Need help on replacing diodes..

[JonathanAnon]

Re: Need help on replacing diodes..

Sorry for digging this up again... but I went at it again today, and it turns out that it IS actually working. The relay was clicking off because the TV was actually switched off.... I turned the TV on to it's back and hit the channel up button and the screen lights up...

So in summary: it was the video board... standby voltage was reaching the MCU, and the signal from the control panel was also reaching the MCU inputs... BUT the MCU was not switching on. I replaced the video board with a good MCU and she powers on .... thanks to everybody who helped.

[JonathanAnon]

Re: Need help on replacing diodes..

Bought the 17MB15E-3 board for 30 quid online.. threw it in, and the main relay is now clicking on... It shuts down immediately, but it's nearly enough for me to know that I was right about the video board :-)

Gonna get stuck in to it again over the next week.

[JonathanAnon]

Re: Need help on replacing diodes..

Just digging up this thread again, as I need to make a decision on what to do about this screen.. I've been mulling it over and I cant decide what to do...

Essentially a brief is as follows:
- TV wont switch on.
- Standby voltage works fine.
- Buttons on front work fine.
- Button power on signal reaches pin of MCU on video board but no BL_ON signal is passed from the other side of the MCU. Standby voltages on power input pins of MCU seem okay also.
- So it seems that the problem is with the MCU (or EEPROM)

so I'm faced with the option now of chance buying a new video board (I can get one for 50 euros online) and hope that there is nothing wrong with the inverter board and the screen.. Is there anything else I should try before taking a leap of faith..

[JonathanAnon]

Re: Need help on replacing diodes..

Made a bit of progress today.. I confirmed that the signal from the panel buttons reaches one of the bottom pins on the MCU..

Turns out R693 was on the other side of the board.... So I put the black lead to cold ground and the red lead to R693... It read 1.42V (on the near side to the MCU) and did change from this value when the panel buttons were pressed..

[JonathanAnon]

Re: Need help on replacing diodes..

Where's R693? Probe there.

I couldnt find it tom. I spent quite a while looking for it, and checking for continuity from pin 49 to ALL the nearby components, but I can find where it connects to.. hence, I dont see any other option but putting a probe on the pin itself..

And I just know what's gonna happen if I try this, I'm gonna short it out against one of the adjacent pins..

[tom66]

Re: Need help on replacing diodes..

Where's R693? Probe there.

[JonathanAnon]

Re: Need help on replacing diodes..

oh man, those pins are tiny... I've traced where the voltage from the panel buttons come back to the pins of the MCU and the path is okay, but I'm afraid to put the multimeter across the pin 49 above... The probes are too big...

I tried to be a smart lad and find where the next component on the path from pin 49 is and measure from there, but I cant find where it goes... Anything I can do about (safely) getting my probe on the small pin..

[JonathanAnon]

Re: Need help on replacing diodes..

PIN 49 on the SDA5550M MCU is called PDP_GO1/BL_ON_OFF



I'll check and see what happens on this pin when I hit the button on the front.

[JonathanAnon]

Re: Need help on replacing diodes..

Perhaps bad firmware or somehow microcontroller not starting.

Hey tom.. yeah I guess that the next thing would be to test the input pins of the MCU to see if it is receiving the signal. I have a pretty good schema for that main board so I'll try find it...

I know for the LCD monitors that the BL_ON is usually about 5V.. What should I expect to see roughly coming from the main board to the PSU to trigger the BL_ON?

[tom66]

Re: Need help on replacing diodes..

Perhaps bad firmware or somehow microcontroller not starting.

[JonathanAnon]

Re: Need help on replacing diodes..

Hi lads, I got to spend a few hours at this again today.. very frustrating..

Like I said above I'm at the stage where I have full standby (all standby voltages and standby light), but not switching on... I have no remote, the chap I'm helping fix it for said that he press the button panel on the side of the TV to control it... He said that the Program+ button starts the TV.

I'm trying to track the BL_ON / POWER ON from the button panel to the front to the PSU. I've traced the buttons back to a connector on the main board.. (schematic below)

http://www.eserviceinfo.com/download...0TFT%20TV.html


The button panel connects to two pins on the main board. The voltage across this is 0.8V, and changes to 0.35V when I press the Program+ button. But I cant find where this signal is passed FROM the main board TO the PSU. I have measured ALL the voltages across all the outputs pins from the main while pressing the Program+ button, but nothing happens.. any suggestions?

[JonathanAnon]

Re: Need help on replacing diodes..

Hi lads, just a quick update.. I had the power board here out of the TV, cos it was too cold to work in my workshop, and the TV is too big to bring in.. I tested alll the components that are usually replaced with repair kits and the like and all checked out okay.. I soldered a few dodgy looking joints..

Put the board back in the next day and the standby light came on. I checked all of the 5v and 3.3v standby voltages from the schematic above and all are working....

I'm a bit confused by this, cos I dont think that anything I did "fixed" the standby issue.. But I do remember that the inverter board cable was not attached when I opened the TV originally, so it is possible that the standby light was simply not connected (giving me the impression that there was no standby)..

I'm gonna move on to checking where the startup voltage is passed from the front buttons to the VGA board to the PSU and see if it is receiving the turn on signal.. I'll keep ye posted.. :-)

[JonathanAnon]

Re: Need help on replacing diodes..

Just downloaded that, Duncan, that will be really useful thanks.. couldnt find it in any of my google searches.. I'll search that site in future.

[Duncanbest]

Re: Need help on replacing diodes..

The SM link

[JonathanAnon]

Re: Need help on replacing diodes..

I assume it is close to zero, Tom, as there is no standby LED light.. I have to play with the band tonight.. .. but I'll get a fresh look at it again tomorrow..

[tom66]

Re: Need help on replacing diodes..

So, what voltage does the pin marked 5VSTBY or similar measure at?
If it's zero or near zero the problem is there. Standby power is used to make the relay actuate.

[JonathanAnon]

Re: Need help on replacing diodes..

Are you getting any standby output from the PSU? There should be a 5V standby pin somewhere.

No Tom, no standby power, which I why I was assuming that it was something to do with the section from the rectifier back to the input cord.. See at this point I had been working with the presumption that standby also use the DC voltage provided by the main bridge rectifier... I didnt realise then that part of the 240V AC is passed down to the standby circuit BEFORE the main rectifier, and the standby carries out it's own rectification on the 240AC .. This voltage is successfully rectified to 330DC, and this is as much as I know at the moment..

I've googled it a lot and this model seems to have quite a few similar errors..

http://www.tv.quuq.org/index.php?ind..._view&iden=189

http://www.avforums.com/forums/plasm...0-died-15.html

I had initially ruled out a lot of the components mentioned on the basis that the problem had to effectively "to the right" of the rectifier... But I'm gonna go back through and check all those aforementioned components when I look at it again, on Sunday..

[budm]

Re: Need help on replacing diodes..

Can we get good clear pictures of the bottom side of the board?

[tom66]

Re: Need help on replacing diodes..

Are you getting any standby output from the PSU? There should be a 5V standby pin somewhere.

[JonathanAnon]

Re: Need help on replacing diodes..

Okay got a few minutes more to look at this.. There is no voltage fed back to open up the second relay as I suspected...

From the second relay, part of the live signal is passed through a few diodes and is jumped the whole way across the board to the what seems like a standby circuit... Along this jumping journey there is a 450V filter cap in the middle of the board reading 328V DC so I assume the diodes carry out the rectification (though there is only two of them).. When it reaches the far side of the board There is a cap there that reads 18.6V, so it seems to be okay up to that point..

The problem must be in this section... I have read online a lot of people having problems with Q201, which I cant actually find as of yet!!!! But I will keep looking... thanks for the help so far, I will report back if I make any more progress..