Megmeet MLT666 R2.8 cockroach short

[attainteddragon]

Re: Megmeet MLT666 R2.8 cockroach short

you can check them in diode mode. leg 1-2 2-3 1-3
then reverse leads.
if you get one that looks sus, pull it out and retest.

[budm]

Re: Megmeet MLT666 R2.8 cockroach short

Z2 ZENER show as shorted because it is in parallel with with very low Ohm (<1 Ohm) Q2 Source resistor. You should lift one leg off the board and test it again.
By the way, Q2 is for PFC Voltage booster circuit.

[mikoz]

Re: Megmeet MLT666 R2.8 cockroach short

Primary Top side mosfet test results:

Q14 Black 1-2=1 2-3=564 1-3=715
Q14 Red 1-2=1 2-3=1 1-3=1
Q2 Black 1-2=1 2-3=518 1-3=668
Q2 Red 1-2=1 2-3=1 1-3=1
Q3 Black 1-2=707 2-3=1 1-3=1
Q3 Red 1-2=1188 2-3=703 1-3=1013
Q8 Black 1-2=011 2-3=002 1-3=001 shorted?
Q8 Red 1-2=002 2-3=002 1-3=001 shorted?
Q9 Black 1-2=001 2-3=001 1-3=001 shorted?
Q9 Red 1-2=001 2-3=000 1-3=001 shorted?

Primary Solder side mosfet test results

Q12 1-2=1 2-3=1 1-3=595
Q12 1-2=1 2-3=1 1-3=1
Q1 1-2=1130 2-3=556 1-3=878
Q1 1-2=628 2-3=575 1-3=670
Q5 1-2=1 2-3=699 1-3=1
Q5 1-2=696 2-3=1 1-3=1639
Q4 1-2=1 2-3=689 1-3=1
Q4 1-2=687 2-3=1606 1-3=894
Q6 1-2=1 2-3=004 1-3=1 shorted?
Q6 1-2=1 2-3=004 1-3=1 shorted?
Q7 1-2=697 2-3=1 1-3=1
Q7 1-2=1 2-3=701 1-3=1472

Q8 = 6N60C (FQPF)
Q9 = 6N60C (FQPF)
Q6 = 4403?

[mikoz]

Re: Megmeet MLT666 R2.8 cockroach short

FQPF6N60C's are end of life apparently? Could somebody recommend a suitable alternative for me?
I have just had a look on rsonline and they say similar is the FDPF5N60NZ but the continuous drain current is 4.5A instead of 5.5A of the 6n60C is that going to be a problem?
Ive attached datasheets for 5n60, 6n60 and 7n60 below.

[mikoz]

Re: Megmeet MLT666 R2.8 cockroach short

pulled Q8 And Q9 out and here is the results.

Q8 Black 1-2=1823 2-3=553 1-3=001
Q8 Red 1-2=553 2-3=1818 1-3=001
Q9 Black 1-2=001 2-3=001 1-3=001
Q9 Red 1-2=001 2-3=001 1-3=001

So I guess Q9 is dead short then.
Is there anything I should test while these are out?

[mikoz]

Re: Megmeet MLT666 R2.8 cockroach short

I'm onto the last piece of this puzzle now. Does anybody know what the pink casing denotes as this 0.5R resistor is open. Is this a fusible resistor?
I have salvaged a green one with the first four band colours the same but it has a fifth black band? It measures 0.8R?

This is R74 on this PSU. Would it hurt to try it out?
Picture below...

[budm]

Re: Megmeet MLT666 R2.8 cockroach short

That is MOX (Metal Oxide) fusible resistor. The green one, when you remove from the board, is it marked on the board with Rxx? Just to be sure it is not an inductor. If it is 0.8 Ohms resistor, you can use that for now.

[mikoz]

Re: Megmeet MLT666 R2.8 cockroach short

Yeah the green one was marked R20 on the donor board. I've soldered it and the replacement 6N60C mosfets in now to R74, Q8 & Q9 and will report back after I plug the TV in.. if all is well?

Wish me luck..

[mikoz]

Re: Megmeet MLT666 R2.8 cockroach short

OK I'm done.. I give up.. Fuse blows as soon as its switched on at the wall... Tossing it after harvest. All replaced parts still test good but there is obviously still a short someplace.

[Caleb]

Re: Megmeet MLT666 R2.8 cockroach short

Have you tried putting a light bulb in place of the fuse and using freeze spray to see what is shorted? It works quite well for finding pesky shorts.

[mikoz]

Re: Megmeet MLT666 R2.8 cockroach short

Hi Caleb,

Would I just solder a 100w bulb directly to the board? And Freeze spray? (Any particular brand) Does that increase resistance on the problem component and flicker the bulb or something? Could you please provide me some instructions as I've never heard of that method before?

Mick

[martino]

Re: Megmeet MLT666 R2.8 cockroach short

Have you tried putting a light bulb in place of the fuse and using freeze spray to see what is shorted? It works quite well for finding pesky shorts.

Yes,plz describe your method,very interesting

[Caleb]

Re: Megmeet MLT666 R2.8 cockroach short

I have a light bulb socket with alligator clips wired in to each leg, 100 watt bulb goes in socket one alligator clamp on each side of fuse holder. Thereby using the bulb as a large resistor, cutting down on the current flowing through the board. Any shorts will pull juice and make the bulb glow brightly. Normally when you plug in a working tv the bulb would glow for about 3 seconds as the caps charge then go out until you hit the power button. When you have a continuous short the bulb will glow and not go out, this works great for finding shorts in the primary as the short should be the hottest spot on the board, add a little freeze spray and it will thaw out first pinpointing the shorted component. One thing to note if your short is in the secondary it may not show up using this method as most power supplies wont turn on with the light bulb.

I noticed you said fuse blows as soon as it is switched on at the wall? does that mean it didnt blow until you turned it on? or it blew when plugged in? If it didnt go until it was turned on it kinda sounds like the primary may be ok now and you have trouble down the line.
If you have a current limited power supply you can do the same thing on the secondary side with the primary unplugged just match up voltages (12v to 12v or 5v to 5v) the short should warm up first. I havent gotten around to buying one yet but am planning on building a temporary one from an old atx computer power supply, you can use an automotive lightbulb to limit current on the low voltage stuff.

[budm]

Re: Megmeet MLT666 R2.8 cockroach short

This is what I made: It basically put the lamp in series with the load.
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

[mikoz]

Re: Megmeet MLT666 R2.8 cockroach short

We have a power switch on the wall. Lead plugged into TV with wall switched on fuse blows, I had not touched the TV switch yet.

[Caleb]

Re: Megmeet MLT666 R2.8 cockroach short

Budm has the fancy version, I may have to find some parts to make one of those! Any old canned air held upside down will double as freeze spray. From your description it sounds like its still got trouble in the primary so you should be able to use this method