Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

[SuperSkippy]

I'm trying to fix a Toshiba 30" TV that a friend has given me, mainly as an exercise to see if I can. So far it is an unsuccessful exercise.

The problem:

When you apply power to the TV the standby light goes red, then after a couple of seconds it turns blue. The backlights fire briefly but then go out. There is no picture. The blue light remains on, but there is nothing else- no picture, no sound, nothing else.

Clues:

PSU:
I've attached a picture of the PSU I nabbed from someone's eBay listing for a replacement. As you can see it has two outputs that go into the main board, both labelled as 24V. One is going the entire time the set is powered. The other is only turned on when the TV is turning on, and I think powers the inverter board. The voltages are at 24V according to my multimeter.

I've tested the caps on the secondary sides, and they are all good caps.

The PSU maybe makes a slight crackling noise when the second power supply is going.

Inverter board:

I've attached a picture of the inverter board- this is exactly the same as the one in the TV.

I've tested all of the transformers and all of the resistances of each of the coils are correct.

I've tried firing it up with one backlight attached per time and all of the backlights fire at least a bit, before shutting down. I don't have a spare tube.

The inverter board has a couple of connectors for power input. The top one has the power from the main board, and also the turn-on, turn-off signal. I think the bottom one is turned on and off by the main board when it wants to fire up the lamps.


I've put an oscilloscope on the power input to the inverter board (the second connector, without the on-off signal or other information from the main board). It's a fairly flat 24v when it is on, except when it's trying to fire the lamps when you get a 3v ripple on it.

What would you do?

Ask if my ramblings don't make any sense.

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

It's 24V when on, but 3V ripple added to it when the backlights fire up?
This sounds like a power supply problem, possibly bad caps.

What does the 24V line dip to? If it is 24V with 3Vp-p of ripple, it could dip to 21V... most inverters cut out at 22V.

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

Time for a stupid question whilst I work on this.

If I just unplug the backlights, will the TV work, only dim, or will the inverter detect the missing backlights and make the TV go into shutdown mode?

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

The second option. It will do two seconds to black.

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

It's 24V when on, but 3V ripple added to it when the backlights fire up?
This sounds like a power supply problem, possibly bad caps.

What does the 24V line dip to? If it is 24V with 3Vp-p of ripple, it could dip to 21V... most inverters cut out at 22V.

I've had a look. There is definitely around 3V peak to peak ripple on this output. It's not bad caps on the secondary (I think) since I've checked them all with an ESR meter and they all seem perfect.

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

What's the voltage across that large Rubycon capacitor? It could be failed PFC.

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

what's the voltage across that large rubycon capacitor? It could be failed pfc.

pfc?

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

PFC essentially makes the input current wave more friendly to the electricity company. (No, you didn't ask for it and it wasn't an option.) It essentially improves the power factor.

But to do so it boosts the 230Vac or so to around 380-420Vdc. It lets the manufacturer use a smaller primary capacitor due to how it works. When PFC isn't working you get only around 320-340V on the primary cap, but there's a lot more ripple, due to that smaller capacitor (and you can't easily measure this ripple.) This ripple is reflected in the outputs of the PSU.

I had a Sanyo Vestel-based TV with failed PFC, the bulk cap measured 335V, and the 24V had about 1.5Vp-p of ripple.

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

Information you asked for:

So the voltage on the big filter cap is 330V at turn on, and it shoots up to 420V when the power supply tries to power on the backlight inverter.

Information you didn't ask for:

I've osilloscoped the output of the secondary transformer. If you recall, I said that when the set is first turned on, this bit of the PSU turns on very briefly, then it switches off. The TV then tries to turn on, the backlights flash, at which point there is massive ripple on the output, then it settles down and there is about 1.5V ripple.

When the oscilloscope is put on the output of the transformer, but before the diodes, it gives a wall of pulses when it's trying to turn on the backlight. When it's no longer trying to fire the backlights, it goes to pulses every so often. (not describing this very well- by every so often I mean many times a second). From this I conclude that the feedback circuit is working OK on the PSU, and it's trying to send more power when the backlights are on.

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

Can you get hold of car brake lights or indicator lamps... two can be tied in series to provide a load to the power supply. If the lights cause the ripple to show up again, then the power supply is definitely at fault, otherwise, it could be excessive load from the inverter (which I would think of as being unusual.)

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

Can you get hold of car brake lights or indicator lamps... two can be tied in series to provide a load to the power supply. If the lights cause the ripple to show up again, then the power supply is definitely at fault, otherwise, it could be excessive load from the inverter (which I would think of as being unusual.)

Right, so I've attached 2 bulbs to the inverter power supply. I don't get the high ripple I get when I attach the inverter. I only get the "normal" ripple".

I'm going to investigate the inverter or the bulbs now, I think.

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

So, if you attach only the inverter (without bulbs?) you get low ripple (<200mVp-p would be fine for 24V line.)

And it is same with car bulbs.

But when you add the the inverter (with bulbs?) you get high ripple? This makes sense -- the inverter is drawing normal load, but the PSU is unable to supply it.

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

So, if you attach only the inverter (without bulbs?) you get low ripple (<200mVp-p would be fine for 24V line.)

And it is same with car bulbs.

But when you add the the inverter (with bulbs?) you get high ripple? This makes sense -- the inverter is drawing normal load, but the PSU is unable to supply it.

Yep, I've had a look, and this is true. With bulbs, or with the inverter on it's own, or with the inverter but with just one bulb, you get "low" ripple. With the inverter with all the bulbs, you get "high" ripple.

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

That screams power supply to me...

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

Do you have an isolation transformer?

I would consider checking the primary capacitor for excessive ripple, but you need to float the TV chassis. (The scope MUST remain grounded.)

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

No isolation transformer. Not that keen on putting my oscilloscope on the primary side either- it's only rated 300V input RMS.

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

What did you do with your TV with dodgy PFC?

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

Replaced the PSU because it had other issues, and the PSU was less than £20.

[SuperSkippy]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

Yeah, that's what I'm going to do. The PSU board is only £22 from eBay.

[tom66]

Re: Toshiba 30" TV 30wl46b TV 2 seconds to black(ish)

If it is cheaper than about £30, I usually buy the part, instead of tracking down a really obscure problem and replacing lots of components.