Sceptre X460BV-F120 2 seconds to power off.

[tom66]

Re: Sceptre X460BV-F120 2 seconds to power off.

Ah, sorry if I wasn't clear, but what I meant is one of those diodes on the heatsink - one will be for 12V, one for 16V and one for 24V, any one will do, and TV should be switched on, making noise. Those little things do look like ferrite beads.

Even though it's not one of the main outputs, I see considerable negative peak ripple -- that is, the negative peaks are varying. I'm not sure why; this should only happen if the main capacitor has significant variation across it. The peaks appear to vary by at least 2~3V (assuming 5V/div)... which won't have a direct translation to the main cap ripple but is way more than I'd expect in standby.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

Let's try this again.

http://i177.photobucket.com/albums/w...D92AD461D1.mp4

[tom66]

Re: Sceptre X460BV-F120 2 seconds to power off.

Yep, those were the cathodes, there are two per diode. And we see the same pulsing on the negative portion of the wave. So check the voltage across the primary capacitor while it is working. Your meter may not be fast enough to register the pulsing, so first try replacing the startup capacitor for the PFC section, let me know if you need help locating that. Also, does the same pulsing occur (perhaps on a lesser scale) with the inverter disconnected? -- just in case the inverter is causing the fault.

Don't you have one of those hook probes for the scope? Makes attaching to component leads so much easier.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

Yep, those were the cathodes, there are two per diode. And we see the same pulsing on the negative portion of the wave. So check the voltage across the primary capacitor while it is working.

With the scope or the meter?

Your meter may not be fast enough to register the pulsing, so first try replacing the startup capacitor for the PFC section, let me know if you need help locating that.

I need help locating that.

Also, does the same pulsing occur (perhaps on a lesser scale) with the inverter disconnected? -- just in case the inverter is causing the fault.

This was with the inverter disconnected.

Don't you have one of those hook probes for the scope? Makes attaching to component leads so much easier.

Yeah, I have one. I didn't think to use it.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

I just checked the primary cap again. With the PSU forced on and all boards disconnected, the DC voltage across the cap fluctuates from 336V-390V. Is this caused by a bad cap or possibly something else?

[tom66]

Re: Sceptre X460BV-F120 2 seconds to power off.

With the scope or the meter?

You'd blow up the scope if you tried to measure across that (without an isolation transformer), so I'd advise you use a meter. The scope is earth referenced, while the capacitor is not relative to earth, but floating around all over the place, about +/-400V.

The startup capacitor will be near the PFC IC; I suspect the PFC IC will be on the bottom of the board, so after you've measured the cap voltage, flip the board over (REMEMBER TO DISCHARGE THE 400V CAP!) and take some pictures around where that inductor, diode and MOSFET I mentioned are, I think it's bottom left corner, near the big cap.

I just checked the primary cap again. With the PSU forced on and all boards disconnected, the DC voltage across the cap fluctuates from 336V-390V. Is this caused by a bad cap or possibly something else?

There's your problem -- and this is hopefully the PFC not starting up/running properly due to bad startup cap. It should be stable 385~400V.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

There's your problem -- and this is hopefully the PFC not starting up/running properly due to bad startup cap. It should be stable 385~400V.

I've got that cap out. I'll have to see if the local guy has one in stock tomorrow. Otherwise, I may have to order it.

I have the set plugged into a surge protector. With the probes of my DMM on the leads of the primary cap, when I turn the surge protector on, the voltage jumps to 394V immediately and slowly drops to 169V. The whine from the transformer gets higher at the same rate as the voltage drops on the primary cap. When the set is powered on, the voltage on the cap jumps to 394V and stays there until the set is powered off.

Why do you suppose it would measure OK when all of the boards were connected and the set was powered on via the ON/OFF button, but fluctuate when the PSU is forced on?

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

So, I'm not so sure that the cap is the problem. The big cap is a 270uf 450V cap. With the questionable cap out of the circuit, I connected two 150uf 450V caps in parallel and jumpered them into the PSU with the same results.

Is it possible for a ceramic capacitor to make the whining noise? I took a three foot section of tubing with one end up to my ear and used the other end to locate the sound. If I touch the end of the tube to the cap circled in the pic below, I hear the sound loud and clear. If I touch the transformer but not the cap, I hear it, but not like I do if I'm touching the cap. I'll see if I can get a good video.

[tom66]

Re: Sceptre X460BV-F120 2 seconds to power off.

Sorry, I meant "there's your problem" as in, that's what's causing the issue with flickering backlights/buzzing etc. Not necessarily the big cap. In this case I reckon the startup cap could be the fault. I've looked at the board and can't definitively narrow down which is the startup cap. It could be any one of of those four down the bottom left of the PSU. So flip the board out and take a picture of the back side.

The whining can come from some capacitors (microphonic effects), but it's more likely to come from coils and transformers.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

http://i177.photobucket.com/albums/w...F1906E473B.mp4

Here's a video of what I believe to be a capacitor whining. I'll get the pic of the back o the board in a second.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

Back of area with those four (1000uf 35v) caps and back of entire board.

[tom66]

Re: Sceptre X460BV-F120 2 seconds to power off.

I have a feeling that cap is part of the snubber network - so it will buzz whenever current flows through the transformer, it gets a share too. Something is up with the switching frequency, too... because it shouldn't be audible. I don't think the cap itself is failed, though.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

Back of board

[tom66]

Re: Sceptre X460BV-F120 2 seconds to power off.

Part number of IC120 please -- top right of board.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

I think it says TKAA FAN6961. That's with two pairs of reading glasses and a magnifying glass.

[tom66]

Re: Sceptre X460BV-F120 2 seconds to power off.

I'm gonna guess you're correct as it is a PFC controller.


Vcc is pin 8... follow the trace from pin 8 and it should lead to the startup capacitor.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

I traced pin 8 to this area. Things aren't marked so well on the back of this board. This is the first cap I come to but it's hard to follow the trace from that point.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

The other side. 50V 10uf. I have several 50V 22uf caps. Would they work?

[tom66]

Re: Sceptre X460BV-F120 2 seconds to power off.

22uF would probably be fine -- value is not critical, also the maximum voltage on Vcc should be about 30V, so a 35V cap should be fine... but I'd go with the 50V to be safe.

[johnboy1313]

Re: Sceptre X460BV-F120 2 seconds to power off.

I replaced all four caps with the correct caps. No luck. The big cap still fluctuates. Turned out to be three 50V 10uf caps and one 50V 22uf.