Toshiba 19DL502B totally dead

[Toshiba19tv]

Hi

My sons small TV/DVD combo has gone dead. No red standby light, no sign of any electric life inside. Played with holding down the power button as the manual says, but nothing. As it is half term school holidays in our part of the world this has not been good news! Not to mention the Faulty Towers DVD still in the DVD drive!

We have opened it up and found a Vestel 17PW05-3 Power Supply board inside. We've got 235V coming in from the socket, no obvious burnt components and now I'm trying to identify the broken bit(s). Happy to invest into a new Vestel board, as long as I'm reasonably sure it is the troublemaker.

From a previous thread covering a Toshiba 22BL502B with the same Vestel power board I gathered how to check the voltage for the panel back lighting, PL706. Pin 1 & 2 go up and down, from just below 0V to something like 6V, oscillating happily within a few seconds, ie pretty erratic. Rest of pins all read zero V.

But, I have no idea how to check whether the Vestel is supplying the main board with anything useful, at PL703.

Any advice would be greatly appreciated.

Cheers folks

[tom66]

Re: Toshiba 19DL502B totally dead

Everythng in this TV is Vestel supermarket junk - it's the same as an Alba, Technika, whatever. Toshiba contracted it out under their good name, ripping off customers in the process (IMHO.)

I have heard a common issue with these PSUs is a shorted diode.
Can you post a picture of the whole PSU?

[Toshiba19tv]

Re: Toshiba 19DL502B totally dead

Struggling with the attachment functions, full pic should be there now

[Toshiba19tv]

Re: Toshiba 19DL502B totally dead

It's a LG screen ....
Shall I post a close up of any specific component?

[tom66]

Re: Toshiba 19DL502B totally dead

Yep, pretty standard LG screen - nothing special. They're OK. You'd get the same in any Vestel TV -- I think Toshiba made their own remote for these sets though!

To test: you need to set your meter to the diode range and put it across each diode. Then reverse the probes. For each diode, note the reading. A low reading <100 in both directions is indicative of a short circuit.

[Toshiba19tv]

Re: Toshiba 19DL502B totally dead

The three fellows grouped together all read zero in both directions, whereas the two grouped together gave values between 400-1600 or so. I had the power disconnected whilst measuring. So the three are all dead?

[vinceroger69]

Re: Toshiba 19DL502B totally dead

to comfirm 100% you really need to unsolder each diode and retest.

[Toshiba19tv]

Re: Toshiba 19DL502B totally dead

Job for tomorrow!
Thx for the support so far, Ace!

[tom66]

Re: Toshiba 19DL502B totally dead

Yeah, pretty typical on these (and other variants) is one of those three go bad, you can find whichever ones are OK and run briefly (max 2 minutes) with it removed. The diodes are in parallel, which is bad design because diodes have a negative TC (temperature coefficient)... this means as they get hotter, their voltage drop falls, which allows any one diode to conduct more current, causing it to get hotter, causing more current... before "POP" and it's dead.

[Toshiba19tv]

Re: Toshiba 19DL502B totally dead

Makes sense, one goes down and takes the rest with it, coz they have to work harder now.

So what's next once they are out, assuming they are dead. Is the code printed on them sufficient to order new ones?
Next stop eBay/RS trade counter/Maplin depending on patience levels to wait for a delivery?

[tom66]

Re: Toshiba 19DL502B totally dead

More like, one goes bad and they all appear bad. So only one is actually bad and set will work with one less diode briefly. However, once one fails, the others aren't far behind. Basically one diode ends up taking all of the load and fails because of this.

I think they are UF5408 or similar. I would replace all at once, because of matching between diodes. RS is best, eBay 2nd, Maplin don't stock them in my experience. I'd also consider some thermal epoxy between diodes to help keep them matched at the same temperature.

[Toshiba19tv]

Re: Toshiba 19DL502B totally dead

Ok, as they are in parallel I'm measuring always the same circuit, slowly starting to get the drift.
We have a RS nearby, I'll certainly change the lot, not doing this again next week. I assume putting in a bridge from thermal epoxy, creates also additional surface dissipating the heat a tiny bit better.

[Toshiba19tv]

Re: Toshiba 19DL502B totally dead

No longer dead!

Took me a while to get to it, but it's alive & kicking again. 3 new diodes did the trick.

Cheers & thx a a lot!

[tom66]

Re: Toshiba 19DL502B totally dead

Nice one. Did you add the epoxy?

[Toshiba19tv]

Re: Toshiba 19DL502B totally dead

Not yet, but on the to do list, together with another epoxy job.

Cleaned out the vents, though!

[tom66]

Re: Toshiba 19DL502B totally dead

I just got a 32" Hitachi (also a Vestel) to fix with shorted diodes, unfortunately it did some damage on the primary side too. But shouldn't be too hard/expensive to fix.

[877]

Re: Toshiba 19DL502B totally dead

Well I am so glad I found this forum

I have EXACTLY the same problem, and followed the advice by the knowledgeable tom66, I have found one of the three grouped diodes is faulty! So I ran it with just two diodes for a minute and all is well. Saved me £30 for replacement PSU board, or £150 for new TV. So well chuffed!!

The diode I removed has SB5B0 printed on it, the closed match I have found is a SB5100 from RS which comparing data sheets seems a good match.

I will of course replace all three diodes and try and find some thermal epoxy from my nearest RS (Birmingham).

Thanks again to the legend tom66

[madmike1984]

Re: Toshiba 19DL502B totally dead

Hi, Sorry to bump an old thread. I just joined the forum as I have the exact same problem. Have ordered the new diodes and was just wondering where I apply the thermal epoxy after soldering?
Also is thermal epoxy the same as thermal paste that you use on a heat sink/processor of a PC?

Thanks very much

Michael

[877]

Re: Toshiba 19DL502B totally dead

Hi Michael,

The thermal epoxy is better as it goes hard like a glue, run a line of it across the black middle part of three diodes so that they are joined. It basically means the heat will be spread out across the diodes if one is getting hotter than it should (from my understanding). Thermal paste like you use on a CPU would not go hard and might drop off in time, so is not too good. Plus the silver stuff is conductive so could cause a short.

Good luck!

[madmike1984]

Re: Toshiba 19DL502B totally dead

Hi Michael,

The thermal epoxy is better as it goes hard like a glue, run a line of it across the black middle part of three diodes so that they are joined. It basically means the heat will be spread out across the diodes if one is getting hotter than it should (from my understanding). Thermal paste like you use on a CPU would not go hard and might drop off in time, so is not too good. Plus the silver stuff is conductive so could cause a short.

Good luck!

Thanks very much for replying, I will let you know how I get on