diode hair line crack

[jimbob8467]

hello, i have a power board 17pw26-4 from a Toshiba 40 inch lcd tv. the problem is it comes on and roughly between 50 seconds and 1min 10sec seconds it switches off, i have metered the caps and all is well but i have found 2 big diodes with a hairline crack from cathode end to the anode , i did a diode test on them and they measure 388 to 389, they don't measure anything the other way, not familiar with diodes and have tried to post a pic but don't know if it has went on here, any help appreciated

[Mad_Professor]

Re: diode hair line crack

If they don't measure anything the other way, then they are good.
http://en.wikipedia.org/wiki/Diode

What method did you use to meter your caps, did you test for capacitance and check for ESR values?

The way you describe your symptoms of your TV sounds like an inverter or a bad CCFL in the TV.

I would look there for problems.

[kc8adu]

Re: diode hair line crack

mold line on the case.
normal.

[jimbob8467]

Re: diode hair line crack

ok found another diode hiding behind the 2 large caps to the top left side corner of the inverter (coil) on the power board, i measured this with a multimeter XL830L on diode setting, normal way i get .428 and i am getting a reading the other way also, and it counts from 365 all the way up to 1.999 then the meter cant go any higher and just gets a reading of 1, i used a newcason xc6013l meter to read the caps not too sure how to do a ESR check because i dont think this meter will check that?? the diode is a F54 series can not see it properly as i have not taken it off yet thanx for the reply, is this diode faulty???

[philliesfan30m]

Re: diode hair line crack

it might be in parallel with something else you should take it out or remove 1 side and check it to make sure

[philliesfan30m]

Re: diode hair line crack

what do the rest of the boards look like? can you take a picture of the back of the tv with all the board and then individual pics of all the boards? like mad professor said it could be something else so you could be trying to troubleshoot the wrong board. do you get backlights at all and a picture for the minute it does come on?

[ant3202]

Re: diode hair line crack

take out / desolder the diode and measure...
reverse the polarity and measure..

only one side should have readings,
if both have readings, diode is likely damaged.

[jimbob8467]

Re: diode hair line crack

everythig works perfect for about 1 minute 20 secs, picture is there and sound up and down channels ok, then it switches itself off for about 40 secs then switches back on for about 1 minute and keeps going like this, i will take some better pics of the tv and post here later on thanx for the help guys

[Th3_uN1Qu3]

Re: diode hair line crack

If the diode was cracked it wouldn't power up at all, there's something else going on.

[tom66]

Re: diode hair line crack

You should try checking if the 24V on the output is stable. I have had bad PFC circuit cause low 24V. As bulbs in set warm up, the current draw increases which increases the amount of 24V load (dropping it and other rails like 12V and 5V) and eventually causing main board to reboot or turn off.

[Mad_Professor]

Re: diode hair line crack

Yeah sounds like bad caps in power supply or inverter board.

[multimeter]

Re: diode hair line crack

a vestal in a toshiba jeesh what next

[JonathanAnon]

Re: diode hair line crack

I would agree with Th3_uN1Qu3. It is not likely that you would get the TV to power on if those diodes were fault. Since they are checking out fine, I would move on to testing other parts.

I would start with testing all the secondary capacitors, off the board if possible.. Although they all look fine the ESR in just one of them may get high shortly after powering the TV and cause the TV down.

Also, check the back of the board for cold joints, particularly on inductors, transformers and ICs.. posting a good image of the back and front of the power board would help us to help you.. Like tom was saying, it is not just coindicence that it is powering on and then failing at the same point. It is most likely that one of the components will work from cold, but when it starts to warm up it fails. Maybe you could use hairdryer / freeze spray to help you to isolate the faulty component(s)