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Have schematics + waveforms, please help find the dead part!
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FalconFour



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

PostPosted: Sun Nov 22, 2009 8:32 am    Post subject: Have schematics + waveforms, please help find the dead part! Reply with quote

Okay, I really really need help here! I've disappointed my roommates long enough with this broken TV. I really need someone to point me in the direction of what component to poke with and figure out what to replace. It's an RCA CTC169 chassis, for those familiar - CTC169CA2 chassis, to be exact; model G27221WK.

It began doing this after leaving the set unplugged for about 12 hours. It simply wouldn't turn back on after that, but prior to that it never had any similar issues. I have the feeling I'm so close to the solution but I'm missing something that only the internet can help me with... =(

Here is the schematic of the affected area. You might notice it's only a section of the full schematic, but I can 110% guarantee the problem is in that area. I've removed the switching transistor and the exact problem remains (so it's within that area).

Here are the waveforms... these are tested just past R4003 and ground - the power supply to the regulator control circuitry. These should be at 12v all the time in order for the system to run.

Plugged in, constant power. (I think this was actually at 0.2s but the screen was recalled at 0.5s)


Unplugged, filter capacitor discharging - note that the recharge slows down over time... (also taken at 0.2s)


At plug-in - right after plugging the AC line in. This was taken at 0.5s.


(Yes, that is a digital oscilloscope kit that I assembled... no, I don't have anything to compare its quality with)

I've already replaced R4003, C4118, and U4101 (which was blown out during testing - der, oops). CR4106 and CR4101 were taken out and tested by powering a 12v DC fan with them. Power only flows in one direction, so I assume they're OK. R4149 and R4126 were removed and tested OK. C4153, amazingly enough, I removed and it tested OK as well. Not sure what a capacitor is supposed to do when connected to both sides of a component (as in CR4104/CR4106) so I honestly don't know what to expect there.

... Anyone?

edit: UGH, the topic title keeps getting cut off.
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jts1957



Joined: 24 Nov 2008
Posts: 2472
Location: Far, Far Away

PostPosted: Sun Nov 22, 2009 11:08 am    Post subject: Reply with quote

90% regulation power solver replacements are as follows:

C4108--39 mfd/100 volts--RCA #208002
C4102--2.2 mfd/100 volts--RCA #195691 (High Failure Item)
CR4111--Reg. Kit--RCA #202055
CR4116--Diode--RCA #200157 (High Failure Item)
CR4118--Diode--RCA #165590 (High Failure Item)
C4104--15 mfd/63 Volts--RCA #189982 (High Failure Item)

Notice: If C4102 and C4104 are defective, the unit will not start up.

We always carry the following items with us when making service calls on
dead sets for the RCA CTC169 chassis:

Fuse F4001--5 amp/125 volts--RCA #175425
Resistor R4001--1.8 ohm/15 watts-- RCA #200444
IC U4101--Regulator IC--RCA #200419
Transistor--Q4101--Chopper--RCA #200165
Zener Diode--CR4122--3 volts/1 watt--RCA #200158
Capacitor--C4118--470 mfd/35 volts--RCA #190496
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FalconFour



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

PostPosted: Sun Nov 22, 2009 11:26 am    Post subject: Reply with quote

Wow - THANKS for the useful reply!! Very Happy So glad to have a direction!

Those high failure item locations you mentioned, I hadn't even looked at as potential issues. I was probably looking in all the wrong places!

Since it's so unlikely that RCA even exists as a parts-distributor anymore, can those components safely be replaced with "generic" alternatives of the same rating? Particularly the C4102/4104 capacitors?

Also, C4116 and C4118 are past the transformer on the low (currently dead) side. Would it be worthwhile to replace these as well, even if they were working fine before this failure? If so, what should they be replaced with?

And yeah, my roommate just had a total drunken blow-out over the broken TV (it's his TV and he sort of almost not-really half-blames me for unplugging it and breaking it...). The quicker I can get the parts, the better for the fabric of the universe Laughing
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jts1957



Joined: 24 Nov 2008
Posts: 2472
Location: Far, Far Away

PostPosted: Sun Nov 22, 2009 12:02 pm    Post subject: Reply with quote

From a quick look at some of your posts, seems like you are more heavily into computer/computer monitors rather than television. I'm just the reverse, with computers being my weakest (but still learning).Very Happy

Any parts catalog/site should have the required electrolytics.Cool

Get the supply running again before "shotgunning." Also remember, when a part is destroyed during troubleshooting, you then may have more than "the usual suspects" to check. Wink

Stick around here long enough and you'll also discover some other peculiararities on this forum ("edit" button comes and goes and can't search back more than one page on "unanswered posts," to name two).
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FalconFour



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

PostPosted: Sun Nov 22, 2009 12:25 pm    Post subject: Reply with quote

jts1957 wrote:
Get the supply running again before "shotgunning."
Not quite sure what you mean here... Shotgunning?

As for the part, well, one of two dumb late-night things I did was a little screwed up. One, I bought a new replacement for that 35v/470uF capacitor. But I installed it backwards like a dang moron. Bulged it out and I promptly caught my mistake. But the reason it got enough power to blow up like that was because, as the service manual suggested, I tried giving it just flat 12v regulated power to see what the chip did. Evidently, it blew out. Laughing But I tested nearly every component after I made that dumb move (mostly resistors) and they seemed to be OK, no other overheated components or burns elsewhere...

Not much else to do until the parts store opens on Monday though... I'll probably yank and test those caps tomorrow though. Smile
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jts1957



Joined: 24 Nov 2008
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PostPosted: Sun Nov 22, 2009 12:43 pm    Post subject: Reply with quote

The indiscriminate replacing of parts just cause they're "in the same area/circuit." Wink
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FalconFour



Joined: 22 Nov 2009
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Location: Fresno, CA

PostPosted: Mon Nov 23, 2009 3:21 am    Post subject: Reply with quote

Huh, well, here's the results I've got so far.

C4104 (50v/15uF) wouldn't pull up a reading on my (up to 700uF) meter. It hung it every time I plugged it in, and gave weird readings if I had it plugged in while I turned it on (the instructions were kinda vague on how to use it...). Wasn't able to get a reading on it at all, let alone in the ball park. I'm looking at that one as my faulty component... even though it looks 100% OK... it's strange that it wouldn't test.

C4102 (100v/2.2uF) tested at 2.17 uF. Don't think that's the one to blame but if I find a replacement at the store, I'll buy it anyway.

I'm going to pull out C4108 next and see how it looks. But that C4104 has got me thinking...
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FalconFour



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

PostPosted: Mon Nov 23, 2009 3:33 am    Post subject: Reply with quote

Bleh, looks like this is one of those times when the edit button "goes". Maybe it's a forum permissions configuration issue. Confused

Anyway, meant to edit that it's a 500uF meter, not 700. Innywho... I pulled out C4108 (nasty bugger, right between a surface-mount capacitor) and it tested 47.1-48 uF (is it more proper to write "mfd"?). Went back and re-checked C4104 and rechecked it once more... still no result on it. I've got a good feeling that's the problem, although it's kinda an odd problem to have that be the cause of all this headache Wink

I'm almost thinking since capacitors are so cheap, I should replace them anyway. But being as though it is a mere parts store, I probably won't be able to find the exact value. Is it better to substitute a higher voltage (so I've heard)? Is that acceptable in this case? Or is a higher rating better?

Of course, it also begs the question: if a higher can be substituted, is that "better" at all, or should the original ratings always be stuck with?
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jts1957



Joined: 24 Nov 2008
Posts: 2472
Location: Far, Far Away

PostPosted: Mon Nov 23, 2009 3:45 am    Post subject: Reply with quote

O.K. to go 1 or 2 "steps" up in voltage (if the lytic can physically fit), I'd try to stay with the SAME value though.

Remember, electrolytic have 4 main failure modes (five, if you include "bulging"), bad Value and ESR being the most common defects. Dielectric absorption (DA) and Leakage are less common failures. For example, a defective lytic CAN have Good value but bad ESR. Cool
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FalconFour



Joined: 22 Nov 2009
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PostPosted: Mon Nov 23, 2009 4:48 am    Post subject: Reply with quote

With that in mind (good value but bad ESR) then it's settled, I'm just gonna replace all 3 of 'em Laughing

So I'll try and grab the same values for everything but if they only have a step up higher voltage that's about the same size, I'll grab that instead. There's a bit of wiggle room in that area of the board so I shouldn't have much trouble as long as I stay within a few MM...

What's funny is that I'm basically saving my roommate a few hundred bucks of repairs (more than the value of the TV of course) and it's like "did you fix it? is it working yet? hmm hmm?"... he keeps thinking he could get it fixed quicker than I can. Laughing Seeing how close to the solution I am... I don't even WANT to let it go! Even if he doesn't pay me back for the parts & work, it's well worth the experience, IMO...
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