Sanyo DP42848 42" LCD TV: sound, but no picture

[sjfehr]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

Ah, that should have been obvious. IC800 is on the mainboard and was the first troubleshooting step in the service manual, btw.

I'm getting 24VDC on several pins on the invetrer boards, and 2.2-3.2VDC on several other pins, and 0V on the rest. Unfortunately, I have no way of knowing which pin is BL_ON.

[budm]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

BL_ON is the backlights on signal generated by the main board to turn on the backlights. I do not know what IC800 is or where it is at.
For the backlights to be on, the inverter board needs 24VDC for power and the BL_ON signal (>3VDC) to turn on the PWM IC which drives the Gates of the MOSFETs which will drive the inverter transformers which will produce high voltage to drive the CCFL's.

[sjfehr]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

What is BL_ON? I'm getting 5V on IC800 pin18, and 5V on connector K8B pin 1, both of which indicate the inverters should be on.

[budm]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

I would check the pin on the inverter board for 24v input, and the BL_ON (INV_ON, or ON/OFF) pins. Tell us what you are getting right now

[sjfehr]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

Well, it's gone black again. Same as before, though this time it shows a few seconds of video before going dark. Sound continues to play. What should I check next?

[gilly1984]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

Update: just reinstalled the 2nd inverter board without the cage to try to measure the output to the backlighting. And the damned TV just started working again! Crossing my fingers and hoping it was just a loose connector and the troubleshooting process corrected it. Also hoping I didn't fry my meter, as there was quite a bit of sparking and wierd things on the display...

Odd, though, that the symptoms persisted through playing with the PSU, mainboard and 1st inverter. And started working when I reinstalled the 2nd inverter, all the connections of which were firm and I didn't mess with the pins at all like I did the one cable on the PSU. I'll keep an eye on it and see if it continues working & post back with an update.

This exact thing happened to me, bought a bravia which would switch off after a couple of seconds, took the inverters out to see if anything was wrong, couldnt find anything. Plugged everything back in and its still working months after, switched on most of the day too with cbeebies on

[sjfehr]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

Yeah, I counted the transformer windings and looked at the insulation, and figured it was operating somewhere in the 200-300V range. So, set my meter to 600V and... well... didn't give me useful data, but appears to still be working. Even if the voltage isn't that high, the frequency is likely well outside the range of this meter.

[budm]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

Never try to measure the high voltage output of the inverter transformer, it can be as high as 2500V which will blow your typical DMM input circuit, you will need AC high voltage probe.

[sjfehr]

Re: Sanyo DP42848 42" LCD TV: sound, but no picture

Update: just reinstalled the 2nd inverter board without the cage to try to measure the output to the backlighting. And the damned TV just started working again! Crossing my fingers and hoping it was just a loose connector and the troubleshooting process corrected it. Also hoping I didn't fry my meter, as there was quite a bit of sparking and wierd things on the display...

Odd, though, that the symptoms persisted through playing with the PSU, mainboard and 1st inverter. And started working when I reinstalled the 2nd inverter, all the connections of which were firm and I didn't mess with the pins at all like I did the one cable on the PSU. I'll keep an eye on it and see if it continues working & post back with an update.

[sjfehr]

Yesterday, out of the blue, our 42" Sanyo DP42848 LCD stopped working. No lighting strikes or anything, it just stopped working properly. It's out of warranty.

When turned on, we see a black screen with the blue "HDMI1" input notification in the corner: the screen appears to be uniformly lit, so it sure looks like the LCD and backlighting are all working properly. A few seconds later, at the point where it would normally start showing video, the backlights turn off and sound comes on. If I shine a light in, it looks like the LCD is showing video, but it has no lighting. Sometimes, it just simply turns itself off, which might have been my troubleshooting triggering some other safety.

I did a bunch of googling, followed troubleshooting steps posted by others in this forum and in the service manual one of you posted (thanks!!). I'll try to summarize here, but I'm kinda stumped at this point.

No obvious burnt components on the PSU, mainboard or inverter boards, and no capacitors are bulging or leaking. The HV side of the inverter transformers looks ugly, but it's hard to tell if it's burnt or just resin.

Power Supply: with cables unplugged, I measured 24V to one of the inverter boards and 5V to ground on CN2. I'm unable to measure 24V to the other inverter board or 12 or 24V on CN2, but I suspect it's because power is intentionally turned off when they're unplugged. I was able to measure 12V and 24V to ground on mainboard on solder points near K8B, so I'm pretty sure at this point the power supply is OK.

Main board: While the TV is running (sound, etc.) I'm getting 5V on IC800 pin 32 consistantly. I've had difficulty measuring it on pin 23, at some times measuring 0V, 3.5V, and 5V, but finally was consistently measuring 5V when sound was on. Might have been my error reading a wrong pin, or might have been while the TV was in standby or some sort of limp mode.

Inverter boards: only thing I've found so far is that the HV side of the transformer nearest the edge of the board has no continuity between terminals on either board. The inboard transformer on both sides does have continuity. I measured 0.2Ohms on the LV side of each transformer between 3-4, & 4-5, but open circuits between 2-3 and 6-7. All were consistently like this. Is this my the problem or is it a red herring? I've not yet tested voltages on the inverter boards with the TV turned on due to all the shielding in the way.

What's my next step?