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Magnavox RS2744 A402 Turns off    
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tom418



Joined: 03 Aug 2014
Posts: 18
Location: New England

PostPosted: Sun Dec 21, 2014 4:11 pm    Post subject: Magnavox RS2744 A402 Turns off Reply with quote

Recently ( a few months ago) thanks to help from this board, I was able to get my Magnavox RS2744 A402 working again- Thanks JTS1957.

It's been working OK, except that once in a while, it will turn itself OFF. I've found that unplugging it, and waiting a minute or so, and then plugging it back in , will allow me to turn it back on. Sometimes, I can leave it on for hours and it will run, sometimes it will turn itself off just after a few minutes.

BTW: This new problem does NOT occur when I use the other socket in the duplex outlet it's plugged into. But my DTVconverter box works flawlessly in either socket. A heat problem?

Power supply problem again? I replaced F401, C432, and Q502 back in October. It was running fine until last week, then the random turns-off problem.
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JTS1957



Joined: 21 Jan 2009
Posts: 2266
Location: Far, Far Away

PostPosted: Sun Dec 21, 2014 6:47 pm    Post subject: Reply with quote

tom418 wrote:
BTW: This new problem does NOT occur when I use the other socket in the duplex outlet it's plugged into.


'If it hurts when you do that ... then do that.' or in this case: If it DOESN'T do that when you do that ... then do that. Rolling Eyes And 'that's' the truth!!! Smile
tom418



Joined: 03 Aug 2014
Posts: 18
Location: New England

PostPosted: Mon Jan 19, 2015 11:33 am    Post subject: Magnavox RS2744 A402 Turns off Reply with quote

Well, JTS1957, I was HOPING that I had a defective electrical outlet, but the TV is once again, dead.

So, I'm making repairs again. What I've done so far:

2.5A Fuse open. (Replaced)
Solder connections to the degauss coil resistor defective (repaired).

Q502 Shorted (Replaced)

IC 410 (no way to test, I replaced it this time). JTS: you said it may have been stressed last time I posted here. could this have wiped out Q502?

Parts I have, but did not replace yet:

Q400 (TIP 47) 129V source transistor
R438 .5w 680Kohm resistor (this connects to fuse)

JTS 1957: In this video, this guy is fixing a magnavox 25 CRT, similar to mine.:

https://www.youtube.com/watch?v=pzYraW8oWf4


He's changing T504 (horizontal driver transformer), and Q400. You had mentioned, in your reply to me previosly, that I may need to change the FB transformer T501 in a worst case scenario. Should I be doing that?

I'm just afraid that changing out Q502 is not solving a problem, but just curing a symptom.

Besides the FBT, what else could be frying Q502? BTW: MY Q400 looks brown. Burnt?

Thanks.
JTS1957



Joined: 21 Jan 2009
Posts: 2266
Location: Far, Far Away

PostPosted: Mon Jan 19, 2015 1:24 pm    Post subject: Reply with quote

tom418 wrote:
IC 410 (no way to test, I replaced it this time) Usual fails either with a dead short or after replacing fuse and HOT, set will try to start but instantly shuts down. If run at say 80VAC, will play, but hum bars or 'snakes' through picture. JTS: you said it may have been stressed last time I posted here. could this have wiped out Q502?
He's changing T504 (horizontal driver transformer) resoldering - NOT changing. If yours is like the one in YT video, I'd pull it and resolder the 4 points where a wire from a winding connects to a post and then reinstall onto board, and Q400 Yes - IF checks bad. You had mentioned, in your reply to me previosly, that I may need to change the FB transformer T501 in a worst case scenario. Should I be doing that? Not at this time.

I'm just afraid that changing out Q502 is not solving a problem, but just curing a symptom. Make sure heat sink mounted parts have sufficient thermal compound. Monitor heat build-up on the heat sinks over a 30 - 60 minute period. A bad solder joint ANYWHERE in the set, especially the power supply and horizontal deflection circuits, could be the cause.

Besides the FBT Intermittent arc or a intermittent shorted winding are quite rare. More usual fail is instantly blows HOT., what else could be frying Q502? Or parts in the same areas that have values way out of tolerance. BTW: MY Q400 looks brown. Burnt?
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