Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

[budm]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

BTW, did you download the service manuals?

[Howard_Woodard]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Yes, I did -- thanks for the link. Still pretty skimpy on mainboard info.

My wife sent me on a grocery errand. I'll follow-up on this when I get home.

[vinceroger69]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

this may help should be a service manual

[Howard_Woodard]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Ok, just got back home and looked to confirm what I had done before responding to you.

(1) Inverter On/Off (pin 12) is grounded to signal (cold) ground on the PSU side.
(2) DET_5V wire between PSU and main board is cut. PSU side left to float and mainboard side pulled up to 5v.

Get no backlight [as expected] but the TV goes into it's boot-loop mode.

[Howard_Woodard]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Btw, when I first turned it on this morning it rebooted one time and then stayed up -- no auto-rebooting -- and everything worked like you would expect. I left it on and came back to it in about an hour and it had shut itself down -- and stayed down. When I turned it on it went into boot-loop and has been in that mode every since. Whatever it is, its not a hard failure. It would be interesting to know what the mainboard is doing, or waiting to do, in that 28-30 second period after the start-up chimes sound. The on/off line never changes while it is looping and the standby 5v seems steady throughout. One of the chips -- I think its UP801S (FAN7530) -- is a comparator and if it detects a difference between the feedback and its expected reference it will stop QP802S from switching which turns off all the rails except for the 5v standby.

[budm]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

1) FAN7530 is the PFC boost Voltage controller, it is turned on when PS-ON is present, so when the TV acts up, does the Voltage on the main filter cap remain around 380VDC?
2) The pin 1 of the PFC is the feedback pin to monitor the boosted out put Voltage, if the boosted Voltage goes to high it will then shutdown the boosted Voltage.
3) Pin 4 of the PFC IC is for over current feedback.
2) When TV is off, the Voltage on the main filter cap will be around 160VDC.
3) The switched 12V (Turned on by PS-ON) also steady at 12V when TV in reboot, correct?

[Howard_Woodard]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Ha ha. I'll have to wait until it fails to see what UP801S looks like when it is looping. When I first plugged the TV in it was looping and I clearly had two different signals going into pins 1 & 4. But every since then the set has worked perfectly.

I did notice that the voltage on pin 4 of UP801S is between 600-700 mv. The data sheet says that 800 mv it will trigger an over-current. Pretty close...

[Howard_Woodard]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

In the pictures attached, the yellow trace is UP801S pin 1, the blue trace is pin 4 and the purple trace is pin 7 at the gate of QP802S.

When working correctly or while in the boot-loop the voltage across CP815 is a clean 379vdc.

Pin 1 stays around 2.88v-3.5v when it the TV is working correctly as well as some of the time when it was boot-looping. It also got as high as 8v.

I've seen pin 4 creep above the .8vdc that triggers an over-current condition only when it is rebooting. When it was working correctly it has stayed in the 640mm-760mm range. The source on that switching transistor is pretty noisy so the ripple exceeds the trigger voltage quite often.

The output (pin 7) changes pulse width as necessary to maintain the output at 380vdc. It narrows when it reboots and then widens as the inverter load comes on.

[budm]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

In the pictures attached, the yellow trace is UP801S pin 1, the blue trace is pin 4 and the purple trace is pin 7 at the gate of QP802S.

When working correctly or while in the boot-loop the voltage across CP815 is a clean 379vdc.

Pin 1 stays around 2.88v-3.5v when it the TV is working correctly as well as some of the time when it was boot-looping. It also got as high as 8v.

I've seen pin 4 creep above the .8vdc that triggers an over-current condition only when it is rebooting. When it was working correctly it has stayed in the 640mm-760mm range. The source on that switching transistor is pretty noisy so the ripple exceeds the trigger voltage quite often.

The output (pin 7) changes pulse width as necessary to maintain the output at 380vdc. It narrows when it reboots and then widens as the inverter load comes on.

You are getting the correct boosted Voltage even when the TV is power cycling and the PS-ON is steady, there is no problem with the PFC circuit, if the PFC shutdown the Voltage on the Voltage on the cap would have gone back down to 160vdc.
BTW, what did you use for the scope ground? The chassis? OR YOU FLOAT THE SCOPE?
The circuit has lots of high Voltage running around in the inverter section (>1000v) so scope ground leads will pick up lots of noise.

[budm]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

When making the measurement on the hot side of the circuit, the scope ground is the negative leg of the main filter cap, not chassis ground.
YOU CANNOT DIRECTLY ATTACH THE SCOPE GROUND TO THE HOT SIDE CIRCUIT GROUND, YOU WILL BLOW UP THE POWER SUPPLY BOARD AND YOUR SCOPE, you have to connected the TV to the isolation transformer.
Are you floating your scope AC plug ground prong?
As I indicate, the backlights circuit is disable and the board is fooled to think that DET-5V is OK but the TV still power cycle, then you have problem in the main board.
You already know that the self test of the power supply board is OK and the backlights stay on a long as the AC is applied to the board.

[Howard_Woodard]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Yes, I understand the difference between power ground and signal ground -- my scope connections are correct and the readings are correct. I have it plugged into a GFI outlet and have popped it a few times when I had one probe on the hot side and tried to simultaneously monitor a point on the cold side. Ha ha, it probably thought the difference in current between L1 and N was flowing through my heart.

Btw, the heat sinks inside the "hot line" on the PCB are part of power ground. But the negative side of the main filter cap is always a good point and easy to reach.

I'm curious as to why the source pin of the switching diode doesn't mirror the gate, just higher voltage.

As for this thread, I'm gonna be done with it. Its an old TV and I was tinkering with it out of curiosity. Besides, as you said, the power supply is doing what its supposed to. And the only documentation I've found on the mainboard is for the video components.

Thanks for all your time and good suggestions.

Regards,

Howard

[budm]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Isn't your scope probe ground connected to the safety ground prong of the scope AC plug?
If it is, I cannot see how you can have the scope probe ground connected to the hot side circuit ground which is not at ground potential.
At this point main board is the issue.

[Howard_Woodard]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Just a final update in case others might have similar symptoms...

After being off for a couple days it came up and acted correctly as it has done before. But rather than turning it off, which has always caused it to resume the endless looping, I updated the firmware. After successful completion of the update the TV has worked perfectly ever since -- no looping or any other unexpected behavior.

[R_J]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Just a note to anyone using a scope and working in the hot side of a power supply, A GFI or GFCI, IS NOT an ISOLATION TRANSFORMER. If your scope is grounded, connecting it to the power supply hot side will damage the power supply.

[Howard_Woodard]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Ha ha. No scopes or other electrical components - or engineers - were hurt in this effort. You're the second to offer such a warning on this thread. I guess I might have assumed too much -- I assumed that anyone using a forum called "badcaps" working on an open TV set was probably knowledgeable enough to understand how power supplies, including switchers and their grounds, work and especially how to use oscilloscopes.

At any rate, I guess it's a good warning for those that aren't competent -- assuming that they know what GFIs, GFCIs and isolation transformers are of course...

Again, thanks for the attempts to help me with this.

[R_J]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Don't use that word.. you know the one, just read a few of the other threads.

[nomoresonys]

Re: Samsung 32 inch (LN32B360C5D) in perpetual restart loop...

Just needed an update, another good reason to try the simple shit first.