17pw26-3 Psu Unstable Output Any Ideas ?

[ReeceyBurger123]

Hi all, working on the Vestel set its a 42" Hitachi lcd with 17mb35-4 with a 17pw26-3 Psu.
Now the Psu was originally dead the main fuse was blown and the mains Ac bridge rectifier was shorted on the negative diode. I replaced the bridge with a known good one and the mains fuse I also checked for S/C on the other parts in the primary side and nothing out of whack apart from low resistance between positive and negative but I ignored this. After powering it on the Pfc Fet and Controller exploded..... I replaced both of these and checked again for S/C and now the set now gives Stby.
The problem now is that the tv acts goofy as the Psu isn't working properly, firstly the backlight would fire on the 24v starts at 35v then backlight turns on and this 35v sags to 18v and the tv re-starts, also the speakers are buzzing loudly with backlight plugged in and unplugged. The 12v is going all over the place with Mobo plugged in and unplugged so I know that Psu is still bad but the PFC is working fine 470v working and 330 in Stby so don't think its related to what I fixed.
Just after some help on where to start I don't have a scope atm and only a DMM if anyone has any ideas please let me know thank you.

[tom66]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Check the transistor that provides Vcc to the controller IC and PFC. It's a small through hole device, near the 8 pin DIP standby controller. Also check the diodes on the output for shorts and bad capacitors.

Your meter may be lying to you if it is autoranging, they can sometimes overshoot on readings which change quite a lot. An oscilloscope (you should get one if you can) can rule this out, or you can use a multimeter in a fixed range (some autoranging meters allow the range to be locked.)

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Hi Tom cheers. I have already recapped this supply I will check that transistor in a min. Also the diode is fine so thats out of the window. If im correct is that transistor the one that you replaced with a Tip40 in your youtube video ?

Also getting a scope soon just had too much todo and been getting along fine with my Dmm but I see your point about the overshooting.

[tom66]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Yes, although in my case I had low output voltages.

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Hmm wander whats causing this, I have another Psu which is a 17pw26-3 the 32" version though that got damaged (corner smashed off unfixable) and I am using that for the parts, btw all the parts are the same for the ones that I have changed so far. Will do some more diagnosing tomorrow seems like its going to be a difficult one haha thanks Tom

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Just reading something can a failed Opto cause such a glitch like this reading up about the feed back on these Psu's ?

[tom66]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Yes, in rare cases although I wouldn't expect this fault.

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Yeah never know with a Vestel though as I say to my co-workers had lot of weird glitches with them lol. Going to check it over tomorrow see what I can find possibly something simple but we shall see haha when Im done fixing it Tom I will post a youtube video on the repair.

[tom66]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

To be honest while some of the parts on this PSU are rather cheap the actual design is pretty good. All the main components are heatsunk, and even they don't run that hot. The output diodes are well rated and the transformer is suitably sized. I've repaired two with no main complaints, both same failure with the FETs short. Only had to replace Vcc transistor on one board.

The 25-4 is the stupid one. The newer LED ones aren't terrible, just a bad example of cost cutting.

[CapLeaker]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Here is a run in I had with a dual voltage PSU in a phone system. The power supply wouldn't put out stable voltage and varied over the place until it went totally out of range and the PSU just shut off after a while. Maybe that gives you an idea? https://www.badcaps.net/forum/showthread.php?t=47479

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

To be honest while some of the parts on this PSU are rather cheap the actual design is pretty good. All the main components are heatsunk, and even they don't run that hot. The output diodes are well rated and the transformer is suitably sized. I've repaired two with no main complaints, both same failure with the FETs short. Only had to replace Vcc transistor on one board.

The 25-4 is the stupid one. The newer LED ones aren't terrible, just a bad example of cost cutting.

I agree the 17pw26-4 seems to be the best apart from caps but only even seen one that wasn't caps.

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Here is a run in I had with a dual voltage PSU in a phone system. The power supply wouldn't put out stable voltage and varied over the place until it went totally out of range and the PSU just shut off after a while. Maybe that gives you an idea? https://www.badcaps.net/forum/showthread.php?t=47479

Thanks CapLeaker much appreciated

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Still not working changed them Opto's still dead also checked all the resistors all good. The 12v is the only one out whack every other rail is good but the tv messes up and restarts guessing because of the 12v. Any ideas any one ?

[tom66]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Optos still dead? Huh?

[CapLeaker]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Did you force the psu on with nothing connected and checked all the voltage rails?

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

I tried with both Mainboard connected and disconnected these power boards will bring all the rails up steady when the Mainboard is unplugged even plugged in its all over the place. Really stuck on this one tbh

[CapLeaker]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

So when you force on the PSU without the main board, all voltage rails are steady, but when you plug in the main board, one of the voltage rails is all over the place? If that's the case, I'd be looking on the main board to see what is using all that power (audio IC, or something else that uses the 12V line). I presume you don't get a picture, just crap symptoms. I think the main board is your candidate!

[tom66]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Yes, I would try loading the PSU with a couple of headlamp/brake light bulbs - Poundland sell them for £1 apiece and they make ready usable 55W loads (two in series make a nice 110W load for 24V). The 12V rail is likely to be only 2-3A so use a brake lamp/tail lamp for that. Then see if they are steady - if so then the PSU is good.

And an oscilloscope *really* helps here if you can just borrow one you can immediately see if the PSU is unstable or if something else is wrong. For example testing the unrectified transformer output can show the switching pattern indicating whether the controller is unstable (optos, feedback etc.) Or if it shows the output voltage of the transformer varying this indicates a primary side problem - coupling capacitor, bad PFC, bad gate drive to FETs etc.

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

I found a 17pw26-5 Psu that had the same output on the connectors and swapped it out and the tv works fine tested for 12 hours and all good so up for sale. Thanks for the input chaps maybe one day I will diagnose the orignal Psu (17pw26-3) also my point is correct you do not need a load on the output for these to run the voltage rails. Thanks again
I will upload a youtube video of the repair soon as well.

[ReeceyBurger123]

Re: 17pw26-3 Psu Unstable Output Any Ideas ?

Well I fixed the original 17PW26-3 the main filter cap was bad WTF ? After replacing it the Psu works fine nice steady output I guess that the Pfc mosfet exploding killed the cap it is possible. Thanks for the input chaps