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#1
10-17-2006, 11:29 PM
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Anyone know how to fix a 2005 Magnavox
Hey guys, another one of my cow-orkers has a dead TV
this one's a 2005 Magnavox TV/DVD Wal-Mart-Special combo model MSD520FF. What I know so far is the Horiz output is shorted or at least I think it's the HO, 3 legged heat sunk transistor near the Fly, probably gotta be the HO. The fuse isn't blown, the diodes in the PS are OK but it won't turn on. There is a heat sunk transistor in the power supply that's getting screaming hot even with the set not operating (but plugged in), also the main filter cap is getting hot but I can't tell if it's just radiant heat from the transistor heating it up (feels more like the heat's from the cap itself) BTW the red hot transistor isn't shorted! there is a small Zener diode in the PS that MAY be shorted, reads .5 ohms but I need to disconnect it to be sure. This probably isn't worth a whole lot of time but since it's only a year old I thought I'd take a crack at it. 
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#2
10-17-2006, 11:39 PM
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Ewwwww........
But seriously, good luck! John
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#3
10-17-2006, 11:50 PM
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#4
10-17-2006, 11:58 PM
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That sounds like the input cap but I would look at a print to be sure, that is if you can find the print. You will have to test the zener diode to make sure and if you're reading with a standard DVM now on ohms scale Im almost sure its shorted OR whatever else is in parallel with that diode itself. If the transistor is getting hot its being driven into conduction but is it a square wave making it up the leg or something feeding it DC? A square wave means something is trying to work and I would venture to guess a cap is soaking everything up through ESR leakage. Most consumer grade switching supplies are of the single ended HEXFET design or IGBT transistors, they regulate by controlling the duty cycle of a square wave around 100 KHz. If that transistor was shorted the fuse would have blown as the transformer itself is only a fraction of a DC ohm but several hundred ohms at 100 KHz, the transistor may in fact be leaky and given the heat stress I put money on that as well. Is there an IC on the board marked uPC 494? These are notorious for failing without provication.
The zener may be a shunt regulator to protect the gate/source of the FET on the heat sink so check the zener AND the FET out of the circuit using the diode check function of your meter if you have it. A good FET will show around 0.600 volts forward between gate/source and will show open in reverse. Also check ALL the caps on the board along with the opto-isolator, that is the square IC with only 3 ot 4 pins coming out of it and it may be white colored. The caps may test good using an ohmmeter but dont be fooled as the leakage may rise with 100 KHz of rectified squarewave coming at them. If they get leaky enough they can saturate a power supply to the point of foldover leaving you with a very smelly supply and little output if they havnt exploded yet. That should get you started and I will watch this thread for a while and offer you a hand grenade to finish the job! 109 Last edited by ARC Tech-109; 10-18-2006 at 12:02 AM. Reason: addl line
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#5
10-18-2006, 10:24 AM
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The only equipment I have to check this thing is a Fluke meter.
Do you think possibly the filter cap is bad dragging down the power supply and popping the HO transistor? Might be worth a shot replacing those parts if they're not to much$$? However if the zener is shorted I have no way of knowing what voltage it is.
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| Audiokarma |
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#6
10-18-2006, 02:18 PM
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You can get an idea by looking for numbers on the diode itself and where it goes in the circuit, would imagine its a reference diode or shunt. Get a mag glass and look for the numbers and if no luck trace back the circuit itself. Im willing to bet its a 6.8v reference or 12v shunt. If its across the source/gate of the switching transistor Im almost sure it will be a 18.5v working as a snubber for scrubbing off the high leading edges of the square wave making its way to the switcher transistor.
109
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#7
10-18-2006, 02:27 PM
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I'd take the lazy approach..... Last year a friend gave me a broken 13" Toshiba TV/DVD combo unit. (POS Orion) Had the SAME problem as yours it sounds like. This sucker would snap if you plugged it in. I just went to a store that carried them and exchanged it for a NEW one. Funny thing tho... the replacements TV worked but had a dead DVD player. I took it back and got store credit. They're ALL junk
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#8
10-19-2006, 11:29 AM
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Funai stinks. When I worked at Sears, we sold several models of Funai with sears brands on them. Bad power supplies galore, and occasional blown hz outputs. Rarely ran long after we fixed them. Many were returned and exchanged.
Be aware that chances are good that your horizontal output probably has an internal damper diode too. Your replacement needs to have one, too.
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#9
10-19-2006, 01:14 PM
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The worst thing about some Funai VCRs is that their power supplies had no overvoltage protection. When (not if) the regulation failed, it would sent way too much voltage to everything resulting in a totally destroyed VCR.
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#10
10-19-2006, 02:30 PM
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| Audiokarma |
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#11
10-19-2006, 07:22 PM
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#12
10-21-2006, 09:50 PM
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I remember the first Funai VCRs... There are more than you think of them still running.
I was only referring to the Sears stuff that were either just TV's, or TV/VCR's. Rotton quality all the way!
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#13
10-21-2006, 11:11 PM
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I think I'll pass on trying to fix this one, maybe I'll try a HO transistor if I can get an NTE replacement from the local Electronics store but I can't see spending more than $10 on this experiment!
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