Metalized polyester film capacitor problem

[link988]

Hi

I am new to all this electronic repair stuff but gotten around to fixing some of lcd monitors and now decided to takle a plasma tv repair

I got around to finding that stan by voltage is at 6.5V instead of 9.5V and every time I touch the testing points with digital multi meter for less then a sec multimiter will spike to 9.4V and goes to steady 6.5V. I traced all components along the line and got around to if I am not wrong "metalized polyester film capacitor" and its saying "470n M 275V" on the top and service manual says its suppose to be 470nF but I measured it with dmm and it shows 360nF.

My question is, are they measured like electrolitic capacitors just put 2 probes to 2 terminals and get the result and does it mean this one is faulty? My dmm can measure up to 50 uF so covers the values needed here.

And is possible for 3 of them to fail on the power filter board (that from different set)?


Thank you in advance for any help.

[PeteS in CA]

Re: Metalized polyester film capacitor problem

link, the markings you quoted mean that the part is a 470nF (.47uF), +/-20%, 275V capacitor. That voltage rating strikes me as curious. Is there a "~" after the "275"? If there is, that is probably an X capacitor, which is connected from Line to Neutral of the AC line, and thus unlikely to be your problem. Also, it may be polyester dielectric, but it could also be polypropylene, and impregnated paper parts could look similar.

Getting accurate in-circuit measurements of resistors or capacitors is dubious, even with high-quality handheld DMMs. So I think your measurement basically means that the part is good.

It sounds to me like that standby voltage is not being regulated, or is being current-limited. If there is an opto-coupler, that may be the problem. Look for a low resistance from the standby output to ground. Check to see if the regulator (or output rectifier) is hot.

[link988]

Re: Metalized polyester film capacitor problem

Thank you for reply.

I tested the capacitor in circuit first and then soldered it out to test and got the same reading of 360 nF. I allready learned a my lesson of not taking parts out to test them spend the whole day on trying to find the faulty part when it was right in front of me just needed to test it separately. So I guess its dead or on its way there.

Do all the characteristics of capacitor apply to these ones as well or there is something different about them?
Is it just the way they made? like more reliable etc.?

There is a "~" after 274 V ~. Just noticed around the pins where they moulded into the cap there is slight brown colour deposit (other 3 that I was asking about got around 1mm circle around the pins on all of them could it be leaking or its just with age).
I will just get a new one just to be sure then.

There is 1 opto-coupler in the stand by circuit will check it out today.

Thanks.

[link988]

Re: Metalized polyester film capacitor problem

0

[PCBONEZ]

Re: Metalized polyester film capacitor problem

Do all the characteristics of capacitor apply to these ones as well or there is something different about them?
Is it just the way they made? like more reliable etc.?

They are a safety device against power surges.
The reduced capacitance happens when they successfully stop a power surge from hurting anything else.
They made to do that without themselves shorting in the process.
Recommend using the same type cap.

They will have both a circuit voltage rating and a surge voltage rating.
I -think- that and capacitance is all you need to know for these.
.

[PeteS in CA]

Re: Metalized polyester film capacitor problem

Depends on wht you mean, link. It has capacitance, and it has a maximum voltage. But every dielectric has its peculiar charateristics, and different types of construction have their characteristics. E.g., electrolytics pack a lot of capacitance into a given case size. But 'lytics are polarized, their impedance goes up with low temperature, and cannot operate as cold as other dielectrics. Some ceramics are high capacitance with horrible temperature characteristics; other ceramics are lower value, but quite stable. The cap you are looking at has a good stability dielectric, and is constructed to handle surge voltages safely. I agree with PC, it sounds like that part may have beenn damaged, though .36uF is within the 20% tolerance ... barely. It must be replaced with a similarly rated safety capacitor! And be sure to match the lead spacing before buying the replacement part.

[PeteS in CA]

Re: Metalized polyester film capacitor problem

I should add, though that I do not think it is the main problem with your power supply.

[link988]

Re: Metalized polyester film capacitor problem

Thank for the help at explaining what that capacitor is and does. Looks like it is not the problem as I found 2 other identical capacitors on the board to be same 355nF and 364nF so my guess if they all would be faulty the TV would not even power up.

My issue is plug it in the wall stand by red light comes up, switch it on with the remote, then light goes green for about 5 sec and then constantly blinking red until I switch it off (remote doesn't respond on switch off command). The display doesn't light up and no sound.

Through some internet search I found that if standby voltage is low like in this case then the blinking occurs. There are no bulged capacitors on power board and 9V stand by is 6.5V roughly.

I am very bad reading schematics and so on but after looking at the one I attached to the post the voltage after the transformer "5500" should be 9V but its 6.5 there, all other voltages like 5V and 3V are there which apparently derived from that 9V and in my case from 6.5V.

Can it be that the transformer is not completely failed but on the way there?

Sorry probably should have started another Thread.