EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: budza on May 06, 2019, 12:52:03 am
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Hi guys,
I have a Hisense 65K560DW TV with problems on the Power Supply Board.
The board number is RSAG7.820.4877 and I have not been able to locate any service manuals or schematics for it.
Forgive me if I explain this wrong or use the incorrect terminology, I am still new to electronics and wrapping my head around SMPS. I cannot find any shorted components or blown fuses, there is no power on any of the rails nor any standby power.
As far as I can tell thus far, the AC power is coming in, going through the rectifier and charging the bulk capacitors, but then that's as far as it goes (the caps are charging to around 400vDC) there is no power on any of the other caps on the output or anything like that.
I usually just fix things like blown MOSFETs or diodes, whereas I cannot find anything to my knowledge wrong so I believe I am in over my head. Any tips of what I should try, or check would be highly appreciated. I have the basic tools such as a DMM and LCR meter if that helps
Cheers
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My mate has a TV he wants me to diagnose with the same PSU. I have managed to obtain the schematic after going through a forum paywall (sigh)... here it is, I hope it helps you.
I have also attached two other files that were associated with this download, they might be relevant to your model also.
Edit: nope, the extras are just some other random PSUs, hadn't looked until now.
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Refering back to the schematic posted by gnif:
If you're getting +400V at the bulk capacitors we can assume the PFC section is working.
First make sure there is really nothing at the 12V standby supply (C921, C922).
If 12V standby really is dead start by checking resistors: R906, R901, R902, R903. (I believe the values on the schematic may be false, would expect values of 100k ohms or 1Mohm, check with component markings.
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In my instance there was no 12V standby, and low voltage (6-8V) across VZ901 (should be 16V). The cause was R915 and R916, one was open circuit, the other was reading 1M, both are supposed to be 10R. These clearly get warm as the board is scorched from heat from them. Replaced them with 1/4W through hole resistors.
The fault exhibited by a very dim standby light flash (as reported by my mate), when I got to it, there wasn't even a flash.
There are also bulging capacitors (C855) that need replacing, might be a good idea to do them anyway even if they are not on yours.
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I find quite amusing the trouble they went to implementing AC detection (brown-out) that stalls PFC and standby supply.
Would the idea behind this be to not stress the PFC with too high duty cycles when the primary bulk capacitors discharge?
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I just do this as a hobby, so not sure, however at a guess it might be to ensure the micro on the main PCB never gets into a bad state due to a brownout.
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Apologies for the necropost but I thought it may be relevant to add what I found here in case someone else stumbles across this like I did.
First up thanks for those schematics gnif, definitely helped me fault find my power supply.
My issue was the standby 12V was fluctuating as low as 7V before going back up to 12V. This was causing the power supply to go into a protection mode of sorts and cutting all non standby voltage outputs 12/18/24V. Only symptoms externally was blinking dim standby led.
Measured across VZ901 but the voltage was also fluctuating there. I discovered the rectified DC output from VD901 and VD902 was not stable with approximately a 40V ripple.
Turns out C915 was missing, there was a space for it on the board but it wasn't fitted (suspect someone had worked on this TV previously as screws pretty loose and one was missing) and C901 was defective. Fitted two new 10nF metal film caps and now a stable 12V allowing the unit to power up.