via closed loop control but to get stable operation in closed loop control a very high gain is necessary.

It is hard to explain why, but effectively, if the feedback circuit were set to a regulated output of 12.5V, then at 12.499V it would be off, and 12.501V it would be on. The gain is often on the order of millions, so the transition point is incredibly fine. This generally makes the circuit more stable.

Now in the real world the circuit will bounce around a little so the feedback voltage will vary. And there is compensation in the loop which slows this all down. The end result is there is an "error" signal that is generated by the feedback which communicates to the PWM IC what duty cycle is needed. It directly determines the duty cycle of the controller. At 4V it is probably at near maximum duty cycle which means it is trying to put more energy into the output. It is not succeeding in this instance because it has no drive energy.

The TL431 device is used to generate the regulation signal on this board. The TL431 is a "virtual Zener" device which acts like a high-impedance when its reference pin is below 2.495V. When its reference pin is above 2.495V it conducts. The capacitors from the reference pin to its anode (positive end of the "diode") control the response rate of the TL431; they ensure the circuit remains stable by adding a bit of "loop compensation". Without those caps, it is very likely the circuit will oscillate, this could be one issue affecting your TV.

I can see that the REF is set via R355, R352, and R353. R355=15k, R352=1.2k, R353=1k. Using a bit of Ohm's law. We want to work out what the output voltage will be when REF is at 2.495V (relative to cathode -
or ground in this instance). So the 1.2k and 1k resistors will have 2.495V across them. This means the current I = V/R, so 2.495V/2200 = 1.134mA. The REF pin draws almost no current - it is "high impedance" - and so it does not affect the circuit. And we can ignore the effect of the capacitor because we want to know the "DC steady state" of the circuit, as capacitors do not have any effect on a DC circuit at rest. So it is now a simple case of taking the 15k resistance and working out what voltage will be across it at 1.134mA. A bit of maths gives us 17V. Now this seems wrong, because when the 2.495V from the ref pin is added, this sets the output voltage at 19.5V.

I suspect that the Vestel schematic is wrong. It is not the first time I have encountered their schematics being incorrect. It could be that 12V is designed to be about 20V but that seems odd. Or more likely the resistor values in the schematic are wrong.

I suggest you measure the actual values of resistors and also the REF pin of the TL431 and use this to determine the output voltage set point.

[jesterace]

Re: Vestel 17pw06-2 made some smoke

Thanks this is really helpful.

I think i've concluded to today that the 12v FB is probably working as it should, maybe you can confirm:

The resistor divider for the TL431 on the real board is:

top resistor = 10,000ohm
bottom resistor = 2 resistors totaling 2500ohm

Doing a resistor divider calc, i think i'm right in saying that in order to get 2.5v out of this you need at least 12.5v in.

According to some datasheets for the TL431, the output voltage is calculated as:

Output Voltage = 2.5V * (1 + 10,000 / 2500) = 12.5v.

So the turn on voltage of 2.5v for this TL431 would require minimum 12.5v from the SMPS to begin regulating/sending error feedback to the PWM IC via the opto?

I tried this with the regulated PSU again and injected 2.6v... the TL431 turned on and the FB pin of the PWM IC went down to 0.09v. When the injected voltage was under 12.5v, the FB pin went up over 4v. I guess this is working.

I wonder if it's worth using this technique to test out the feedback of other rails? I'm not sure what approach to take as all the rails are so up and down that i guess it could be a fault on any of them that is triggering the TV to keep restarting/ticking.

[jesterace]

Re: Vestel 17pw06-2 made some smoke

Finally something good has happened. I just removed L304 to see if i can just isolate the whole 12v circuit on it's own. After doing this, the TV doesn't tick tock anymore (i get normal red/green flashing) and the voltage at L301 is a stable 12.3v! I didn't leave the TV on to try and fully start up incase it do more harm than good after removing L304. I just left it on long enough to probe the 12v. Seeing a stable 12v finally is promising!

So now i can be sure the whole 12v PWM creation, feedback etc must be good. Whatever is wrong must happening after L304.

[R_J]

Re: Vestel 17pw06-2 made some smoke

looks like the +12vcc is being loaded down, Could be bad ic401 or ic 403, try removing L414 then L402, then reconnect L304
You could check if those ic's are shorted pin S2 to D1 and D2

[tom66]

Re: Vestel 17pw06-2 made some smoke

Thanks this is really helpful.

I think i've concluded to today that the 12v FB is probably working as it should, maybe you can confirm:

The resistor divider for the TL431 on the real board is:

top resistor = 10,000ohm
bottom resistor = 2 resistors totaling 2500ohm

Doing a resistor divider calc, i think i'm right in saying that in order to get 2.5v out of this you need at least 12.5v in.

According to some datasheets for the TL431, the output voltage is calculated as:

Output Voltage = 2.5V * (1 + 10,000 / 2500) = 12.5v.

So the turn on voltage of 2.5v for this TL431 would require minimum 12.5v from the SMPS to begin regulating/sending error feedback to the PWM IC via the opto?

I tried this with the regulated PSU again and injected 2.6v... the TL431 turned on and the FB pin of the PWM IC went down to 0.09v. When the injected voltage was under 12.5v, the FB pin went up over 4v. I guess this is working.

Yeah, you have it pretty much correct.

In the method you've tested it, you'll pretty much only see the "0%" and "100%" conditions, i.e. the output is above or below the set point. Think like a thermostat, but trying to regulate a power supply output like that, would be difficult! Only works for very low load currents. Normally the PSU operates in the closed-loop mode where the duty cycle varies with the error from the output, so the FB pin will vary with, e.g. load on the output, or the AC voltage going into the PSU as the PSU will need more/less output duty to handle that change.

So by removing L304, you have removed the load from the main board.

So there are a number of things that could be going wrong here
- It could be that there is a fault on the main board drawing too much current on that line causing the power supply to "hiccup"
- It could be that the power supply itself is not stable under higher loads that the main board requires (bad capacitors, broken stability circuit, controller IC issues, etc.)
- It could be a combination of the two in some rare cases.

[jesterace]

Re: Vestel 17pw06-2 made some smoke

Went ahead and tried to narrow down the 12v fault a bit more while keeping L304 soldered in. I removed IC401/IC400 (this board only has a 5v circuit, no 3.3v). Removing those IC's made no difference. So then i tried disconnecting the +12VCC trace entering Q402/R435, but also no change. Tick tock again.

After exhausting those i think i was only left with VCC_AUDIO which takes a feed from +12VCC near the 12v output diodes. I found a jumper here that i could easily cut. After doing so, the TV will power up without clicking (L304 re soldered).

Only problem is, voltages all go quite low after a few moments. Well, the 12v/24v lines are normal for the first maybe 10-15 seconds and then the set does a slow blink (not a ticking though, more like some error code thing). After that, the combined 12v/5v rail falls to 7v and 24v just slowly falls. I guess the jumper i've cut may be more important than for just feeding VCC_AUDIO, i'm not sure at the moment. The led lamp slowly blinks between red and green now with the low voltages. Despite this, the PSU actually sounds better giving that reassuring buzz that I've been waiting to hear for ages.

Darn it i keep getting a bit closer though! Thanks for the continuing help.

P.S. it was J163 that i cut to try to remove VCC_AUDIO, although the schematic doesn't appear to show this jumper. I can see that the L200 line isn't arriving at the plug socket anymore when i check for continuity, so cutting this jumper would seem to have been a bad choice. I've resolder that up and resorted to removing r264 out of curiosity. That seems to have the same effect.