TV Power Supply help

[chiefcw]

Hello all,

I'm a newbie when it come to repairing electronics but I'm giving it a shot. I need a little help in fixing a power supply for a Philips LED TV 40PFL4908/F7. This is my son's tv and I thought I would give fixing it a shot. The original problem was no power, no standby light. I pulled the power supply board and found a blown fuse. Investigated further and found two bad diodes and some suspect looking caps. I replaced the caps and diodes and it blew the fuse again. I checked the mofset and it was shorted so I replaced it with the original part number. It doesn't blow any fuses now but I have no power on the cold side. I really don't know where or what to check so that's why I'm here. I have checked the voltages across the rectifier diodes and they are at 60 volts, the voltage across the big caps are 160. I've also taken off the optocoupler and applied voltage across the anode and cathode and measured a resistance drop across the other pins when the voltage was applied, it appears to work correctly. No other bad diodes can be found.

View attachment 41388

View attachment 41389

Could someone point me in the right direction?

 

[73's de Edd]

Sir chiefcw . . . .
I no can see your attachments . . .even with my 20/15 vision !

Looks like the party (ies) . .twice removed . . that would validate and host your attachment, have not done so.

The last active attachments that I see just PRIOR to yours, was being # 41386 . . within . . https://www.electronicspoint.com/attachments/555-monostable-png.41386/

Try using an independent host for your attachment and then go to your submission page and seek out the
IMAGE / DIALOG Box just to the right of the SMILEY logo and enter your hosted URL addee within it.
That way, at least, we can then see what supplemental info you were trying to pass to us .
I even had to do this on MY last 475 scope photo / post yesterday.

73's de Edd

 

[chiefcw]

Sorry about that, first time user here. I've set up a shared folder that contains the board pics and manual. Hopefully you can see them here : https://1drv.ms/f/s!AjiYxoIRa3j7gblquxWmWPFrk2_EVQ

 

[davenn]

I no can see your attachments . . .even with my 20/15 vision !

your vision quality must have dropped substantially

they are there, click on the attachment links and they are in full surround colour for your perusal and entertainment

 

[73's de Edd]

I can get the newly externally hosted info fine
BUT the on site info still comes up as
https://www.electronicspoint.com/attachments/41388/
and
https://www.electronicspoint.com/attachments/41389/
Which is only a BLUE-GREY Semitone- BLACK and WHITE message of no access.

 

[davenn]

from the links in post 1

 

[73's de Edd]

Maybe . . . . . . with you being logged in as a moder-ator-i-ficator . . .with your get out of jail free card, you can see beautiful things.
But as John Q Public, either logged in or not . . . .this is what is being seen.


73's de Edd . . . . .

 

[(*steve*)]

It's weird. I see the images in Dave's post and also if I manually edit the original post (but not afterwards).

 

[73's de Edd]

Sir chiefcw . . . . .

found two bad diodes and some suspect looking caps. . . .you know . . .but we don't know WHICH ones they were.

Somehow . . . if being on the cold side of the power supply secondary , either a D65? series diode and its C65? companion electrolytic capacitor . . .OR . . . a D55? series diode and its C55? series companion electrolytic capacitor . . would be suspect.


Looking at the design of this power supply it actually is not using a newfangled IC controller, but old school design of just having a controlled threshold power oscillator.

If the power FET blew hard, it could have fed some collateral overvoltage from its gate into attached parts.
Check the Q602 and Q621 transistors CBE junctions in DVM diode test mode.
Check zeners D607 or 607A . . . of whichever option part that they chose to use.

Schema bottom left corner . . .

Check R611 to see if it was damaged and D608 Zener options.
Check D605 and D623 regulation threshold Zener diodes.
Check regular silicon diodes D621 and D609A
Check your unit to see whether it is using IC601 or IC602 optical isolator option, it seems to suggest that IC602 is being used, with there being no IC601.

but I have no power on the cold side.

What is the situation when you read right across the terminals of the C605 and C611 main raw DC supply capacitors, with you now hopefully reading ~approx 160-180 VDC ?

If you don't come up with any significant findings, I will come back up with a markup of the schemattic of its signal and power flow functions to finally end up with its basic 13 VDC TV supply and its 21VDC LED backlight display related supply and one very minor negative supply.

PRELIMINARY . . . . .

As an upcoming " 'po boys " testing aid, do you have any /or/ can you scavenge up, from any of your . . . . electronique junque boxe . . .scrap ?
Two RED LEDS and a 1K and 470 ohm ~1/2 watt resistors ?

Thasssssssit . . . .

73's de Edd
.....

 

[chiefcw]

Thanks for the reply, here is the info you requested

found two bad diodes and some suspect looking caps. . . .you know . . .but we don't know WHICH ones they were.

Diodes
I replaced the diodes at D655 and D653 on the cold side. Both were shorted
Replaced the zener diode on the hot side at D608. This diode also blew again when I plugged the board in after replacing the diodes. It has been ok after replacing the FET.

Capacitors
Replaced C651,C652,C653,C667,C668. These only had a light film on the top but I went ahead and ordered them from Mouser because they weren't very expensive and I was already paying shipping for the diodes and I wanted the 105 degree vs the 85 that were installed. (also replaced C1008,C1009,C1010,C1011 for the same reason)

Somehow . . . if being on the cold side of the power supply secondary , either a D65? series diode and its C65? companion electrolytic capacitor . . .OR . . . a D55? series diode and its C55? series companion electrolytic capacitor . . would be suspect.

Sorry, not seeing any D55 or C55 parts. Where are you seeing those?

Check the Q602 and Q621 transistors CBE junctions in DVM diode test mode.
Check zeners D607 or 607A . . . of whichever option part that they chose to use.

Q602,Q621 are smd. I get readings all directions, guess they need removed and tested.
D607A is there, it's a smd diode. It checked ok one way but got a reading the other way too. Decided to try and remove it but I don't have a hot air gun so I tried wicking it. Broke a tab off of the diode, aaargh. Guess I'll just try to find a board since the expense is getting more than anticipated and will order these next time I make an order and have a hot air gun.

Schema bottom left corner . . .

Check R611 to see if it was damaged and D608 Zener options.
R611 is a weird one, it is supposed to be 0.3 ohm, but meters at 44K ohm. I forgot to mention that I already replaced it with one from an old board I had laying around. D608 was already replaced, see above.

Check D605 and D623 regulation threshold Zener diodes.
More smd, not shorted but tests show voltage both ways.

Check regular silicon diodes D621 and D609A
Both check ok

Check your unit to see whether it is using IC601 or IC602 optical isolator option, it seems to suggest that IC602 is being used, with there being no IC601.
IC602 is being used, it tested ok as I mentioned in the original post.

but I have no power on the cold side.

What is the situation when you read right across the terminals of the C605 and C611 main raw DC supply capacitors, with you now hopefully reading ~approx 160-180 VDC ?
Yes, I get 160 VDC across those.

If you don't come up with any significant findings, I will come back up with a markup of the schemattic of its signal and power flow functions to finally end up with its basic 13 VDC TV supply and its 21VDC LED backlight display related supply and one very minor negative supply.

PRELIMINARY . . . . .

As an upcoming " 'po boys " testing aid, do you have any /or/ can you scavenge up, from any of your . . . . electronique junque boxe . . .scrap ?
Two RED LEDS and a 1K and 470 ohm ~1/2 watt resistors ?
Yes, I have all of those

I appreciate your input!