Sanyo DP46480 2s to black

[CommTechRepair]

I have a Sanyo DP46480 that when you turn it on the backlights come on after a few seconds, stay on momentarily then shut off. The TV continues to run with sound in the background and I can see video if I hold a flashlight up to it.

So far I've put an oscilloscope on the power connectors from the power supply board to the main system board. There was no ripple and voltages where good. I compared it to the expected voltages that are silk screened on the board.

The power connector from the power supply to the backlight inverter board has 9 pins, 4 marked ground, 4 marked 24V, and one odd man out, white wire, that also has 24V on it. They also have no ripple and no correlation between voltage and the backlight's working. They stay at there intended voltages.

Measuring the BL_ON pin I have spot on 3.3V, it comes on right when the backlights power up momentarily. When the backlights go off the voltage stays at 3.3V.

Using the holes in the back of the TV it appears that when the backlights come on that every CCFL powers on. I see no unevenness in display of the light. Some light from other tubes must bleed through, but I would expect with one not working it would be noticeable by the naked eye?

I have a known good back light inverter that I tried to put in and it does not change anything. Same problem, same symptoms.

The TV works with video and sound, so I would expect the power supply, system board, and TCON board to be ok. The system board is sending it's 3.3V signal to the inverter board. It appears that all the CCFL tubes are turning on before they shutdown, so I don't expect them to be bad? What is left?

Another note:

The BL adjustment pin that runs beside the BL_ON pin sends a PWM signal to the inverter. When it first comes on the pulse width is close to 100%, when the backlights turn off there isn't a change that correlates with it. However the pulse width slowly decays to ~50% over time. I don't know what the expected operation of this pin should be?

[multimeter]

Re: Sanyo DP46480 2s to black

a bad ccfl tube if it isnt broken will light up for a moment,do you have a spare ccfl tube with the same connector as the ones that are in the monitor now?

[CommTechRepair]

Re: Sanyo DP46480 2s to black

It's a large 46" TV, so I don't have any tubes. I don't know of a way to test it in the short amount of time until it shuts down. It must be the same idea however, a bad CCFL tube will turn on even if it's failing?

[tom66]

Re: Sanyo DP46480 2s to black

You may have to disassemble the LCD panel to see the tubes; but be very careful with the fragile, thin LCD...

[multimeter]

Re: Sanyo DP46480 2s to black

yes it will still flash if its failing,you dont need the same size tubes as the ones in your tv,any one will do the job,disconnect one ccfl at a time,plug in the spare,power on keep going until you see the backlights stay on,when they do where ever the spare tube is plugged into that will be the faulty one

[CommTechRepair]

Re: Sanyo DP46480 2s to black

Any hints on how to get to them? I could put some nonconductor over the pads when I push the inverter into place so that it doesn't light that set, but will that cause harm being unbalanced?

Picture of CCFL connections

Sorry I don't know how to link to images without them loading inline

[tom66]

Re: Sanyo DP46480 2s to black

Looks awkward.

Since you own an oscilloscope you can try this trick.

Get your scope probe out and leave it OPEN. Wave it over each transformer - the same distance on each.

CCFLs which are working will give good waves on each transformer. The bad CCFL will produce a substantially smaller wave.

[CommTechRepair]

Re: Sanyo DP46480 2s to black

That trick worked very well. I had to go through each one with a new power cycle since the inverter shuts down pretty quick. I had two questionable waves, not much smaller but by half. After I checked all of them I went back and retested them made sure my distance was consistent and they were fine too. So if the transformer putting out a wave at a consistent level at the same distance is a good indicated that the bulbs are working then they are all good!

I am very stumped! One thing I'm confused about is I expected there to be feedback from the inverter to the system board. There is only two signal wires, BL_ON and BL_ADJ. There are empty spaces in both connectors next to those signals. Do some TV's not bother with feedback?

[CommTechRepair]

Re: Sanyo DP46480 2s to black

On that train of thought the unused pins are silk screened onto the inverter as IPWM, and DET.

Edit: I have a service manual for a similar model using the same system board I found in another post on the forum. The pins are definitely not used.

[CommTechRepair]

Re: Sanyo DP46480 2s to black

I don't think measuring EMF with the scope will tell me if a bulb is bad. It would tell me the high voltage exists on the transformer, but nothing about the current.

Are CCFL like fluorescent lamps I have above me. They start at a very high voltage but that drop to 25V from then on. Maybe they fail when the voltage drops, instead of my assumption the circuit is gone into a protection mode.

[multimeter]

Re: Sanyo DP46480 2s to black

just noticed the inverter has darfon transformers on it,check all the secondary outputs resistance of each transformer,they all should be quite close ,use 2kohm-20kohm setting on dmm,these transformers have a high failiure rate,and will also cause your 2 seconds to black problem

[tom66]

Re: Sanyo DP46480 2s to black

I don't think measuring EMF with the scope will tell me if a bulb is bad. It would tell me the high voltage exists on the transformer, but nothing about the current.

Are CCFL like fluorescent lamps I have above me. They start at a very high voltage but that drop to 25V from then on. Maybe they fail when the voltage drops, instead of my assumption the circuit is gone into a protection mode.

CCFLs in an LCD get a pulse of about 2.5kV to start, then run at around 1.5kV.

I may have been wrong about the transformer trick; try touching the probe on the magnetic core of the transformer. (NOT ON THE OUTPUT! YOU WILL BLOW UP THE SCOPE!!) I am thinking maybe an inductor would help form a coil which would form an air gap transformer, allowing you to measure the CURRENT through the transformer, not the voltage across it.

[multimeter]

Re: Sanyo DP46480 2s to black

Looks awkward.

Since you own an oscilloscope you can try this trick.

Get your scope probe out and leave it OPEN. Wave it over each transformer - the same distance on each.

CCFLs which are working will give good waves on each transformer. The bad CCFL will produce a substantially smaller wave.

stop it ,your making me want to buy your rigol off you

[CommTechRepair]

Re: Sanyo DP46480 2s to black

I don't know exactly which pins lead to the secondary winding. I uploaded a picture of the pins I measured. The measurements were consistent with each other and with a second inverter board I have of the same part. From 0.81ohms to 0.82ohms.

The two smaller pins I suspect is the primary winding just measured a short, I don't have a meter sensitive enough to measure it.

[multimeter]

Re: Sanyo DP46480 2s to black

pop one off,on the other side it should show you what pins go where.resistance should be between 1.8kohm to around about 2kohm,your primarys sound fine you tested,they should be low ohms readings

[retiredcaps]

Re: Sanyo DP46480 2s to black

What is the part number on inverter transformer? Then see if you can find it at

http://www.lcdparts.net/T.aspx

That site will show you the secondary pins.

[tom66]

Re: Sanyo DP46480 2s to black

stop it ,your making me want to buy your rigol off you

You can buy my old 20 MHz analog scope...

[multimeter]

Re: Sanyo DP46480 2s to black

You can buy my old 20 MHz analog scope...

no!,it must be 100mhz,and be able to make the tea also!

[tom66]

Re: Sanyo DP46480 2s to black

no!,it must be 100mhz,and be able to make the tea also!

Just sold my old 100 MHz scope to a good friend... HP 54501A.

[CommTechRepair]

Re: Sanyo DP46480 2s to black

I'm attempting to take the LCD panel off so that I can get access to the CCFL tubes. I don't know anything past what I've learned here:
http://www.lcdparts.net/howto/troubl...LCDTVCCFL.aspx

Can someone confirm what I expect, or correct me?
1. On top is the LCD panel. It's glass and has to be supported evenly or it's own weight will crack it. It's not rigid. The back is more delicate then the front, I plan to lay it face down on a table with some bubble wrap on it.
2. Next is some type of light diffuser. I don't know what to expect here, is it multiple sheets? Can I scratch it? The lcdparts website shows them just lifting it up and leaning it behind it. That is my plan, it must be fairly rigid?
3. Then I have reached the bulbs and I have to find a way to test them!