Blew Up My First Oscilloscope!!!

[Gyro]

The USP of the Rifa caps is higher dV/dT rating than Polypropylene, that's why they still use them.

[Calambres]

Someone should start a database of appliances using those infamous RIFA caps!

[tautech]

Someone should start a database of appliances using those infamous RIFA caps!

You'd be amazed how large it would be, they were used in a very wide range of mains powered gear.

[Zenith]

High value Resistors with high voltages across them used to be a problem with the old CRT TV years ago.  The focus potential used a chain of high value carbon resistors, often in the Megohm range, and they used to shift dramatically in value as they aged.  Sometimes down by several Megs other times similarly upwards.  High voltages across resistors, particularly the carbon based ones,  are definitely not very good for them.

It's often forgotten that resistors have a voltage rating which you can exceed without exceeding the wattage, and if you do you alter the behaviour of the resistor. Carbon composition resistors tended to drift high after a few years anyway

I've seen special high voltage resistors in old scopes, which I think were quite long with a spiral carbon film track. They weren't that reliable, but people tried to replace them with the porcelain bodied 1/4 watt carbon composition resistors which died very quickly. Farnell and others used to stock modern metal glaze resistors rated for 3kV or so, I don't know if they still do. I never had any problems when I used them as replacements

[David Hess]

Some 2200 series scopes also have problems with some 470K or so 1/4 W resistors in the focus circuit.  If the the scope is apart, look for a group of  resistors with a bit of space around them.  The PCB itself might also look a bit stressed.  If I remember correctly, Tek used 5 or so of them in series to handle a 1 KV or so potential.

The 2213/2215 used carbon composition resistors in the focus resistor chain.  The 2235 and 2213A/2215A, which are really a different but related series from the 2213/2215, also used carbon composition resistors in the focus resistor chain.  Oddly enough the 2236, which was released at the same time as the 2235, uses high voltage film resistors instead (or at least the service manual says) and I think all of the later instrument do also.

I've seen special high voltage resistors in old scopes, which I think were quite long with a spiral carbon film track. They weren't that reliable, but people tried to replace them with the porcelain bodied 1/4 watt carbon composition resistors which died very quickly. Farnell and others used to stock modern metal glaze resistors rated for 3kV or so, I don't know if they still do. I never had any problems when I used them as replacements.

I did some research on this in connection with the 2235 series several years ago and found evidence that the high voltage film resistors Tektronix used to replace the carbon composition resistors in the high voltage circuits eventually became the Vishay VR25 and VR35 series of high voltage film resistors which have good pricing and availability now.

[Electro Detective]

Ha, just copped the same RIFA PITA myself on a kitchen appliance... operating in magic smoke mode   

had all the symptoms of a burnt out motor aka 'toss it time' 

Still, I opened it up for parts scavenging purposes and what a MESS    brown gunk EVERYWHERE,
the RIFA must have popped during the loud motor running, then started to smoke gradually. 

Folks, the smell was unbelievable, I had to work on the sucker outdoors with gloves and lots of residue free contact spray and toothbrush to loosen and wipe up the all crud.
If you don't mop up all of it, it will smell like forever, what the hell is in those stinkers ?   


Amazingly the bridged resistor across the Live/Neutral legs and the small 250volt blue cap thingy with one leg to earth/ground were both unscathed and like new, tested and re-used

Repaired kitchen appliance is 30 years young, nice clean motor, heavy duty switch work, hopefully should see another 30 with the new X2 cap in there   

FWIW Jaycar and Altronics in Australia regions have X2 replacements and cheap, smaller in size, and apparently no quality comebacks over the years.


I got the the same value 0.1 that was on the trashed RIFA so as I could reuse the same resistor and cap combo with it,

not sure what the difference would be with other X2 values,
more or less suppression at 50 hz, is bigger better, etc  ?   

[SoundTech-LG]

just another load of fried Rifas going back...

[Electro Detective]

Why the RIFA label and not REEFER ?   

That is how they eventually perform, right ? 

[PixieDust]

Does anyone know how to remove these plastic screws? They appear to be quarter turn to loosen them, but then they don't pull out easily after that. Should I be pulling harder?

If you turn it 180 degrees, they make the receptical bulge out again, making them increase in diameter and hence hold the board in place.

Scrap that, they just need to be turned, not removed.

[PixieDust]

Job done! Replaced the two RIFA with some 100nanoFarad ones. Not 0.068microFarad but I'm hoping it should work. Re-assembled it, turned it on and passed all the tests.

The replacement capacitor in the image is of a different value, but the one I installed is of the same form.

[med6753]

The absolute value doesn't really matter. Just as long as they are X2 caps which they are.

You're good to go.   

[Twoflower]

Isn't that thing on the left side of the picture a line-filter that's also notorious to fail?

[Gyro]

The absolute value doesn't really matter. Just as long as they are X2 caps which they are.

You're good to go.   

There's no reason why you can't substitute X1 capacitors of course - they just have a higher voltage withstand rating.

[PixieDust]

Thanks everyone for your help .

[Electro Detective]

Whoa     hold up guys, there is a slight difference between X1 and X2 suppression capacitors 

X2 bridge across Live and Neutral

X1 are placed in line 

Also if an X2 has an associated resistor and cap with it (and or other parts) I am assuming the manufacturer has chosen the values to tune in a certain bandwidth to suppress
or whatever the result required for that particular power input circuit. 

i.e. till certain that 'any value will work' I reckon it's a better punt to wack in the original X2 value


FWIW: a worthy Youtube to watch      if someone buys up on different value X2 caps and tests them in the same circuit to see what actual effect/difference they make.

In the meantime, I'll keep replacing old RIFAS with the same values the manufacturer installed...less thinking     
 

[Gyro]

Whoa     hold up guys, there is a slight difference between X1 and X2 suppression capacitors 

X2 bridge across Live and Neutral

X1 are placed in line

No, X1 capacitors are rated for L-N use, they just have a higher peak voltage withstand rating (<4kV) versus <2.5kV for X2. They tend to be used more in industrial gear and 415V three phase but it's perfectly acceptable to substitute them in if you have some around (I happen to have more X1s than X2s in my drawer).

http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdf

https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

X1s won't affect filtering performance if they are the same value.

[Electro Detective]

I haven't looked into it too deep (yet) and just roll with straight up same value replacements

still I don't understand why all the cap values available, and X1 and X2 voltage ratings, rather than a 'one size fits all' approach

They must have a reason for the different cap values   

 

[Gyro]

In a word, cost.    X2s are a bit cheaper than X1s.

X1 caps are also slightly bigger physically, although they often share the same lead pitch.

Theoretically an X1 cap should last longer in a mains application as its higher withstand voltage causes it to self-heal (and hence lose capacitance) less. Hence they tend to be used more in industrial applications - though I've spec'd them in consumer gear before now.


P.S. There are also even lower rated X3 caps but I've never seen one.

[PixieDust]

Well, I repaired the scope and used it a handful of times to complete a small project and then put it into cold storage. It has been sitting idle ever since then. I turned it on just now for the first time since it’s last power off almost 5 years ago now and… more magic smoke. I was hoping to use it to learn electronics on something that wouldn’t be too expensive to blow up in case I made a mistake, but it seems like I should just get a new scope when I finally move on to electronics on serious basis. I’m starting to realise that old gear is just now worth the hassle.

[DavidAlfa]

Only 12 years? Rifa caps usually fail in 25yr+ equipment!

[metrologist]

Could be something else but if it's the same component, at least you have practice replacing it 

[DavidAlfa]

"Rifa" in spanish is a draw raffle ticket, usually sold locally by kids for their summer school trip, or in parties, charity events... where there's usually 1st, 2nd and 3rd prices, could be anything from paid vacations to Christmas baskets ( With cheese, ham, wine...) or just a moderate amount of cash like 3000€.
So it's actually pretty hilarous when these go boom, you literally won the "rifa".

[Fungus]

"Rifa" in spanish is a draw ticket

"Raffle ticket" en Inglés. 

[DavidAlfa]

Aha, wasn't sure which to use, both made sense to me 

[p.larner]

i had one go nuclear in a big 5v 90amp smps,i just removed it and the psu still worked ok.