Electronics > Repair

CRT TV power regulation issues

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piepurveyor:
Hi,

I have an old Magnavox PR1397X122 that is having what I think is voltage regulation issues, but I was hoping to get more informed opinions about what I'm seeing to make sure I'm on the right track.

Before these issues happened, the tv was working normally with no problems. I went to power it on recently and discovered it was dead. No power on, no HV hum, nothing. I opened it up and discovered the pico fuse was open (F401 on the schematic). I wondered at first what could have caused it, but I went ahead and replaced the fuse and got power back. However, the 1st thing it did when I plugged it in was immediately power on without my input. The tube was displaying snow because there was no input signal and the OSD was working but when I tried to power it off and then back on, the tv wouldn't power down. The display would dim considerably but the snow was still visible as the tube just wouldn't power off. I have a link to a video of the problem. Note that when the video starts, I'm in a dark room with the TV "off."

I started then fully checking the board visually and discovered that resistor 432 was totally burned out and open. The linked image is of the original resistor. I replaced it and when I powered the tv, the resistor immediately burned out. Puff of smoke. The schematic, which I have for this particular TV, says it's a 1k ohm resistor. At this point I started to suspect the voltage regulator, so I took some measurements from it which I have listed on the PDF file I've attached. I know those values are not correct, but I don't know if it's due to the voltage regulator IC or something else I'm overlooking. I've checked diodes D401 to D404 and they all seemed fine.

Any insight would be appreciated. Thanks.

The burned resistor -
--- End quote ---

Video of the symptoms - https://www.mediafire.com/file/qidj0emyu11gvlr/Img_1233-1.m4v/file
Power supply schematic - https://www.mediafire.com/file/wvrqak427gfm39r/3756n+pg+4+Voltages.pdf/file

fzabkar:
Datasheet for STR30130:

https://pdf.datasheetcatalog.com/datasheet/Sanken/mXyzytzv.pdf

It looks like U410 may be shorted between Base and Input/Output, and Input may be shorted to Output.

piepurveyor:
Ok, I'll check the IC then and update when I find out, thanks.

piepurveyor:
Ok, I checked the voltage regulator IC and, sure enough, the input and output were shorted. I replaced it and the burnt resistor (r432), reconnected everything but it would not power on. I checked the pico fuse (f401) and it was open again. Something is overloading that fuse, and I can't seem to figure what it is. Fault finding is not my strong suit, unfortunately. I took multimeter readings on what I could with the fuse open and listed them here. Also, I've uploaded the entire TV schematic.

Schematic -> https://www.mediafire.com/file/1r7i2tvabpojst8/3756n.pdf/file
Multimeter readings ->
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fmashockie:
Typically, when you have a voltage regulator shorted across the input and output, usually it is not the voltage regulator that is the problem.  There's some short across the power rail that STR30130 is regulating.  You can use a multimeter/LCR meter that can measure milliohms out to a few sig figs to help locate the short or using a thermal imaging camera to see if any component is getting hotter than the rest.


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