Can capacitor dielectric be black??

[JonathanAnon]

Hi,

I've just opened a screen and looking round for the main filter capacitor in the SMPS I spotted this..



I've taken it out of circuit, and it doesnt seem to hold any charge at all.. but the deposit on the top of the cap looks like glue or coolant, even though it must be dielectric.. What do you think?

[Rulycat]

Re: Can capacitor dielectric be black??

Are you sure it isn't just the rubber bung they put on top of high voltage caps?

[selldoor]

Re: Can capacitor dielectric be black??

did you test in circuit as well? - care, high voltages.

[JonathanAnon]

Re: Can capacitor dielectric be black??

This is the best pic I could get close up. The substance has a hollw, plastic feel.

http://omg.wthax.org/small_1.jpg

There is a small radial fuse blown on the AC input so I cant test in circuit... I've held it to a 9V battery and it doesnt hold any charge.. This same battery charges another large filter cap to 8.00v in a few seconds..

[JonathanAnon]

Re: Can capacitor dielectric be black??

Just thinking about it there.. It doesnt make any sense for me to ask for any more help without peeling off the substance and having a look underneath.. So here it is..

http://omg.wthax.org/thumb_without_rubber.jpg

[mariushm]

Re: Can capacitor dielectric be black??

Yeah... change it... doesn't have to be 68uF if you can't find one... could probably go up to 120uF with no worries

While the voltage rating could go as low as 350v, that's kind of risky... I'd say stay with 400v-450v rated caps.

This should work: http://ie.farnell.com/panasonic/eeue...5mm/dp/1673496

And this page has a selection of good caps... choose best mix of diameter / height + temperature rating you can afford: http://tinyurl.com/czn3pwg (farnell search page... short link because url was 500+ characters)

[multimeter]

Re: Can capacitor dielectric be black??

that cap has had it,the disk on the top stops anyone getting a shock,the aluminium case is usualy at mains potential.

[JonathanAnon]

Re: Can capacitor dielectric be black??

that cap has had it,the disk on the top stops anyone getting a shock,the aluminium case is usualy at mains potential.

Yeah, I see that now.. I'd never seen those rubber tops before.. I guess what happened was that when the cap blew it seemed to melt the rubber top and made me think it was dielectric...

Gonna buy a new fuse and a new cap. Just wondering if the blown filter cap lead to the blown fuse, or perhaps a failure somewhere else along the line lead to the blowing of both the fuse and the filter cap...

[b700029]

Re: Can capacitor dielectric be black??

Probably the cap failed shorted, causing it to vent and blow the fuse.
With the cap out of the circuit, ensure the primary is not shorted.

[multimeter]

Re: Can capacitor dielectric be black??

yeah,as b700029 says,the cap has shorted out causing the fuse to blow,when you put in the new filter cap,just ohm out the fuse terminals,it should show open circuit or very very high resistance,and to be safe,connect a 100w light bulb across the fuse terminals and power on,should glow bright then off or very dim,if it does that then your safe to put the fuse in and away you go

[JonathanAnon]

Re: Can capacitor dielectric be black??

connect a 100w light bulb across the fuse terminals and power on,should glow bright then off or very dim,if it does that then your safe to put the fuse in and away you go

Yeah I read this in a book, but havent had the opportunity to try it yet... It's a great idea. I was gonna try it before putting in the fuse.. btw, that's a fast blow fuse isnt it? I've found two identical fuses on the radionics site, but there's nothing I can see to identify F or T ..

Hopefully by the middle of this week I'll have myself a nice working PC/TV screen for less than ten euros... (and you know how little euros are worth these days :-) )

[b700029]

Re: Can capacitor dielectric be black??

T = time delay.

http://www.sourcingmap.com/t315a-315...aign=usfroogle

[ben7]

Re: Can capacitor dielectric be black??

It appears that the vent has burst on the cap and melted some plastic.

Crappy "chang" caps! Replace the ones in the secondary too!

Hopefully the s/c capacitor didn't blow anything else! (besides the fuse)

[tom66]

Re: Can capacitor dielectric be black??

Looks like a Vestel PSU these were notorious a few years back for having fake primary caps which shorted after some time.

[momaka]

Re: Can capacitor dielectric be black??

Looks like a Vestel PSU these were notorious a few years back for having fake primary caps which shorted after some time.

Interesting.
I always though the common failure mode for those high voltage caps is open circuit rather than short circuit.

O/P should definitely check the main MOSFET for short circuit and the Source resistor for open circuit. That way, if more parts are bad, it can be all ordered at once.

[ben7]

Re: Can capacitor dielectric be black??

Interesting.
I always though the common failure mode for those high voltage caps is open circuit rather than short circuit.

Crap-caps love to go short-circuit and draw excess current

[JonathanAnon]

Re: Can capacitor dielectric be black??

I always though the common failure mode for those high voltage caps is open circuit rather than short circuit.

Crap-caps love to go short-circuit and draw excess current

I have them ordered, so put your chips on the table please.. :-)

I have a 400v 150uF cap. Could I use this instead.. I thought the higher the capacitance the better the filter job it would do?

[ben7]

Re: Can capacitor dielectric be black??

I have them ordered, so put your chips on the table please.. :-)

I have a 400v 150uF cap. Could I use this instead.. I thought the higher the capacitance the better the filter job it would do?

Sure you can, as long as it has enough space!

[JonathanAnon]

Re: Can capacitor dielectric be black??

See, I picked up an old XBOX today (which has a 400V 150uF cap), having already ordered the 400V 68uF cap and the 3.25A fuse from radionics. They should be here on Friday.

The 400v 150uf cap is certainly way too big for the TV, it's wayyyyyyyy too tall. But it would have worked as a nice test before I made the purchase.. I could throw it in place of the 68uF cap and put a 60W bulb in place of the fuse... I might just try it anyway..

[ben7]

Re: Can capacitor dielectric be black??

See, I picked up an old XBOX today (which has a 400V 150uF cap), having already ordered the 400V 68uF cap and the 3.25A fuse from radionics. They should be here on Friday.

The 400v 150uf cap is certainly way too big for the TV, it's wayyyyyyyy too tall. But it would have worked as a nice test before I made the purchase.. I could throw it in place of the 68uF cap and put a 60W bulb in place of the fuse... I might just try it anyway..

If you don't have a spare 3.25A fuse, just use a 3A to 2A fuse to test it. One thing is for certain, NEVER put in a bigger size fuse!