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Magnavox RS2744 A402 Power supply    
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tom418



Joined: 03 Aug 2014
Posts: 21
Location: New England

PostPosted: Sun Dec 29, 2019 9:59 am    Post subject: Magnavox RS2744 A402 Power supply Reply with quote

I've had intermittent problems with this tv turning itself off.

I have the service manual. I have a lead connected to Test Point 8 in the power supply. , and I'm reading 170 volts DC with the set plugged in but off (120 volts * square root of 2?).

According to the manual, voltage is supposed to drop to 143 volts when the set is turned on. It doesn't. It drops maybe, 5 volts (or less) to 165 volts.

BTW, after the set turns off, (and I'm unable to turn it on) I still read the 165 volts at test point 8 (as though it was still in "on" mode)

Could this be my problem? Is the set detecting an overvoltage condition and activating protection circuits?

If so, is this due to a bad capacitor, voltage regulator (I replaced IC410 years ago) or what?

Thank you.
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JTS1957



Joined: 21 Jan 2009
Posts: 2395
Location: Far, Far Away

PostPosted: Sun Dec 29, 2019 8:01 pm    Post subject: Reply with quote

Reread your prior threads on set. I think you have another defective regulator.

Once TV is working normally, for future reference, record the test point voltage, once with TV OFF and once with TV ON.
tom418



Joined: 03 Aug 2014
Posts: 21
Location: New England

PostPosted: Mon Dec 30, 2019 4:49 am    Post subject: Reply with quote

Thanks, JTS1957

I remember your "handle", and it was you who helped me out on this the last time.

So, if I understand correctly, the voltage drop when the set is turned on is a function of the regulator?

I'll order another one, (from Mouser, if possible) as opposed to eBay.

I'll post back when all is done.
tom418



Joined: 03 Aug 2014
Posts: 21
Location: New England

PostPosted: Fri Jan 03, 2020 12:32 pm    Post subject: Reply with quote

JTS1957:

I ordered a new Sanken STR30130. (IC 410)

In the meantime, I've taken measurements this time, from Test point 4. which is the output from the voltage regulator. When the TV is on, there is a steady 129 volts. When TV plugged in and "off", I read 14.7 volts, as they should be.

Question: If the voltage regulator is defective, how does this affect the voltage at test point 8 (INPUT of voltage regulator, pin 3)? Could there be another reason for the higher than normal voltage?

Another interesting observation (while I wait for new IC): If I let the TV sit (unplugged) for days at a time, and start using it, it will work fine for a few days , and then the "shutting off" problem re- appears. Once this happens, and the TV turns off, I can un-plug it, turn it on, and it will turn off after only a few minutes.
It's as if a long "rest" period solves the problem for a while.
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