Memorex MLT3221 - No Power

[Gagne1023]

Re: Memorex MLT3221 - No Power

I checked the fuses on the board and found where F2 was bad. It took awhile to get a replacement but once I got it and stuck it in there, powered it on and the screen flashed for a quick second then went black again... After rechecking F2 it had blown again... So any ideas on why it's blowing the fuse.... Also since I was ordering a fuse I went ahead and just replaced those two Caps that u had mentioned

[Dgtech]

Re: Memorex MLT3221 - No Power

Well, there are two fuses on this board - F1 and F2.

F1 = Main power fuse (120VAC here so be carefull)
F2 = Small green component right next to the largest capacitor on this board.

Also, the two caps near the two transformers need to be checked. These might be 10uF, 450V caps. I just changed a pair in a Polaroid. The bad ones measured 4 nanofarads.

[Gagne1023]

Re: Memorex MLT3221 - No Power

IPOS 150
REV:1.8
2006.11.25
200-P00-HM150H

Thse are the specs on the supply board. The Rev number is there... not sure which is the model number. Also i should mention that i could only find one fuse on this Tv.

[Dgtech]

Re: Memorex MLT3221 - No Power

This power supply board has different revisions to it. Please post a picture of your board with the model number and rev number.

[Gagne1023]

Re: Memorex MLT3221 - No Power

I have had the same problem with the same tv. These posts were very helpful since I could just go straight to D10 and check it and that was my problem. I now have 5v across the connection and the tv will turn on. The LCD screen however will not. There is a little red LED light in the back that is on but I'm not sure what this indicates... Any ideas????

[Dgtech]

Re: Memorex MLT3221 - No Power

If your diode was shorted, then yes replace it with the 304. However, without you neither describing the nature of the failure or why you think it is the diode, I cant tell you for sure if it will fix the problem.

[momaka]

Re: Memorex MLT3221 - No Power

Excuse me my diode is a Her303 will it be okay to put a 304 in it.

If you meant Her 304, then yes, you can use that as a replacement.
Her 304 is essentially the same diode as Her 303 but with a higher voltage rating.

[Bakke]

Re: Memorex MLT3221 - No Power

Excuse me my diode is a Her303 will it be okay to put a 304 in it. It is a IPOs 150 H. it says on the heat sink D120-0000-ipos150 H. and on the diode j710 Her 303

[Bakke]

Re: Memorex MLT3221 - No Power

So did you just replace the old diode in the heat sink?

[Dgtech]

Re: Memorex MLT3221 - No Power

The replacement part was an NTE equivilent. NTE576

[momaka]

Re: Memorex MLT3221 - No Power

Hey don't thank me. After all, you found the shorted diode and did all of the work . Good job!

By the way, what diode did you use as a replacement? (you know, just in case anyone needs this as a reference.)

[Dgtech]

Re: Memorex MLT3221 - No Power

Everything works well after replacing that diode. All powered up and scanning for digital channels now. Thanks again momaka. I apreciated your help and guidance.

[Dgtech]

Re: Memorex MLT3221 - No Power

I apreaciate the help momaka. Yes, the readings were out of circuit. After checking Digikey and Fairchild, I finally found it at Allied. I will post the result once I get the part. Thanks again.

[momaka]

Re: Memorex MLT3221 - No Power

I did find a shorted diode at D10.

Is this out of circuit or in-circuit measurement? If in-circuit, measure it again out of circuit just to make sure.

Now I have to find a solution for a match if I cant find the exact component.

Looking at the HER304 datasheet, just about any fast-recovery diode capable of 3A @ 50C should work.
Basically just match the Forward Current (Amp) rating and the Maximum Reverse Recovery Time rating. HER304 has 50 ns (nanoseconds) recovery time. As for the voltage rating - I doubt this PSU has any voltage on the secondary side above 30v, so even a 100v diode would likely work fine.

*EDIT*
Looking at second photo above, the solder joint between the heatsink and the diode seems cracked. Perhaps this is what caused the diode to short?

[Dgtech]

Re: Memorex MLT3221 - No Power

It was my mistake, you were right about the part number. I checked out those components and found that they were good. I changed the cap just to be sure - same result.

I did find a shorted diode at D10. This diode is marked as HER304. I have circled it on the attatched photo. It is attatched to a heatsink and is standing upright. As also seen in the 2nd photo. This is the classic case of "I thought I checked that before". Taking notes would have made all the difference.

Now I have to find a solution for a match if I cant find the exact component.

[momaka]

Re: Memorex MLT3221 - No Power

however, does .158 sound right for D6(both diodes)?

I think there was a misunderstanding. The diode D6 I was talking about is located on the primary side, close to the transformer and Q6. Perhaps I misread the numbers on the board.

Do these fail like this? or do they usually burn open?

Diodes *usually* become shorted when they fail.

However, the reading you got for those schottky diodes attached to the heatsinks (0.158 for both diodes) sound normal since schottky diodes have lower voltage drop (and hence better efficiency, which is why they are preferred for the outputs in PSUs).

Both datasheets are attatched. I cant seem to find the information from these to tell if both are good or not.

For FYPF2045DN, if you look at Figure 1. Typical Forward Voltage Characteristics (on page 2) you will see a graph of Forward Curent vs. Forward Voltage Drop. As the Forward Current increases, the Forward Voltage Drop becomes larger. Since your multimeter is not putting a lot of current through the diode, the Forward Voltage Drop you get displayed on your multimeter's screen is fairly low. Hence the low readings.

[Dgtech]

Re: Memorex MLT3221 - No Power

.

[Dgtech]

Re: Memorex MLT3221 - No Power

D2 looks fine - .5 Vdc on diode check however, does .158 sound right for D6(both diodes)? I'm used to diodes checking .5 Vdc or .7 but not .158. Do these fail like this? or do they usually burn open? Also, the one above it (on the same heatsink) measures .252 as well.

Here are the numbers on these:
D6 = FYPF2045DN
Above component = MBRF10100

Both datasheets are attatched. I cant seem to find the information from these to tell if both are good or not.

[momaka]

Re: Memorex MLT3221 - No Power

Check diode D6? and also small SMT diode D2 on the backside of the board.
If those are good, check resistance between 5vsb and ground. If it's not shorted, change the output capacitors on 5vsb - that would be EC15 and the two capacitors to the right of the inductor L6 in the second picture.

[Dgtech]

Re: Memorex MLT3221 - No Power

OK- I found my error with testing this mosfet. EARLY MORNING BLUNDER. This one is a P - channel vs an N -channel. REVERSE THE POLARITY OF THE METER LEADS. LOL.

I need sleep, or alcohol, which ever comes first!

This mosfet tests ok now. I still have no output on this power supply.