Philips 21PT1357 CRT - bad Nichicon PR

[Th3_uN1Qu3]

This one arrived last night. Owner's description: Works for 5 minutes then picture starts wobbling.

Popped the back off today, and we have a bloated cap. A Nichicon no less... It's a Nichicon PR 47u 160v. It appears to be the cap after the pi filter on the B+ rail. The capacitor read open on the ESR meter. Replaced with a Ruby, TV is now working fine. A little searching reveals that this particular cap appears to be a common issue with this model.

Otherwise, the TV is pretty nicely built, all Nichicon and Rubycon caps. Unfortunately this series is notorious for bad geometry from the factory, and this was no exception. Needed a few tweaks in the service menu. The picture quality is very good tho. If i'm correct, this set should be around 6-7 years old, the manufacturing date wasn't printed on the back, and i can't be bothered to look for it elsewhere.

Interesting bit is the other cap on B+ is another Nichicon PR 47u 160v which is fine - but it's in a larger can size as you can see. The Ruby i put in is in that larger can size as well. I'm leaning towards a design mistake, the cap's ripple current rating could have been exceeded, and that's what caused it to bloat. I can't see other reasons - it's definitely not heat.

[tom66]

Re: Philips 21PT1357 CRT - bad Nichicon PR

Hmm, interesting. Is it definitely before the pi filter? Otherwise, Nichicon seem to be the least reliable of all the good brands (but still very good.)

[Th3_uN1Qu3]

Re: Philips 21PT1357 CRT - bad Nichicon PR

The bloated Nichicon PR was after the pi filter like i mentioned. It's marked C2409 in the service manual which you can find on elektrotanya, just google "philips chassis L01.2E.AA" and you'll find it if you want to take a look. It filters the B+ rail going into the flyback. In between the two caps is a 27uH inductor.

Otherwise it would really have made no sense to put a larger can after the pi filter, than before.

[tom66]

Re: Philips 21PT1357 CRT - bad Nichicon PR

Sorry, I meant to say "definitely after the pi filter"

It does seem like some PSUs are unstable with a higher capacitance on the output of a pi filter, than on the input, but I have seen a few power supplies which do this. Might be related to the compensation?

[Th3_uN1Qu3]

Re: Philips 21PT1357 CRT - bad Nichicon PR

I don't think i've yet seen a TV that has feedback taken after the pi filter. I think it's simply the case that the high energy pulses taken by the flyback resulted in excess ripple and low voltage on the outputs that are derived from it.

[tom66]

Re: Philips 21PT1357 CRT - bad Nichicon PR

I have seen a poorly designed wall wart with 200mVp-p of ripple which had feedback after pi filter, moved feedback, ripple dropped to a tenth of that.