Sceptre 32 inch , PSU problem?

[khaosduke]

Hello all, I've been reading the basics documents and have a Sceptre 32 inch with no power, no lights nothing. The PSU is a meg meet mp116.

I'm trying to figure out what to test first, all I have is a multimeter. I know I should first check the +5V rail because that is always on so long as the unit is plugged in.

I didn't see any obviously blown capacitors, possibly some bulging ones? but I don't have an ESR meter to check.

My question is, is there anything else I could probe on the PSU to make a definitive conclusion that it's the PSU?

[budm]

Re: Sceptre 32 inch , PSU problem?

https://www.badcaps.net/forum/showthread.php?t=39740

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

I'll post pictures once I get back to it and opened up again. Hopefully tonight.

[budm]

Re: Sceptre 32 inch , PSU problem?

BTW, there are more than one version of this mp116:
http://www.shopjimmy.com/catalogsearch/result/?q=MP116

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

Here are some pictures, I tried to get decent pictures which was difficult with the glare. If you need anything close up, a particular part of the board; I'll be more than happy to try again.

I tested the +5 and got nothing out of it.

Also I just noticed there seems to be a patina around the hot side of the underside of the board, like it was burnt.

Thanks again!

[budm]

Re: Sceptre 32 inch , PSU problem?

"I tested the +5 and got nothing out of it. 5V or 5VSB?
If 5VSB then you can try heating up the start up caps EC6, and the one with orange sleeve next to the jumper J7 (both of these two caps are sitting between the main filter cap laying its side and the small transformer for the standby power supply), then apply the power and see if the standby power will start running.

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

I'm sorry, I tested the +5 standby and got nothing. I'm going to try heating up the caps you mentioned and report back.

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

I tried heating up the 2 caps, nothing past 100F and still getting 0v out of the 5VSB.

[budm]

Re: Sceptre 32 inch , PSU problem?

OK, lets make sure we have voltage at two legs of the main filter cap, it should have about 170VDC on the two legs, if it does, then we need to test the voltages right at the two legs of those two caps that you heated up.

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

I get a voltage, consistently -10.4~-10.5 mV at the legs of the main filter cap. 0v at the other caps. When it's warmed up a bit I get -15mV at the main filter cap and still 0v at the others.

The main filter cap gives it's readings regardless of if there is power applied, so I'm going to assume this is the resting potential of the cap?

[RCA2000]

Re: Sceptre 32 inch , PSU problem?

If you are not getting your hot B+ at least 160 volts -- at the big cap--there is an input problem on the PS.

Perhaps a shorted FET on the supply, or if this one uses a boost converter--a shorted boost transistor?

You DID check the fuse--right? if open--you then need to OHM that same big cap and see is there is a low resistance short there. It should read 1K or higher. It is reads a few ohms--a short exists on the hot side of the supply--likely a FET.

[RCA2000]

Re: Sceptre 32 inch , PSU problem?

I looked at the picture closely. this one DOES does have a boost converter. I have worked a LOT on this SMPS_-for the "kids" to then put in a set.

It often had a shorted boost diode and FET on the boost converter. Sometimes a shorted FET on the standby (small transformer) or one or both of the ones on the run supply (large transformer). I also saw some shorted diodes on the "cold side" on the 5 and 12 volt lines--and that will totally hold down the voltages. But you would still have full voltage on that input cap on the hot side if that was the case.

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

Hehe, well ... that went well. I checked the fuse and it was blown so I did of course what you're not supposed to do and jump it. That blew out the R12, the big resistor and one of the smaller ones. The cap seems fine it even read some residual voltage. Now the problem here is...


There is something right at the input of the board that would cause the fuse to blow? It's right at the terminal so maybe the connector is bad?

[RCA2000]

Re: Sceptre 32 inch , PSU problem?

No--likely you have a bad FET or bridge, but likely a FET, either the boost converter or one of the SMPS FET's. The boost converter is supposed to boost the line voltage to nearly 400 volts during the "run"mode--it supposedly makes the SMPS more efficient this way. if it shorts--it puts the full hot B+ straight to hot ground--blowing the fuse.

But by shorting the fuse--and blowing the surge resistor--you may have done more damage. It might be better now--to get another PS that is good on Ebay.

If you still want to troubleshoot--I will walk you through it.

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

I'm confident knowing the PS was bad the TV would work, however I like the learning process and it's why I undertook this to begin with. So I would very much enjoy going through it, if you don't mind of course.

[RCA2000]

Re: Sceptre 32 inch , PSU problem?

You already know you have a short on the hot side. So, Start by checkng that boost converter FET on the hot side. It is on that heatsink---in front of the big cap.

Likely you will find that shorted.

If so--check the diode off of it. It is likely shorted too. This is blowing the fuse.

If that is not shorted--then move to the 3 FETs--near the center of the board. These drive the 2 SMPS transformers. one is for standby and the other pair drive the larger, run tranny. If these are shorted--they too--will blow the fuse, and sometimes the boost converter and "flywheel" diode off of it.

Any of these shorted--must be replaced--before you can move on.

If NONE of these are shorted--check the bridge rectifier, near the sink on the hot side. It is possible-but unlikely--it is the shorted component.

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

I tested the MOSFET, it's a K3568. It's shorted out from drain to source. The diode next to it seems to be working fine, also just checked the 3 FETS you mentioned off the center of the board and they appear to test fine.

I didn't take them off the board but it might be interesting to be replace the boost converter FET and the blown resistors and fuse as they are all low cost parts anyway.

[budm]

Re: Sceptre 32 inch , PSU problem?

When the MOSFET goes bad, you need to verify that the IC that drives the Gate of the MOSFET is not damaged or bad, otherwise your new MOSFET will be damaged. I usually replace the drive IC at the same time.

[khaosduke]

Re: Sceptre 32 inch , PSU problem?

The IC that goes is on the underside of the board which has a resistor (the one above the one of the SMD's I blew out) is marked 1606B it's a PWM/Power factor controller. I have no way to really tell if it actually works.

In addition to the 0.1ohm surge resistor that blew , one of the tiny 1 ohm SMD's around the leg of the bad MOSFET is also blown. Provided that I replace the IC, the MOSFET, the 2 resistors and the fuse, at that point what else would I have to test?

[budm]

Re: Sceptre 32 inch , PSU problem?

If you trace the Gate pin, there should be some resistor/diode network between the Gate pin and the Out put pin of the IC, if those are blown, then the IC is more likely to be gone. I would not take a chance of blowing new MOSFET and cause more damages. You will have to get the P/N of the IC.
I also use lamp in series with the AC line to find out if I still have shorts in the circuit after new parts are installed.
These are what I made beside using Isolation transformer and GFI outlet for safety:

http://s807.photobucket.com/user/bud...tml?sort=3&o=0

http://s807.photobucket.com/user/bud...tml?sort=3&o=1

http://s807.photobucket.com/user/bud...tml?sort=3&o=3

http://s807.photobucket.com/user/bud...tml?sort=3&o=7