RCA DLP m50wh72s

[mitchkramez]

new to the forums here, but have a question.

i have the tv mentioned in the subject above, it was given to me because it wouldn't turn on. after opening it up i found 4 bad capacitors on the power supply and brought it to a local electronic shop and had them replaced. the tv now turns on, but there is a hissing sound (not buzzing, but like air coming out of a tire) and, frankly, it's kind of irritating.

the hissing is louder when the picture is brighter, so it's loudest when the picture is full white. the sound is also NOT coming from the ballast. (i've had the back off and listened to both)

physically, i do not see any other bad capacitors on the board, and it's definitely coming from the power supply board, audibly, i've narrowed it down to the section of the board where the power enters (which is opposite of where the bad caps were).

I'm wondering if anyone has any input as to what this is, or possibly any part numbers. I've been told to check for excessive ripple, which I understand how to do, but I'm not sure what i'm looking for as a result on my meter... thanks in advance...

[Wizard]

Re: RCA DLP m50wh72s

the shop used wrong capacitors, common if they don't know enough importantance of using good caps for SMPS stuff. They have to be low ESR type.

Cheers, Wizard

[mitchkramez]

Re: RCA DLP m50wh72s

while i'm sure that's possible, it seems unlikely, this shop ONLY fixes TVs and similar electronics, the hiss is not coming from the caps they replaced, it's coming from another area...

[Wizard]

Re: RCA DLP m50wh72s

hiss can be from color wheel. Be certain. The color wheel si right in front of HID lamp.
Wear polarized sun glasses!! If this noise is from there, expensive part.

Cheers, Wizard

[kc8adu]

Re: RCA DLP m50wh72s

since the hiss is coming from the psu it likely has more bad caps or the ones installed by the shop are not low esr types.
pictures would help.

[mitchkramez]

Re: RCA DLP m50wh72s

could high esr caps cause the hiss to come from another place on the power supply? or would the caps themselves be hissing?

[mitchkramez]

Re: RCA DLP m50wh72s

i'm planning taking some pics tonight, but from what i can tell, there are two large caps near the transformer that appear to be generating the sound... they're huge, about 1.5" in diameter if remember right (i'm at work now)... they don't really look puffy... just noisy? could it be these are affected by the other caps that were replaced.

[Wizard]

Re: RCA DLP m50wh72s

Stop clattering, and get a length of small diameter rubber vacuum hose used in car and holding one end to your ear and other end point at many items in turn till you find the source of hiss.

Also give us the printings on the capacitors that your shop replaced so we can tell you if it good or bad replacements.

Cheers, Wizard

[mitchkramez]

Re: RCA DLP m50wh72s

Wizard,
Using the method you mentioned, i was able find the source, or at least i think the source of the sound (printed right below this part is LP707:



Here's a shot of the whole board with the caps that were replaced marked in red:



And here's an up close shot of the caps... All four are 1000uf 35v +105C and say CE NHG 80140G:



What is that part the sound is coming from? Is that the culprit or is something else causing that to hiss?

[PlainBill]

Re: RCA DLP m50wh72s

Wizard,
Using the method you mentioned, i was able find the source, or at least i think the source of the sound (printed right below this part is LP707:

Here's a shot of the whole board with the caps that were replaced marked in red:

And here's an up close shot of the caps... All four are 1000uf 35v +105C and say CE NHG 80140G:

What is that part the sound is coming from? Is that the culprit or is something else causing that to hiss?

NHG appears to be a Panasonic series designed for linear power supplies, NOT for switching power supplies. That could be the source of the problem (unless Wizard says otherwise).

PlainBill

[Krankshaft]

Re: RCA DLP m50wh72s

That component that is whining is called an inductor.

Replace the caps with the proper low ESR series and see if it quiets down. I would also replace any caps before and after in circuit with that inductor.

I have had switching power supply transformers emit a high pitched whine due to bad caps. Replacing the caps with Pana FCs fixed the whine.

Good the shop used Panasonics bad that they didn't use the right series.

Like the shot Rubycon 85C caps in my TV that caused erratic channel changing a good brand but using the improper specd caps (temperature wise should have used 105Cs) caused a failure a few years later.

[mitchkramez]

Re: RCA DLP m50wh72s

krankshaft, when you say "whine" do you mean hiss, like the sound of a tire leaking air?

[Krankshaft]

Re: RCA DLP m50wh72s

Whine or hiss doesn't matter it's the same thing producing the sound the windings on the inductor vibrating the determining factor of the sound whine or hiss subjectively is the frequency of those vibrations.

Caps can do this too but since you said it's coming from the inductor I'm going in that direction.

Back in the linear PSU days this wasn't such a problem since they operate at 60 Hz a nice quiet hum if you put your ear close enough.

Switchers on the other hand operate in the Khz (thousands of hertz) range and their windings are much smaller which makes way for some really interesting sounds.

[mitchkramez]

Re: RCA DLP m50wh72s

How certain are you about the NHG's being for linear power supplies? I'm wondering if i should call up the shop and see if they can replace them with the right ones or if i should just try it myself...

[Krankshaft]

Re: RCA DLP m50wh72s

Here is the product datasheet:



Nowhere on that sheet does it say "low impedance" Panasonic usually puts that right at the top of the datasheet under Features.

Like in this datasheet for Panasonic FC series:



I use them frequently and know their default datasheet layout.

Barring that this means that the cap is what we call a GP or General Purpose cap.

GP caps are used at low frequencies like filtering the output of a transformer based (linear) supply at 60 Hz for example and NOT on the high frequency Khz output of a switching supply.

Low ESR caps can be used in place of GP caps but not the other way around.

[mitchkramez]

Re: RCA DLP m50wh72s

krankshaft, how can i know what's before and after in the circuit? or should i just replace those 4 for now?

[PlainBill]

Re: RCA DLP m50wh72s

krankshaft, how can i know what's before and after in the circuit? or should i just replace those 4 for now?

I'm not Krankshaft (but can be plenty cranky if someone is trying to shaft me).

My general policy is to check the existing caps against the good / bad caps list in the stickies on this site. If they are on the good list I will replace any that are visually bad or have a high ESR rating. If the manufacturer is on the bad list, I replace them all.

Yes, I've replaced quite a few caps that meet specs. And I'm disabled, so my time isn't worth a lot, but the extra confidence using good quality parts is worth a few extra bucks.

For what it's worth, in quantity 1, the caps the TV shop used cost $1.03 each from Digikey. Panasonic FC series would cost $1.11 each. Panasonic FM series (better specs than FM) would cost $.95 each. You can replace a LOT of caps for $20.

Without maligning the shop you took the board to, TV techs cover a wide range of abilities. Some are Wizards at troubleshooting and repair. Others are barely smart enough to know which end of a screwdriver to hold (and can't be trusted with a soldering iron).

PlainBill

[Krankshaft]

Re: RCA DLP m50wh72s

I see Nichicons and Rubycons all good who makes those brown caps?

The real question is what was the brand of the caps they removed? If they are still on the board swap them out too.

[Wizard]

Re: RCA DLP m50wh72s

The service manual doesn't call out what kind but I KNOW for SURE they need to be low ESR capacitors for SMPS. Change most of them out including those incorrect caps. I knew this is very common with many repair shop not too famillar with SMPS design requirements.

Hiss/whine is very similar noise due to high frequency. Some make noises when there is bad caps or incorrect replacements is used. Very rarely the coil is the problem. Do the capacitors first and ditto to small capacitors in feedback and transistor drivers (this is all transistors design).

Oh, RCA is NOTORIOUS for delicate circuit traces, desolder components *carefully* and put NO stress on the traces. (Carefully heat the solder and use a tool to unbend the legs of capacitor and carefully desolder with solder sucker/solder wick. They lift too easily if too hot or too long. That job I tend to set my soldering station at around 500-550F with fat screwdriver tip.

By the way, this is first time I saw this failure of yours. All this I see did not have this problem. And this board is on left side with component side facing the side grille and is cool running. Interesting. Keep us updated.

Cheers, Wizard

[mitchkramez]

Re: RCA DLP m50wh72s

Thanks guys for all your info, I went to the shop this morning and they *seemed* to agree that fm/fc caps could rectify the buzzing (though they don't seem as confident about it as you guys), they're going to take a look at it again and replace them and then find out what exactly is causing the inductor to hiss if it's not those caps... I'm bringing it back to them on tuesday, and they'll try to get it back to me before the weekend. Will let you know what they find. (for the record, the company is: http://www.mobile-e-service.com/ ... i'm really surprised at this point that they would have used those caps in this application)