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Old 14th Oct 2020, 5:11 pm   #221
Electronpusher0
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Default Re: Beware Rifa Class X Capacitors!

I just bought a Levell TM6B broadband voltmeter from e-bay and it came with a Levell P8S power supply with the mains lead cut short.
I already have a P8S but would never actually use it.

Out of curiosity I opened up the "new" one and found an exploded Rifa bomb. I do not know what else it took out but certainly the main smoothing capacitor has also blown.

Interestingly it looks like the Rifa is not the original capacitor, the schematic calls for a 1uF and the Rifa is 0.47uF
The capacitor in my other P8S is NOT a Rifa and is the correct 1uF.

Peter
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Old 14th Oct 2020, 5:52 pm   #222
Maarten
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Default Re: Beware Rifa Class X Capacitors!

Even in your working one, you might want to stick a Zener diode across the 150uF electrolytic. Maybe a 24V 1,3W one, given the 25V WV.
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Old 14th Oct 2020, 6:19 pm   #223
Hartley118
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Default Re: Beware Rifa Class X Capacitors!

I’m sceptical as to how much filtering they actually achieve. Once the RIFA fire has died down, I’ve always found that the product is working just as well as it was before, with no noticeable increase in mains-borne interference. And if a product needs mains RF filtering to bring it within EC directives re radiated RF, surely something a little more refined involving ferrite would be more effective.

Do designers just toss in RIFA capacitors on the mains ‘just in case’? Do we have practical evidence of a benefit?

Martin
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Old 14th Oct 2020, 7:14 pm   #224
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Default Re: Beware Rifa Class X Capacitors!

Quote:
Originally Posted by Hartley118 View Post
I’m sceptical as to how much filtering they actually achieve. Once the RIFA fire has died down, I’ve always found that the product is working just as well as it was before, with no noticeable increase in mains-borne interference.

Martin
The Rifa in this application is not filtering it is mains dropping, followed by a low voltage bridge rectifier and 25V capacitor (which also burst) see schematic. I have not checked how much other damage was done.

Good idea about the zener Maarten but I am not sure I would be happy using the power supply in any case, I just kept it as a curiosity.

Peter
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Old 22nd Oct 2020, 12:38 pm   #225
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Default Re: Beware Rifa Class X Capacitors!

I have just started work on a bush vhf71, which has had some caps changed during the course of its life. One of these was a Rifa mains filter capacitor.

Upon powering up for the first time, the lamp limiter bulb shone brightly and before I could shut everything down the Rifa mains filter cap went "bang" and split open with a horrible smell.

Having noticed it was a Rifa (after the event) I remembered having come across this thread before and that they were prone to failure.

I feel confident replacing wax and hunts components on sight in untouched sets, but run into difficulty (mostly inexperience I suppose) when more modern components have been thrown into the mix

I would like to propose a 'sticky' thread with a list (and pictures) of notoriously unreliable replace on sight components, if that is feasible.

Regards
Gabriel
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Old 22nd Oct 2020, 4:14 pm   #226
Maarten
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Default Re: Beware Rifa Class X Capacitors!

The rule of thumb never changed, you just have to apply it less often as there's only 2 types of impregnated paper capacitors in common use anymore. Rifa PMx and Wima MP3. They're the modern waxies, literally, except that they use epoxy instead of wax. They both look roughly the same, in a semi transparent potting.
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Old 24th Oct 2020, 3:07 pm   #227
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Default Re: Beware Rifa Class X Capacitors!

Thanks maarten
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Old 24th Oct 2020, 3:17 pm   #228
Maarten
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Default Re: Beware Rifa Class X Capacitors!

I forgot, in the 1970s and 1980s, mixed dieelectric capacitors were somewhat popular for pulse applications, and might still be around for niche use. They look more like normal foil capacitors but usually consist of polyester and paper. They won't measure leaky until the polyester gives out, but will often measure somewhat high in capacity.
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Old 5th Nov 2020, 12:32 pm   #229
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Default More scary capacitors

Attached is a photo of the mains filter capacitors removed from the switch-mode power supply of an instrument that I am working on.

The two larger ones are 220nF each, class X. Well known Rifa things connected in parallel across the mains input. They've not emitted magic smoke yet, but..

The two smaller ones worry me more. They look to be similar in construction but are 4.7nF class Y, connected between each side of the mains after the filter choke and ground. Now if they fail short-circuit (I know they are not supposed to, but the class X often do) it's nasty.

Needless to say I'm replacing the lot.
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Old 5th Nov 2020, 2:34 pm   #230
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Default Re: More scary capacitors

had my first Rifa explosion the other day. This was in a Goodmans amp, not my usual fare at all, which is mostly valve radios and record players. I thought it might be a Rifa but it didn't look in bad condition, so I switched on. A few seconds later I was greeted by a plasticky stench, mains plug quickly withdrawn but not before an extremely loud bang.
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