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RCA Scenium ATC221 vaporizes fuses immediately

 
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Sat Mar 20, 2010 3:49 am    Post subject: RCA Scenium ATC221 vaporizes fuses immediately Reply with quote

Well, I've had good luck with this forum in the past, that last TV I got fixed thanks to this forum is happily running and has for a few months now... now it's complimented with a Sony LaserDisc player and a good collection of 120 discs, works like a perfect set.

But I recently got an upgrade, an RCA Scenium D40W135D 40" rear-projection HDTV (chassis ATC221). Got it for free from someone, he said it "made a pop noise, shut off, and wouldn't turn back on, so [he] bought a new one". It's one heck of a sweet machine, and from what he later told me, I rescued it from being smashed into a thousand pieces by a tractor (to "fit it in the garbage"!!). The humanity!

It can't keep eating up dead space in my living room though! Right now it does nothing but pop fuses - that's _it_. Plug it in with a good fuse, and the moment the plug makes contact with the socket, BANG, a flash of light and the fuse is vaporized.

I've got schematics from schematics.me (<.< so don't spammy up this topic, thx) if anyone wants a page or two to help me figure it out. I've tried unplugging the two 300v outputs from the rectifier/capacitor board, and when I do that, the fuse no longer blows (the DVD player is the only thing getting power, and the chopper IC on its power supply gets barely warm, indicating it's running fine). So it's something past that board, on one of the two boards, either what I assume is the "convergence board" (a board along the width of the unit mounted near the mirror that connects to each CRT), or what I assume is some sort of high voltage board with a flyback transformer on it. Those are the only two places those 300v connections go. If it's worth a try, I can waste one more fuse to isolate which board is causing the problem, if it's safe to only have one of the two boards plugged in...

Any help, any pointers would be much appreciated. Thanks!
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Sun Mar 21, 2010 2:48 am    Post subject: Reply with quote

OUCH. And for what it's worth... the large (still called "unreg B+"?) capacitors work just dandy. Even after being unplugged for 2 days straight, they still held at least 280 volts. Know how I figured that one out? Fishing for the connector to plug the 2 300v connectors back into, and I touched the contacts on the board. OW.

Yes, some guidance would be much appreciated before I blow something else up...

edit (yes I found a way to edit... f'ing board bugs): Meanwhile I'm using my multimeter to slowly discharge them. God only knows what would happen to my meter if I just use the 10a plugs to snap-discharge them...

edit edit: Okay, it's not on the high-voltage board (with the flyback) either! I plugged only that one in, and with only 84 volts left on the capacitors it made a small spark (when I plugged that connector in) and dropped to 14 volts. Unplugged the meter, leaving the power to the "convergence board" (I think?) unplugged, took a deep breath, and plugged the AC line in. No pow! It came up fine. Hesitantly, I went for the power button. I heard a click, then I hit it again to shut it off. Static sizzling/crackling. Within 10 seconds of plugging it in, I unplugged it. Well, that part works fine, so I've narrowed it down to that convergence board everyone seems to be talking about. I'll pull that out in a few once I'm more confident the high voltage has gone away Wink
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Sun Mar 21, 2010 7:29 am    Post subject: Reply with quote

Okay, so TP30, RP31, and RP30 on the convergence power supply (which I've confirmed - no thanks to the users of this board so far... - is the name of the unit with the problem) are all shattered. Looking at the schematic I can almost make out why that's true. It also confirms why the previous owner said it made a "POP" - the explosion of the driver MOSFET. I just need to figure out what caused it to fail, and keep it from happening again. I think I might need to do a little parts-hunting too, to find the replacement parts. Also a little random component-testing in the area as well.

And as last time, now that I know where the issue is, have a schematic. Maybe it'll help out the late-comers.

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PostPosted: Mon Mar 29, 2010 5:24 am    Post subject: Reply with quote

Not even a late-comer? Good god this place is a ghost town.

All the capacitors in the area check out on a capacitance meter, and I replaced the two blown resistors with the same values. The 10 ohm one was totally obliterated, just fell apart when trying to desolder it. Replaced them all with the same values, just a higher wattage (the only ones I had).

It still eats away at me that I can't figure out what the hell would cause this kind of failure... and, even when it's operating, where does the power flow to? The schematic doesn't show any direct link to ground from the Source pin of the MOSFET. So all the power going through the transformer, by control of the MOSFET, goes through a resistor or a diode. That doesn't make much sense to me. I can't figure out where, in that system, enough power could flow to cause the chip to explode. And what other components were affected by the short as well.

Almost feels like I'm just writing a journal here, though. Hey, you, on the other side of the screen, reading this. Drop down there, hit reply, and say something. Yeah, you. Do it.
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Mon Mar 29, 2010 5:24 am    Post subject: Reply with quote

Ugh, that was me... I'd've expected it to ask me to log in if I didn't enter a username above. Guess not.
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Mon Mar 29, 2010 5:55 am    Post subject: Reply with quote

FalconFour wrote:
Ugh, that was me... I'd've expected it to ask me to log in if I didn't enter a username above. Guess not.


edit: Mmm, shorted zener diode. Good thing I already ordered a replacement.

edit: EDIT DAMMIT NOT QUOTE
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Sat Apr 03, 2010 5:07 am    Post subject: Reply with quote

Well'p, got it to stop eating fuses, and everything's plugged in. Replaced a few parts and it seems to have stopped killing itself. Plug it in and hit the power button, it springs to life with a CRT sizzle, and the power LED comes on. Anticipation! Never met, though... the screen stays black. Then, about 10, 15 seconds later, the power LED starts flashing.

I counted off 49 (maybe 50) flashes before it stopped blinking and stayed on again. The service manual indicates that it should blink a pattern of 0-9 blinks, pause, and 0-9 blinks to indicate an error code, but there's no pause. Tried the service menu mode, and it turns on after holding the buttons, but it doesn't show anything on the screen - still black. Then the power LED flashes again. Curiously, if I turn it off and back on, the blinks reduce down to 1 instead of 50 after the 10-15 second wait.

I think I know what general area to look in - there's a cable coming off the area where that MOSFET was blown out, and it powers what I believe is called the "convergence amplifier". I'll check out those voltages and see if that's to blame. May very well be more components left to replace...
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