SMPS TV Secondary Diode shorted.

[tvtech]

Hi All.

I do not ask many Technical questions here. I can normally work out stuff myself. I should be able to after so many years with CRT otherwise I would be considered useless .

Anyway, lately we are getting Secondary Diode failures. Specifically on the 12V rail on the TV....
Replace the offender...and it goes shorted again because it is running too hot.

Set comes in. Customer reports dead set. Dead indeed. Normal procedure. PSU fine et al. Won't go into all details here. I believe this is a specific TV CRT SMPS problem..

OK...I have had all kinds of failures on the Secondary side from LOPTX arcing over and damaging Regulation so set trips and all. I have seen it all. I thought....All there were associated with the LOPTX and B+ and so on. You know Line running and all....

This MF is different. Picture is perfect. Sound is perfect. TV is looking and sounding 100%. But it insists of taking the 12V rail and destroying the poor Secondary Diode......

So, I soldered Copper cooling fins on the Diode....It got through the day and all is good so far.
Another Three days of soak testing needed. I don't like my solution though. I need to know why.

Any clued up person here that can maybe provide answers ?

As always
tvtech

 

[Nigel Goodwin]

What replacement diodes are you fitting?, and what was the original?.

 

[tvtech]

Hi Nigel

Original was FR 154. New FR 307. Fast rectifiers both. The FR307 took strain too. And melted even through it is rated @ 3Amps...

That is when I knew something is/was wrong. Anyway, stay well

You know what what I am on about. Us tvtech's always want to find out WHY things happen.
Makes our lives easier next time round when similar stuff happens.....understanding is all.

This time, I have not a clue.

Be Good,
tvtech

 

[debe]

Had a simmilar problem years ago & it was a faulty electrolytic cap with high ESR causing over heating.

 

[Nigel Goodwin]

Had a simmilar problem years ago & it was a faulty electrolytic cap with high ESR causing over heating.

Yes, stick a scope on the output of the diode, and check for ripple (always a concern) - but more importantly check the speed ratings of the diodes, while the new one might be 3A it might also be slower than the original diode, this means it gets too hot and fails. Speed is critical, and I often find it's better to fit a smaller faster diode instead of the original larger one.

 

[tvtech]

Hi Guys

Thanks for your replies. In trying to puzzle out the problem initially.....I did exactly what both of you suggested here before posting. The only logical solution as per experience....

Well, some things just don't make sense

This set in now on it's second day on the soak bench and is working 100% with the cooling Mod. So, one more day and I will release it.

Thank you Nigel and debe. You guys try your best when I am stuck. Much appreciated

Regards,
tvtech

 

[olu1yomi]

Pls where can i get a pdf of repair and maintenance'pls any help

 

[tvtech]

Pls where can i get a pdf of repair and maintenance'pls any help

Hi olu1yomi

Welcome to ETO. I see it is your first post.

Best bet is to start your own Thread and be specific about what area of Electronics you are dabbling in. I wish you the very best.

Regards,
tvtech

 

[tvtech]

Hi Guys

Third day of soak testing and all is still 100%. Customer fetched the set this afternoon.

I will watch this repair very carefully once in the Customers hands. I will see what happens....

As always.
tvtech

 

[Mr RB]

I would have tested it's current. With an analogue ammeter clipped into circuit, or a 0.1ohm resistor etc after the diode and current viewed on the 'scope.

A lot of TV's have fusible resistors on the rails anyway, you can measure there, and if I have the schematic I write the rail currents next to that resistor (so I can compare a new set with other sets from before).

It's possible you have a faulty downline component that is drawing excess current, and is the real fault.

 

[tvtech]

Hi Roman

I did do a test with my YEW on it's 1200mA DC range and it reported 300mA through the diode. Well within operating limits.
What I will do is wait for another of the same sets to come in for repair with a different fault and check the current through the same diode. Should be interesting to compare

Regards,
tvtech

 

[Mr RB]

Cool. Please let us know what you find out.