Memorex MLT3221 - No Power

[johnnieboy2]

Re: Memorex MLT3221 - No Power

All I got was some low levels of pulsating voltages, millivolt range. Definitely replacing those startup caps when I get them. Short on the output, can you elaborate, what part... another board?

[PlainBill]

Re: Memorex MLT3221 - No Power

Your input is most appreciated. I really don't like the 'shotgun' approach I'm using but without a schematic and lack of Digital TV experience it's that or the junk pile.

That said, what would be the conclusion if I have or don't have the 13v?

Troubleshooting the standby power supply is rather straightforward. Dgtech is suggesting you check the power supply pin for the SMPS controller. Googling the part number and datasheet should lead you to the datasheet, which will identify the proper pins. If Vcc is missing, it suggests a problem with a bleeder resistor or the rectified mains supply. If it is low and steady, it suggests a bad SMPS controller. If it is pulsing, it suggests a short on the output or a bad startup cap.

PlainBill

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Your input is most appreciated. I really don't like the 'shotgun' approach I'm using but without a schematic and lack of Digital TV experience it's that or the junk pile.

That said, what would be the conclusion if I have or don't have the 13v?

[Dgtech]

Re: Memorex MLT3221 - No Power

As you know, your 5V DC standby voltage is most likely your problem. With that being said, the problem rests on the power board. There are only a few components that make up the 5VDC. I dont have a schematic, and I'd give my left pinky toe for one, but I think the PWM might have something to do with the standby 5V. Check to see if that main chip, near the main output connector, has 13 volts on one of the pins. I can't remember what pin it was. I'm gonna try to find a thread with more info to help out.

[johnnieboy2]

Re: Memorex MLT3221 - No Power

I've ordered a set of caps for the P/S board, 16 in all...will update when installed and tested. If this doesn't do it then it's !!! And in case of failure...anyone looking for components off this thing?, Let me know.

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Thanks for explaining that.

[PlainBill]

Re: Memorex MLT3221 - No Power

I can do that, but out of curiosity, does this board have a 5v reg? If not how is it developed?

This is a switching mode power supply (actually two of them on one board). Regulation is accomplished by the interaction between a TL431A voltage reference, an opto-isolator, and the SMPS controller IC. In very general terms the SMPS controller pumps energy into the primary of a transformer, the secondary voltage is rectified and filtered, and the voltage reference instructs the smps controller to vary the amount of energy dumped into the primary based on the how closely the filtered voltage matches the design voltage.

While it sounds (and is) complicated the entire arrangement can provide over 200 watts of well regulated (within 1%) power at more than 80% efficiency from a package about the size of a cigar box. By comparison, 30 years ago I workec for a company that built 200 watt linear power supplies. They were 18" x 8" x 8" and less than 33% efficient.

PlainBill

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Can anyone tell me what R56 value should be? It's located just to the right of EC7, one of the famed failure caps.

[johnnieboy2]

Re: Memorex MLT3221 - No Power

I can do that, but out of curiosity, does this board have a 5v reg? If not how is it developed?

[retiredcaps]

Re: Memorex MLT3221 - No Power

Well, I can't put out the cash for the meter, that would outweigh my valuation of the TV and I don't expect to be able to use such a meter much in the future.

Agreed. Then just change out those caps with something like Panasonic FC to rule them out as suspects. It shouldn't cost more than $2 USD for those 3 caps.

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Well, I can't put out the cash for the meter, that would outweigh my valuation of the TV and I don't expect to be able to use such a meter much in the future. Are there specific publications like the old Sams Photofacts available today for these TV's?

[retiredcaps]

Re: Memorex MLT3221 - No Power

Fluke 76, everything out of circuit.

No Fluke multimeter can measure a capacitor's ESR. Some Fluke multimeters can measure capacitance, but a cap may have the "correct" capacitane, but have high ESR.

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Fluke 76, everything out of circuit.

[retiredcaps]

Re: Memorex MLT3221 - No Power

Those two, 10ufd and 22ufd both were both good. I also checked the nearby 47ufd cap nearby under the same heatsink while I was 'there', it's good too.

Was your tests done with an ESR meter?

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Is there a 5v reg on this board? I don't recognize any of the marking on any of the devices as being std for a regulator.

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Instead of spinning my wheels troubleshooting individual components, I found the whole board available on shopjimmy.com for $70 and a few dollars shipping. It wasn't worth spending any more of my time checking caps in/out of circuit etc. The replacement board fixed it. If anyone wants my old board for parts (burnt traces, etc?) dated 2006, $20 and it's yours. I don't know what component is bad on it, but the largest cap on the board is very domed. On the new board, it's totally flat across the top. I suspect at the very least, that large cap will have to be replaced. -Web in Myrtle Beach

That's a great way to go probably if you're certain all your problems rest ONLY with the P/S board. Alas I'm not certain, so $70 might just be the beginning for me...so on with the desoldering

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Those two, 10ufd and 22ufd both were both good. I also checked the nearby 47ufd cap nearby under the same heatsink while I was 'there', it's good too.

[WY3XinSC]

Re: Memorex MLT3221 - No Power

Instead of spinning my wheels troubleshooting individual components, I found the whole board available on shopjimmy.com for $70 and a few dollars shipping. It wasn't worth spending any more of my time checking caps in/out of circuit etc. The replacement board fixed it. If anyone wants my old board for parts (burnt traces, etc?) dated 2006, $20 and it's yours. I don't know what component is bad on it, but the largest cap on the board is very domed. On the new board, it's totally flat across the top. I suspect at the very least, that large cap will have to be replaced. -Web in Myrtle Beach

[johnnieboy2]

Re: Memorex MLT3221 - No Power

Had already checked D10 out of circuit, it's fine.

[Dgtech]

Re: Memorex MLT3221 - No Power

With power off, check the resistance of this diode (D10) that I marked in this picture (RED BOX). it is standing up and connected to a heatsink. Check front to back then back to front. You should get low resistance one way and high the other. In my set, this is the one that went bad. It read around 5 ohms both directions.
From my research into this set, I found that the diode mentioned above would fail whenever someone would use this TV as a monitor on a computer. Basically a design flaw.