Toshiba 40BV700B & 17PW26-4

[fruitbat04]

Hi there,

I managed to pick up the above TV very cheaply with a described fault, in that after an hour or so, it powers off. This is pretty much true, but more specifically it's fine for an hour but then goes to standby and then back on again about once every minute.

To me, that sounds like dodgy capacitors on the power board, but visually they look OK. I have an ESR meter and the electrolytics all seem fine as well. Basic voltage tests show the 24 and 12v rails seem pretty stable even when it reboots.

I still suspect the power board which is a 17PW26-4 and seems to have lots of problems here. I know they can be picked up fairly cheaply, which is what I will probably do if it can't be fixed, but being a bit of a geek, I'd like to try and get it fixed at component level if possible!

I've got an ESR meter, Mulitimeter and basic Oscilloscope to help debug the problem but would love a bit of advice as to the next steps to take.

thanks

[Alastair E]

Re: Toshiba 40BV700B & 17PW26-4

Ah--Vestel Crap!

Check the voltage across the main reservoir electrolytic, It should be 390V +-5V
--Needs to be across the elect. and not ref to chassis, as its on the live-side.

If its only 330 or so, the PFC circuit has failed/intermittent. I had one recently that would start up--when it felt like, because the voltage was low.
--I changed the whole PSU as I had one here, but suspected the PFC chip was faulty.

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

Can you tell me which points to use? I think I did see on the internet earlier (may have been this site) a picture with the pins highlighted, but I can't find it now.

Edit : Sorry, I've found it. Seems pretty stable around 388v for that.

[tom66]

Re: Toshiba 40BV700B & 17PW26-4

When the TV fails, see if the 388V drops out in any way. Does it begin dropping before failure for example.
Try using heat to make the TV shutdown quicker - the fault is probably thermally related.

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

Well I'm not sure if it's supposed to be 100% stable at 388v. Basically every time the DMM updates, it might move 0.5v - Nothing significant and all within the 5% spec. There's no noticeable drop when the TV restarts.

You're right though, it's certainly something to do with heat (or maybe capacitance) because if I turn it on from cold, it's fine for long. It gets to the stage were it's restarting after just a couple of seconds though.

I don't really fancy hooking up my oscilloscope of 390v to check the exact stability!

[tom66]

Re: Toshiba 40BV700B & 17PW26-4

Try checking the lower voltages instead - 24V, 12V, etc.

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

Yes, they seem stable as well, even when it restarts.

Slight bit more debugging though. I've tried with the TCon/Inverter/Control panel boards all unplugged and it does the same, so it is definitely the power board (or possibly the main board although that seems unlikely).

[tom66]

Re: Toshiba 40BV700B & 17PW26-4

I wouldn't rule out the main board at all, it's quite a common problem with Vestels.
Do you have a hairdryer you can use - start running it over the boards in the set and see if you can provoke the fault. It has helped me before with intermittent faults.

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

Can do one better, I've got a desoldering air gun. I'll put it on the lowest temperature and give it a crack.

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

So this is really interesting. Not sure if this is a new fault developed or was always present. Basically the TV doesn't turn off properly! I gave it another quick go on my lunch break and noticed that when turning on power from the mains switch, the TV boots up after a few seconds automatically (standby light goes out then a few seconds later the screen comes on).

If I then switch it off using either the remote or control panel standby button, it goes off, then a couple of seconds later comes straight back on!

[tom66]

Re: Toshiba 40BV700B & 17PW26-4

Try unplugging the control panel buttons - could be a stuck button (it's capacitive sensing but can still pick up erroneous presses.)

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

It is capacitive sensing, however they make a beep when you press them, and that's not happening in this case. I will try that suggestion though.

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

With the control buttons unplugged the standby light just flashes and you can't power up. If you power it on, then unplug the buttons, strange things happen like the menu opening and then the options scrolling left.

However, since I've put it all back together, it's no long switching on automatically. I'm currently running it in to see if it powers off as it was before. I'd love it if the power is all a red herring and just reseating a cable has fixed it! It'd also give me a bit more confidence that I wasn't missing anything checking the ESR and voltages.

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

Sorry to keep posting, but I do feel like I'm making progress of sorts.

So the fault did come back after I left it running. Basically it ran for about 10 minutes from cold and then started rebooting again pretty much straight off and on every 20 seconds or so.

I took it all apart again and tried running it with the back off, but stood up straight on the stand. It's now been running for almost 2 hours which is longer than I've ever had it running so far.

So now I'm back to suspecting the power board and something involving heat. Would that sound reasonable?

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

Exactly the same board and power supply as this old thread, and the same symptoms.

https://www.badcaps.net/forum/showthread.php?t=23155

Strange thing is, if I've got the service menu up, it seems not restart! Bizarre!

[tom66]

Re: Toshiba 40BV700B & 17PW26-4

Have you yet tried heat? It is more likely main board than power board IMHO.

[stj]

Re: Toshiba 40BV700B & 17PW26-4

i'v had something like this on a smaller tv that i think may have been vestel.
it was a voltage regulator shutting down when it got hot.

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

Well I managed to pick up a power board very cheaply so just tried replacing that. So far, it hasn't powered down. However, it still powers on when you switch it off. Also, occasionally the buttons on the front panel do the wrong thing, so I do wonder if the powering in is do to with that. I wish it would just let you unplug that board but it doesn't let you power on when it's unplugged

[fruitbat04]

Re: Toshiba 40BV700B & 17PW26-4

I think I've figured out a workaround for the "Wont stay off issue". It also makes me think it is the button panel is definitely faulty.

There are 3 wires, white black and yellow which go between the main board and the panel. Yellow is constant 3.3v, white is ground and black sits at 3v, however if you press any button it drops very briefly. I'm assuming each button gives a different voltage drop, so the main board knows what button has been pressed. I observed powering off the TV with the remote control and after it's gone off, there's a slight 'blip' in voltage a few seconds later and it tuns back on.

So what I've done is hold a jump wire between yellow and black, so the pin that monitors the buttons is constantly at 3.3v. This stops all the buttons working (don't really care about that though) but when the TV is powered off by the remote, it now stays off. The second you pull out the jump wire, it comes back on though.

So I'm guess this would fix it properly

http://www.ebay.co.uk/itm/BUTTON-UNI...item4613db5f36

Seems expensive for what it is though. The whole power board was half of that. I think I might just solder yellow to black and tell the person I'm giving the TV to that the buttons don't work. I can't see how you remove that part either as it seems glued on.

[anotherdude]

Re: Toshiba 40BV700B & 17PW26-4

whats the actual part number?

have a few of these knocking round from smashed screen vestels whic i'm assuming is the same as this