CRT TV power regulation issues

[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 -

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 ->

[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.

[fmashockie]

1) You're measuring 133VAC mains voltage?  You're in the US right?  Shouldn't be that high.  Should be ~120VAC.

2) At C405 per the schematic you should be seeing ~157VDC.  And only 6Vpp ripple.  You can check that with an oscilloscope if you have one or put your multimeter in AC mode and measure across C405. 

3) Are you sure you are seeing a short between input and output on this regulator?  It is pins 3 and 4.  If so what resistance are you measuring across those pins?  I would be checking D420 and D421.  And Q400.  Check to see if those components are shorted.

[piepurveyor]

Sorry for late reply. First off, I switched multimeters and determined the one I had been using was malfunctioning. I've taken measurements again and got some small differences but nothing major.

Plug -> 123.8 VAC
C405 -> 170 V (measures that across it as well. No AC voltage)
                  +                -
D402 -> 124.4 VAC      88.3 V
D401 -> -74.8V           124.3 VAC

I checked the 2 diodes, D420 and D421, both seem OK as they have .622 on the forward bias and are open in reverse.

Q400 I removed from circuit and tested, it was OK. No shorts and had .572 on the collector and .607 on the emitter in diode test.

I tested R400 as well. It was I think 12.5 ohms or so cold and increased as it was heated (used a hot air gun). Seemed OK but I don't know for sure.

I'm still scratching my head here.

[fmashockie]

Hmm yea I'm not sure then how you are seeing a short across the input and output of that regulator.  I would verify that by taking some resistance measurements (with power off) between input and output.  Also check for continuity between base and input, base and output, and check for shorts to ground (input to ground, output to ground, and base to ground).

[piepurveyor]

The original regulator I did completely replace, as in I installed a new one. The old one was for sure shorted input to output, it is ~1.6 ohms and has continuity.

When I probe the new regulator while it's in circuit (not powered), the only continuity I get is from input (pin 3) to ground, it's around 1.9 ohms. So, pin 3 to 1 has continuity as well as pin 3 to any pretty much any common ground point.

[piepurveyor]

Ok, final update. I got everything fixed. Had to wait on a few replacement parts to arrive. The problems were the main voltage regulator, Q410, it was shorted between input and output, which also burned out R432. The 2.5A pico fuse was blown as a result. I replaced all of those and also C405 had gone bad, so I replaced it as well. TV looks great again, thx for the help everyone. 

[fmashockie]

Nice work! I'm glad you  got it working.  But I'm just trying to understand what happened here - always trying to learn from repair.  You replaced IC410 twice - the 1st time you did it, it was still short between input and output.  The second replacement fixed the issue?  Did you also replace C405 at the same time you replaced IC410 the second time? 

Looks like maybe C405 failed short which caused a short to ground. Maybe that damaged IC410; both the original and the 1st one you replaced.  And with that short between input and output, R432 couldn't handle that massive current draw and it burned up.

[piepurveyor]

I actually only replaced the voltage regulator once. I did take out the other small IC, I think it was Q400, but it tested just fine so I reinstalled it. The 1st time I replaced the large voltage regulator, IC410, it was with a new part. Per your suggestion, I took the original out, tested it and discovered that it was shorted. I got a replacement and installed it, but the pico fuse blew again. I had tested everything between the fuse and the AC line-in, and the only part I was unsure of was the big capacitor, C405. I just thought to try replacing it, because again, everything up until that point was testing OK. I swapped it out, replaced the fuse a final time and with the new voltage regulator in it powered on no problem. So, yea, the capacitor was at least part of the issue though I'm not sure where everything started.

[fmashockie]

Got it.  Thanks for the clarification!