Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

[davcom1]

Hi all,

After successfully replacing inverter caps on my Acer monitor last year thanks to info found on this forum, I've now acquired a 'broken' Vistron 32" LCD TV.

When powered on, the screen "strobes" rapidly as if the back light is failing to stay on.

This time I suspect it might be the power board but then again, I'm a complete novice.

After inspecting the caps on all the boards, the only one with anything of note is the large 150uF 450V Samxon on the psu.

Does anyone have any other ideas??

Photos attached and as ever, thanks in advance for any assistance!

David

[tom66]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

That primary cap could be the issue. There have been reported cases of that cap going bad in LCD monitors and causing rapid or occasional flickering.

[selldoor]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

When the set is on and flickering see if the voltage on the Big cap fluctuates.

I was going to suggest looking at the caps on the inverter board but they are Rubycon so should be good. What is the part number of the voltage regulator on that board Cant make it out but may a U designation its near the white barcoded label

[retiredcaps]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

After inspecting the caps on all the boards, the only one with anything of note is the large 150uF 450V Samxon on the psu.

It looks like something leaked out or it is some sort of corrision?

[budm]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Electrolyte is leaking out, I bet if you set your DMM (True RMS) to AC mode and measure the AC ripple voltage on the that cap, you will see quite big AC voltage reading on that cap.
What is you country voltage?

[davcom1]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Hi guys,

Thanks all for your input. Country is UK so 240V 50Hz.

I've already ordered a replacement for the main 150uF 450V cap on the PSU and will swap and test when it arrives.

My DMM is a mucho-el-cheapo thing... I'd normally only measure across low voltages, otherwise non-live components.

tom66 - thanks, that ties in nicely with my findings

selldoor - have taken an addition photo close up although no sure if it is what you wanted!

You're a great bunch on here!

David

[tom66]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

What cap did you order as a replacement - should be a decent one for long term reliability.

[selldoor]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Hopefully the new BigCap will fix it - I was wanting the part number of U1 which I guess is a voltage controller but no matter if the cap fixes it. Oddly enough there is a small black
fuse near the U1 in addition to the 2 larger white ones near the connector. Keep it in mind if there is still a problem.

[retiredcaps]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

My DMM is a mucho-el-cheapo thing... I'd normally only measure across low voltages, otherwise non-live components.

If you are uneasy about measuring high voltages, get yourself some aligator clips to go on the end of your probes so you don't have to use your hands.

Even though I'm an advanced multimeter user now, I still use aligators clips for high voltages. Sometimes, it isn't possible given the layout.

[davcom1]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Just a quick update to this thread.

Received the new capacitor and replaced the suspected faulty one on the power board.

TV is now working 100% again - great result and thanks to all who offered advice!

Happy soldering!

David

[tom66]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Fantastic and thanks for posting back the fix. I do not trust Samxon caps. I would recommend replacement of all of the caps on the power supply soon for best reliability.

[davcom1]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Erm... would you believe, it just went bang!?

Worked fine for a good few hours yesterday but turned on today (which reminded me to post the fix here) and after 30 mins or so, literally went bang!

I'm sure I heard a piece of something explode at the same time too.

Taken cover off and power board heat sinks are hot.

Perhaps the Lelon cap from Ebay was not the best choice!? It did have the same rating at the Samxon and was very similar in size...

I might just try a replacement power board instead!

[tom66]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Lelon caps are crap and will fail. Use better caps, such as Nichicon or Panasonic primaries.

[davcom1]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Tom - do you think the bang was something on the power board? The power LED on the front no longer lights up.

[selldoor]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Can we have new pictures of the power supply board. Check if the big cap is in the right way round?
Main fuse is first test if you cant see anything obviously burnt.

Lelon Cap from ebay!! ughh!!

Can you update your profile with country and mains voltage.

Pictures should be front and back and close up of connectors, (max resolution 2000x2000 and 2MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Examples of what is needed
https://www.badcaps.net/forum/attach...1&d=1290283049

https://www.badcaps.net/forum/attach...7&d=1280167246

[davcom1]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Hmm... plug fuse (5A) has blown.

Removing the power board shows major problem... its cooked!!

I can't see how I have managed to mess this up so badly. I was certain I kept the polarities matching the old cap...?

All the heat damage is around the replaced cap so what did I do??

For amusement only, photos attached as requested. Cap pins indicated with arrows.

A replacement power board is only £30 but is it likely that the other boards will now be wrecked too?

Thanks,

David

[vinceroger69]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

could there of been a lot of flux from your solder on to the new capacitor joints maybe worth cleaning the area up on the back of the board with some cleaner to remove the flux if there is any as it could of shorted out as above the left arrow it looks like a spark has arked across 2 points?

[budm]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

That the blown ground trace which is part of the Source pin high current ground return, I think yOUR MOSFET is shorted.

[davcom1]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

Ok, that's a good point. I didn't use loads of flux but didn't clean up very thoroughly either.

So effectively, the board heats up as it is working and then liquifies the residual flux which then runs and causes a short?

Would that be enough to cause the scorching and blow a section of the PCB away?

Silly question but, is it safe to short across the main capacitor (now removed from the mains of course) to make sure I don't get a jolt?

budm - thanks for the input although once it gets to MOSFET, I'm lost, sorry!

[selldoor]

Re: Rapid flashing screen Vistron LCD TV - PSU or Inverter or?

No dont short the Big cap - test it with a meter on DC 600 to see if it holds any voltage. If less than 20 test again with meter on 20 . If less than 5 then short it out with a resistor or a light bulb.
If you can take it off the board post a picture of what is under it - looks like it was the right way round when it is discharged test it to see if it is shorted.
Looks like there are some links under there id like to see tha damage to the top of the board.