Sanyo DP 42848 , no picture, sound OK

[djcmx49]

Hello all, My name is Don. I have been a bench A/V repair tech. for over 25yrs. but have been out of it for past 5yrs or so. I have a co-workers 42 LCD sanyo tv which has no picture but sound is OK.

When you first turn on tv the backlight comes on for a second then goes out. The green power lamp stays on constant. Sound stays up fine. All power supply voltages are good. +24,+12,+5, and stay up. All caps on PS. board and both inverter boards look fine...

Now here's the strange thing when I look at H/V connector out to CFL backlamp on master inverter board, left one looking from back, with digital VOM the backlamp stays on for 5 - 10 seconds longer while I hold the meter leads to it??? I can see the picture and it is OK. So something is detecting a fault and cutting out inverter drives.?? If anyone has an idea please suggest. I can try to add pictures if needed, Thanks , Don

[PlainBill]

Re: Sanyo DP 42848 , no picture, sound OK

Hello all, My name is Don. I have been a bench A/V repair tech. for over 25yrs. but have been out of it for past 5yrs or so. I have a co-workers 42 LCD sanyo tv which has no picture but sound is OK.

When you first turn on tv the backlight comes on for a second then goes out. The green power lamp stays on constant. Sound stays up fine. All power supply voltages are good. +24,+12,+5, and stay up. All caps on PS. board and both inverter boards look fine...

Now here's the strange thing when I look at H/V connector out to CFL backlamp on master inverter board, left one looking from back, with digital VOM the backlamp stays on for 5 - 10 seconds longer while I hold the meter leads to it??? I can see the picture and it is OK. So something is detecting a fault and cutting out inverter drives.?? If anyone has an idea please suggest. I can try to add pictures if needed, Thanks , Don

This is a case of 'Two seconds to Black. If you want to troubleshoot it yourself, here is the guide courtesy of retiredcaps. It's had several rewrites, so look towards the end of the thread.

If you want help, pictures help us help you. Attach them using 'Manage Attachments' - below the text entry area.

PlainBill

[retiredcaps]

Re: Sanyo DP 42848 , no picture, sound OK

Check the secondary inverter transformer windings. They should be within 3% of each other.

If they are Darfon branded, one of the windings maybe open.

And PlainBill beat me by 1 minute.

[PlainBill]

Re: Sanyo DP 42848 , no picture, sound OK

Check the secondary inverter transformer windings. They should be within 3% of each other.

If they are Darfon branded, one of the windings maybe open.

And PlainBill beat me by 1 minute.

And your answer (and signature) complemented mine nicely.

PlainBill

[djcmx49]

Re: Sanyo DP 42848 , no picture, sound OK

Thanks for the help guys I appreciate it ! I will check all caps in inverter boards again and windings... If I can't find anything I will post up pictures...A schematic would really help. Th e shop I used to work for didn't sell Sanyo so I don't think they are an option. Getting my hands on a service manual would be nice. I have a feeling it's a sense circuit fault?? But it could be too much drain on inverters from ccfl too. I assume the sense circuit detects current drain from ccfl's ? I don't have spare ccfl to troubleshoot with...Thanks again,

Don

[bbjunkie]

Re: Sanyo DP 42848 , no picture, sound OK

Getting my hands on a service manual would be nice.

http://elektrotanya.com/?q=showresul...ia=tv&kat2=all

Enjoy :-)

Click on the "Get Manual" link in the bottom right corner once it appears (can take a short time after the page loads to appear)

If it doesnt work, use Firefox

Jim

[djcmx49]

Re: Sanyo DP 42848 , no picture, sound OK

Thanks Jim... Awesome.!

[djcmx49]

Re: Sanyo DP 42848 , no picture, sound OK

Schematic downloaded fine. However it does not have Power Supply or Inverter Boards. It seems they consider them non-serviceable...crap !

I now think BackLamp is at fault here. Makes quite the Buzz noise for the two seconds it's on...still must rule out control circuit though...

[djcmx49]

Re: Sanyo DP 42848 , no picture, sound OK

Here are some pics...



By djcmx49 at 2012-01-17


By djcmx49 at 2012-01-17


By djcmx49 at 2012-01-17


By djcmx49 at 2012-01-17

[djcmx49]

Re: Sanyo DP 42848 , no picture, sound OK

Transformers all measure the same, only measurements I can get are 24.4 ohms between two contacts and other is 2.4 meg.ohm. that seems high but as I say all 4 same...?? I cannot make heads or tails on how it is actually wound regarding pin layout?

[bbjunkie]

Re: Sanyo DP 42848 , no picture, sound OK

I had a tv recently that made a buzz on startup, turned out it was a bad connection to one of the CCFLs (was easy to tell the noise was coming from the end of one of the tubes)

Make sure the set is off when you are checking as there are lethal voltages at that point (1.5kV @ 10mA isn't uncommon)

Also, the connection to the tube is quite fragile so be extra careful there too.

[tom66]

Re: Sanyo DP 42848 , no picture, sound OK

Also, check the fuse on both boards (the little white thing near the power connector), it should measure close to 0 ohms. (Don't confuse 1 on the left hand side with "1.0". 1 on the left hand side, or OL, means the fuse has failed.) Do this while powered off.

As it's a master-slave configuration, the slave could be shutting down which may shut down the master (or vice versa.)

[djcmx49]

Re: Sanyo DP 42848 , no picture, sound OK

OK so I went back at it and decided to go back to my original findings where I would put meter leads to conn. on master inverter to the ccfl. When doing this the backlamp stayed on for 10 or so seconds. I of course looked at connections carefully but they were not loose nor corroded. I decided to solder them directly to pcb as they went to another plug into panel assy' so I could still remove inverter pcb if need to...The backlamp works !!!! stays on fine... I would have never guessed it was that simple of a solution. The lamp/inverter still squeals at start up for about 30 seconds but then ceases. It is faint but noticeable but as I said it goes away. The lamp does have a slightly darker band in bottom third of picture so the lamp assy' may need replacing but I'm not sure the owner will want to spend the money on replacing it. It is not noticeable unless screen is bright white...Thanks for all the help guys !!!

Don

[DakotaKid]

Re: Sanyo DP 42848 , no picture, sound OK

I pulled mine out of storage today and it exhibits the 2 seconds to black as you described in your original post. Could you explain in a little more detail, or even a picture or two if you took them before you reassembled, exactly what you soldered direct and which plugs you bypassed. I realized it didn't fix 100% with the darkness but it's interesting nonetheless.

From what you've described, it sure sounds like a burned out bulb or faulty CCFL connection.

Thanks.
DK

[DakotaKid]

Re: Sanyo DP 42848 , no picture, sound OK

I checked all connections and pins 3 & 4 on the K8L connector and there is 3.3V present on both of those pins just as there should be. Pin3 goes to pin 12 Vblon on the master inverter board and pin 4 goes to Pdim on the master inverter. 3.3V is the correct operating value for these. The 24V lines are also in spec on mine. From the power and mainboard there is really no other place for there to be a problem that I can see.

The AUO spec sheet that was provided by budm shows the inverter connector info that I listed above and also states specific cautions about leakage thru the high voltage lines going from inverters to the CCFL tubes. I believe the problem in mine is narrowed down. Namely I think there is either overcurrent demanded by the CCFLs by blown bulb, weak bulb, bad connection, leaking voltage in wiring that is causing the inverters to shut down and communicated master to slave via the small ribbon cable connecting them. Or, there could be defective components on an inverter board, most likely the master. It could be inverter transformer, VR, IC, caps, etc. Toshiba (32AV500U for example) uses a similar setup and had a huge problem literally blowing components off the board, typically the BD9897FS IC would blow apart and the FDD8447L VRs would fail so bad you could see the discoloration from heat. It's a difficult repair if it's the SM IC. However, the IC on the inverter board has a normal shutdown function to protect the board from damage and that could be triggering if there is overcurrent demands from the CCFL array.

I am going to test my theory by hooking up external CCFL as explained in the great "2 seconds to black" writeup that the big guns here have provided. Hopefully that will show me if it is indeed the CCFL array that is causing the problem.

Meanwhile, I would appreciate any thoughts or input or approaches. In addition, it would be great to have schematics of the M / S inverter boards to better understand what can trigger the IC shutdown protection. I think I had some schematics for the HIU812 inverter boards for the Toshiba which are similar but not the same as these.

I was thinking I'd keep this thread open even though the previous issue was somewhat resolved to give future fixers a single thread with a lot of info. I hate having to search thru 30 threads on the same TV so I am hoping to build on what's already been done. If anyone disagrees please let me know.

thanks.
DK

[DakotaKid]

Re: Sanyo DP 42848 , no picture, sound OK

Update...it seems it is my slave board that is the problem. I found that the slave board 8A sub mini fuse was blown so I replaced it with an inline fuse. With the fuse in place, I get no backlight at all from slave or master. I also did a CCFL tube test with a small 8W flouro tube and with the fuse out/blown. I hooked the test tube up to the master inverter board and it lit but same 2 seconds to black. The slave board, of course, couldn't light the tube with the blown fuse on the 24V lines. Out of circuit, the 3 caps (35V, 330uf) measured about 344uf each with about .02 ESR so they appear to not be the problem. I am going to check the 4 FET's (Niko Semi P1504 and P2504). I guess it is also possible that the inverter IC on the master board is bad within the feedback circuit. I'll post test results.

[DakotaKid]

Re: Sanyo DP 42848 , no picture, sound OK

Tested 4 FET's as shown in attached, two appear to be bad with readings across the pins ranging from .2 Ohms to 11 Ohms, well below the suspect threshold, while the other two test out with much higher resistance. The attached pix show the bad FET's, one is a Niko Semi P1504BDG and one is a P2504EDG, datasheets attached too. I am having trouble locating replacements on Digikey and Mouser so if someone with more understanding than I have of what's tolerable I would appreciate the advice.
Thanks.
DK

[retiredcaps]

Re: Sanyo DP 42848 , no picture, sound OK

Tested 4 FET's as shown in attached, two appear to be bad with readings across the pins ranging from .2 Ohms to 11 Ohms

Desolder both and verify they are shorted out of circuit.

ebay has both chips available. I suggest you replace all 4 instead if the 2 fets are bad out of circuit.

[DakotaKid]

Re: Sanyo DP 42848 , no picture, sound OK

Got my parts, repaired the slave inverter and it now works like new. Another success. Thanks to everyone who helped.

[brainno11]

Re: Sanyo DP 42848 , no picture, sound OK

OK so I went back at it and decided to go back to my original findings where I would put meter leads to conn. on master inverter to the ccfl. When doing this the backlamp stayed on for 10 or so seconds. I of course looked at connections carefully but they were not loose nor corroded. I decided to solder them directly to pcb as they went to another plug into panel assy' so I could still remove inverter pcb if need to...The backlamp works !!!! stays on fine... I would have never guessed it was that simple of a solution. The lamp/inverter still squeals at start up for about 30 seconds but then ceases. It is faint but noticeable but as I said it goes away. The lamp does have a slightly darker band in bottom third of picture so the lamp assy' may need replacing but I'm not sure the owner will want to spend the money on replacing it. It is not noticeable unless screen is bright white...Thanks for all the help guys !!!

Don

Thank You DJCMX49 my master inverter connector to the ccfl was bad , so I soldered it like you said , and bingo!