Sigh, another dead VESTEL 17PW25-4

[Amjama]

Re: Sigh, another dead VESTEL 17PW25-4

Ok, is 17pw26-4 same as 17pw82-3? Getting confused, my initial question was is 17pw82-3 compatible with 17pw25-4.

cutting traces! What have you been involved with!?

[tom66]

Re: Sigh, another dead VESTEL 17PW25-4

17PW26-4 and 17PW82-3 are based on the same design & I believe they are interchangeable but I cannot be sure.

I did modify a 17PW26-4 I had lying around to fit a set with a failed 17PW25-4. It worked, but it required a lot more than I thought it would. The power supply pin out was the same, but the 5V & 3V3 rails were different.

Basically, 17PW25-4 has no 3V3. The 3V3 is generated on the main board. The 5V rail can supply 5V @ 2A for USB etc. This is always available, the 5V stby and run rails are identical on the 25-4. The 17PW26-4 has a 3V3 & 5V rail, the 3V3 is the "dominant" rail and can supply around 1-2A, the 5V is a low current rail at around 50mA or so. There is also a high-current 5V rail but it is only available after the PSON signal is asserted.

So what I did was initially to get the 26-4 to work in place of 25-4 was to cut the 3V3 supply off and only feed the main board from the 5V standby supply. The problem is this 5V standby is only rated for maybe 50mA or so and the main board draws far more once it has "booted" up causing a brownout reset.

I tried connecting the 5V main & 5V standby rails, this now meant the 5V standby could supply a lot more current. The problem with this is the 5V rail had a lot of load on it before the PSU started fully (because this PSU uses boost PFC, the rails start a little later than 25-4 which starts nearly immediately) & it had a dip on it when powered on. And about half the time, the dip in the 5V rail from the turn-on current would cause the main board to "reset" so the TV wouldn't power up... It would work sometime though, so it gave me some hope...

I figured that the load on the 3V3 was not high enough. Reason being the 5V rail is generated by linear regulator from 12V standby. And the 12V standby is unregulated, it varies from about 11-13V depending on 3V3 load. Under sudden load, it can dip and cause 5V to dip too. So first I tried adding lots of capacitance to 12V rail. This still didn't work as the capacitance required would have been huge for the dip that I was getting. So I added a small 100mA load to the 3V3 (two 68 ohm resistors in parallel) which kept the 12V rail a little higher. This worked well enough to get the PSU to start every time. I was quite happy - but I had spent far too much time on it!

The 26-4 ain't too bad of a PSU, it's got crap caps but otherwise is well designed, it's a shame Vestel tried to use the 25-4 PSUs as they're dreadful. I have no idea who designed the 25-4 but they really are a terribly bad example of design to the lowest possible cost. The errors in the design make me think it was someone who was not very experienced. The diode voltage overshoot on turn on (https://www.badcaps.net/forum/showth...137&highlight=) is just one example of poor design. The power supply also suffers serious stability issues, and produces strong audible noise in operation - especially when the LCD backlight is dimmed due to the PWM dim carrier beating with the PFC circuit.

[Amjama]

Re: Sigh, another dead VESTEL 17PW25-4

The guy from eBay, said that the 17pw82-3 is a direct replacement for the 17pw25-4. Still doing abit research before I buy that.

However after your experience of trying (and succeeding) of using 17pw26-4, perhaps I shouldn't try and mess around lol.

I am struggling in repairing the 17pw25-4 but can't even do that! All my reference voltages are fluctuating, so nearly giving up on that. My second plan was to find a replacement for my psu 17pw25-5.

Yes I read your thread of the bad design of of the 17pw25-4. Very interesting. I will try once more to try n repair it if not I will use it as door holder if is good enough for that!.

I can imagine all the hard work you have put for both type of power supply (25-4, 26-4) you should redesign it and call it tom66-4.

Cheers

[exlink]

Re: Sigh, another dead VESTEL 17PW25-4

A freebie 32" LCD here. HITACHI brand, but a rebadge Vestel junker by Argos (who are better described by a four letter word beginning with "s".)

17PW25-4 which is dead, no standby, no run power.

Blown mains fuse, primary MOSFET and assoc. components. Also has a shorted diode on the output side (which is what blows the FET in the first place.)

These power supplies are terrible! But the design is so poor, nothing you can really do to fix it, only make it as good as possible.

Because Vestel do not match the output diodes, failure is common.

They probably put all the diodes into a bin, with those pre-turned leads, then pick them randomly!! No lot assignment or matching! This of course does not match them anywhere close to each other... and 7 of them are all in parallel. ARGH.

I measured them, readings were: 443, 444, 443, 460, 433, 438, and 000 (shorted.) Readings mV.

Now, what can be seen here - obviously the shorted one is failed, but it can be also seen that the diode reading 460mV was probably taking close to none of the load (because diodes have an exponential I-V curve, a small change in forward voltage will lead to a large change in current), and the majority of the load was probably taken by the one which shorted, failing that it would have then been split between the 433 and 438 diodes. The rest probably didn't take anywhere near a significant portion.

Terrible design! And don't forget about the -2.1mV/degC variation for diodes. That's negative, so as it gets hotter, the forward voltage drops further causing more current and more heat, a rapidly destructive cycle.

This is the 24V, 5.5A backlight rail, but these diodes experience ripple current of around 2X~3X due to it using a flyback topology. No idea how they snuck a 150W flyback past the EMC guys...!

What can be done? I think the best I can do (as VESTEL love putting crappy UF5402 in parallel) is to match them before install by choosing diodes within +/-2mV and also by using some thermal epoxy to closely match these devices.

This isn't even going into the other issues these supplies have, such as the wildly unstable control loop (1.5Vp-p output ripple on 24V when backlight is at 50%, due to poor compensation and increasing capacitor ESR) and the noisy buzzing they make when operating at certain brightnesses (related to the backlight level.) Plus the odd standby controller fault due to pulling about 15W from the 5V standby rail with no heatsink on the standby IC (when operating...)



/rant over

Hello Tom and every tech brains here

How is going great. I have a problem with one of this vestel crappy psu and though am not really that good tech of a guy but i really need solution to this.

My tv keeps flashing blue led light and will not start. Sometimes when you up plug it and plug back in 2 or 3 times, the blue led will now steady (standby mod) and it will now power on. This have been going on for about 3 months and it pisses me off to sit and wait for your tv to take like 20 - 30 mins to com on.

However, for about three days now, the set is not coming on again no matter how long a wait and plug and unplug, it doesn't just come on. Have taken it to two repair shops, one said is the main board (picture board) the other said it is the power board. Don't which to follow as both are costly to repay.

I opened and check the power board and found out it is 17pw25-4 and i have read a lot of negative threads on that board. Their is nothing like blown diodes or capacitors or resistors their and am puzzled.

What should i do? I have attached the picture of it to this post.