Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

[DXseekerMO]

Hello everyone!

So from the free pile yesterday I got a Samsung PN51F8500AFXZA that wouldn't turn on. Stand-by light did not illuminate. Found fuse FP801S open, the DC legs of both Bridge Rectifiers shorted and MOSFET QP802S short drain to source. Removed QP802S and short on Bridge Rectifier cleared. Confirm QP802S is short drain to source. No short found on QP804S.

Control IC ICP801S is TI SPC1012T.

Questions:

Is there a way to test ICP801S with a multimeter for short/failure?

Neither DigiKey or Mouser stock SPC1012T. Is there a means to find an equivalent they do stock? If not does anyone have a recommendation for a supplier?

In the last picture I am pointing to a semi-solid gel like substance that covers QP802S, QP804S and ICP801S which has to be removed to replace the MOSFETs and control IC. What is this semi-solid gel and should I replace it after installing these items?

Lastly, I know these control ICs sometimes survive a shorted MOSFET, and with only one shorted drain to source does anyone have an opinion on the likelihood ICP801S is actually defective?

Pictures enclosed. Thanks!

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

54 views....no replies, so I ordered 4 MOSFETs and 2 fuses. I guess I'll know soon whether or not the control IC is good

[tvtimmy]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

Must be a tricky power supply. Let us know what happens with the new parts.

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

Must be a tricky power supply. Let us know what happens with the new parts.

Looks like it could be a comparatively simple problem. I found only one bad MOSFET. The parts are going to ship Monday, I hope

[dick_barton]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

You could check you have the supply voltage on SPC1012T and if there is voltage at Vref and also the resistance to ground of the gate drive outputs

There is a pinout at

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

You could check you have the supply voltage on SPC1012T and if there is voltage at Vref and also the resistance to ground of the gate drive outputs

There is a pinout at

Thanks for the link!

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

Ok the parts came in yesterday, I just installed QP802S, QP804S and the fuse.

I installed the Power Supply and plugged the TV in. The stand-by light came on then flashed 2 times, went back on steady then flashed 2 times, repeat.

Slowly but surely the 2-flash on the stand-by light started to go away and the TV slowly started trying to turn itself on (as it was on when the failure occured).

About 5min later the TV turned on fully with a picture,

About 2min later the screen started "pulsing", as in getting brighter and dimmer. At the same time there was a "clicking" noise coming from one of the components attached to the center heat sink.

Eventually the picture became plagued with black dots and I could tell it wasn't receiving the proper amount of voltage and current, so I shut the TV off.

It looks like the Power Supply needs a bit more work, but I'm unsure of where to go from here. Any suggestions?

[R_J]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

What is the voltage across the main caps? Did you check the gate drive resistors/diodes on the two mosfets that were shorted? Try working on just the power supply board without it connected in the tv and check the output voltages, VS Va etc.

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

What is the voltage across the main caps? Did you check the gate drive resistors/diodes on the two mosfets that were shorted? Try working on just the power supply board without it connected in the tv and check the output voltages, VS Va etc.

Yes, I checked all of the Gate Drive resistors/diodes, they all checked ok.

I pulled the Power Supply back out and inspected the solder joints on the components I installed. There were a couple not-so-good looking joints so I re-flowed them and re-installed the PS in to the tv.

I plugged the TV in, the SB light flashed twice a couple times, the TV turned on and the picture was still pulsing. I changed channels, and miraculously the pulsing stopped!

The TV has been on now for 45min with a solid picture plus VS and VA 1 volt under prescribed voltage. I haven't checked the voltage at the main caps but I will.

It must have had a lousy solder joint, either that or it went in to "self-healing mode"

UPDATE/ADD Main cap voltage: 393V

[R_J]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

The main cap voltage indicates the pfc is working, if the tv acts up again, check that this voltage is still 395vdc, if it drops the rest of the power supply will not function properly. The main cap voltage without the pfc operating (ie in standby) will be around 165vdc

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

Must be a tricky power supply. Let us know what happens with the new parts.

You could check you have the supply voltage on SPC1012T and if there is voltage at Vref and also the resistance to ground of the gate drive outputs

There is a pinout at

What is the voltage across the main caps? Did you check the gate drive resistors/diodes on the two mosfets that were shorted? Try working on just the power supply board without it connected in the tv and check the output voltages, VS Va etc.

R_J I will check the main cap voltage if the TV faults again.

For now all appears well so the TV is re-assembled and in-use/testing in my living room!

Thanks everyone for your input, suggestions and help! I appreciate it!

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

You could check you have the supply voltage on SPC1012T and if there is voltage at Vref and also the resistance to ground of the gate drive outputs

There is a pinout at

The main cap voltage indicates the pfc is working, if the tv acts up again, check that this voltage is still 395vdc, if it drops the rest of the power supply will not function properly. The main cap voltage without the pfc operating (ie in standby) will be around 165vdc

OK guys, so the TV faulted again. Now the display "pulses", in that it gets brighter and dimmer, alternating about 4 times a second.

R_J, I checked the voltage at the Main Caps with the board started and found an unstable 365V, alternating between 340V and 376V with no particular pattern.

As I thought about it the shorted QP802S also resulted in there being a short upon the DC side of the rectifier diodes, so I checked the voltage at both of them. I have an AC input voltage of 117.5V and with the Power Supply not started the DC voltage is very slightly unstable, alternating between 161.1 to 161.4V. With the Power Supply started the DC voltage at the rectifier diode is an unstable 135 to 160V.

Does this indicate the rectifier diodes are bad, and this is possibly the source of my unstable voltage?

[R_J]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

When the power supply is ON, check if the NT801S is warm/hot, When the power supply is ON, the relay should short out NT801S, If the relay contacts are weak/dirty there will be a voltage drop across NT801S and there should not be when the board is ON, NT801S should only be warm in standby.

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

When the power supply is ON, check if the NT801S is warm/hot, When the power supply is ON, the relay should short out NT801S, If the relay contacts are weak/dirty there will be a voltage drop across NT801S and there should not be when the board is ON, NT801S should only be warm in standby.

I think I should back up for a sec here and ask a question. First, when I checked the voltage on the rectifier diode I did so by probing the + and - terminals on the RD itself with the meter in DC mode. Is that the right way to check it?

The reason I ask: I turned the power supply ON (not stand-by) and checked the rectifier diode at it's + and - terminals in DC mode. I now get a stable 114V. However when I place the + probe on the negative side of the main caps and the - probe on the negative terminal of the RD I get 397v

Interestingly enough VS voltage is now a stable 212V and Main Cap voltage is a stable 397V..... with the power supply unladen (not installed/hooked up to the TV.

To answer your question R_J I ran the PS in the ON state (PSon and Vs on jumpered) for 30min and thermistor NT801S did not get hot.

Everything I see here screams a bad connection somewhere, but at this point I'm not really sure what to think! lol

[R_J]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

When you measure the voltage across + & - of the bridge rectifier, it should read 160vdc, When you measure across the main caps, you will get 160vdc in Standby, but you will get +397vdc when the power supply is ON, this is because the PFC circuit boosts the Dc voltage across the main capacitors, There are other diodes (DP808 & DP809) that isolate the bridge rectifier + from the main capacitor +
Check the solder connection especially the ones that use rivets, they can develop a poor connection between the rivet and the copper trace.
You should have 160vdc across the bridge, not 114v, to get 114 across the + & - of the bridge would suggest 87VAC into it (across ~ & ~)

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

When you measure the voltage across + & - of the bridge rectifier, it should read 160vdc, When you measure across the main caps, you will get 160vdc in Standby, but you will get +397vdc when the power supply is ON, this is because the PFC circuit boosts the Dc voltage across the main capacitors, There are other diodes (DP808 & DP809) that isolate the bridge rectifier + from the main capacitor +
Check the solder connection especially the ones that use rivets, they can develop a poor connection between the rivet and the copper trace.
You should have 160vdc across the bridge, not 114v, to get 114 across the + & - of the bridge would suggest 87VAC into it (across ~ & ~)

Hmmmm.....when the Power supply is in stand-by the voltage is 163V across the bridge, but when the Power Supply is on there is now 116.5V across the bridge, in DC mode on the meter. Is this normal?

EDIT/ADD: there is 118.6V AC across the AC terminals

[R_J]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

Check the AC voltage right at the bridge recitier to see if it is 118v
The only thing I can think of is that the high frequency of the pfc circuit is causing your meter to read a wrong value.
There are two bridge rectifiers in parallel so I doubt a diode is going open, and if one of the diodes shorted it would likely blow the main fuse.

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

Check the AC voltage right at the bridge recitier to see if it is 118v
The only thing I can think of is that the high frequency of the pfc circuit is causing your meter to read a wrong value.
There are two bridge rectifiers in parallel so I doubt a diode is going open, and if one of the diodes shorted it would likely blow the main fuse.

When I checked the voltage and got 116.5V it was right at the Bridge rectifier, in DC mode on the meter. I also checked both BR's, same result. I will check both of the diodes and see if they happen to be open or short.

EDIT: I just realized you said check the AC voltage at the BR, I did, it was 118V AC

[DXseekerMO]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

Check the AC voltage right at the bridge recitier to see if it is 118v
The only thing I can think of is that the high frequency of the pfc circuit is causing your meter to read a wrong value.
There are two bridge rectifiers in parallel so I doubt a diode is going open, and if one of the diodes shorted it would likely blow the main fuse.

DP808 and 809 are not open or short, and they do not reverse bias.

The riveted solder connections appear to be ok, along with the solder joints on the components I replaced.

[R_J]

Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

I can't see how the voltage would drop from 160vdc to 116vdc. What make/model meter are you using to check this voltage? Try using a different meter if you can.