Preventive maintenance - or just leave it?

[arneh]

Hi,

I hope that you could give me some advice here:

Yesterday I bought a Philips/Fluke PM3055 oscilloscope for ~$50. The oscilloscope seems to work just fine, but when checking the web for a user manual I noticed a small video about how to repair such an oscilloscope:



In this video a seriously blown RIFA capacitor is replaced.

Now, my question is this:

I see that it's a real pain to get the PCB out of the scope. There are some plastic clips which are quite nasty to get at - without breaking them. What do you think? Should I just leave the scope as it is, or should I do the effort of getting the PCB out and replace (or remove) the RIFA cap?

Any advice would be appreciated.

[ambrosia heart]

This scope was manufactured in 1986.  Today it is 35 years
old. According to my experience, if you use it more than 1
hour per day. It goes wrong within 1-2 year(s). If you use it
infrequently, it will also goes wrong within 5 years.  I could
Just leave it alone until it goes wrong, if I were you.

P.S. other brand not philips but kenwood.

[Terry Bites]

If it aint broke dont fix it.
You might want to check the calibration from time to time.

[george.b]

That depends: do you prefer having to replace it cleanly now, or with a bang, smoke and stink later? Because that RIFA cap will fail at some point, which is probably not too far in the future.

I'd do it sooner rather than later.

[arneh]

Thanks all!

I think I'll try to open it carefully and see if I can manage to get the PCB out. If not - I'll think I'll just wait for it to blow up:-)!

[CaptDon]

I wonder does that RIFA capacitor really serve any useful function other than helping the equipment pass EMI radiated emissions certification? I have seen some of those caps blow in such a way as to burn off one leg and become an open circuit. This went totally undetected until about once each year we would open up our test bench gear for calibration and cleaning. There seemed to be no ill effects from the RIFA not being there. I know I will get flamed for this, but I started just clipping the un-exploded RIFA's off of the line inputs. What always worried me was that many of these ALWAYS had line voltage on them even when the equipment was 'off' and I didn't want any suspicious midnight lab fires when no one was around. I have seen them burn holes in power supply PCB's that you could stick your finger through. That is fatal to a multi-layer PCB!!

[george.b]

I wonder does that RIFA capacitor really serve any useful function other than helping the equipment pass EMI radiated emissions certification?

No, that's pretty much it.

There seemed to be no ill effects from the RIFA not being there. I know I will get flamed for this, but I started just clipping the un-exploded RIFA's off of the line inputs.

It'll work without it, but it's probably not the best of ideas to have your gear not suppress EMI properly.

[Gyro]

It doesn't sound as if you've opened the scope yet. It might be worth at least taking the lid off and checking the capacitor with a small (plastic) dental mirror and torch. You might find that it has already been removed by a previous owner or replaced with a more reliable MKP type. At worst, you should be able to check the degree of package cracking.

[james_s]

Here in 120V land I usually just leave them alone, but I also switch off power to my bench when I'm not using it. With 240V mains I would replace any RIFA capacitors, they seem to fail a lot more often at the higher voltage.

[srb1954]

I wonder does that RIFA capacitor really serve any useful function other than helping the equipment pass EMI radiated emissions certification?

It is more likely that the class X RIFA capacitor is there to meet the conducted emissions certification for f < 30MHz. The self resonant frequency of such a capacitor is generally too low to be effective at controlling radiated emissions at f > 30MHz where the capacitor's self inductance becomes dominant.

Aside from meeting emissions testing requirements the class X capacitor also provides some useful suppression of incoming mains transients. I would recommend that you should replace the RIFA capacitor rather than just removing it so as to retain some additional protection for your equipment.

[Doctorandus_P]

I would open it up and give it a good look.

Those old RIFA caps (and integrated mains filters) have a bad reputation and are worth replacing.

I'd also have a good look at the rest of the PCB. Electrolytic capacitors are among the most unreliable parts and defects can often be seen visually. If their top is bulging or cracked, or if they're leaking electrolyte on the PCB then they should be replaced.

This does not mean that they are OK if they do not show these symptoms.

I would localize "preventive maintenance" to mostly the (SMPS) power supply section.

This was a decent scope in it's days, but the user interface has always been quite horrible.
If you're willing to put up with it's user interface (I won't) then USD50 is a quite reasonable price for it. The amount of time you are willing to spend on it will mostly depend on your own goals. If it's just a tool for you, then don't spend much time and effort on it. If you want to learn more about electronics and repair, then spend some more effort on it.

[Mark]

That depends: do you prefer having to replace it cleanly now, or with a bang, smoke and stink later? Because that RIFA cap will fail at some point, which is probably not too far in the future.

I'd do it sooner rather than later.

I agree with this.  Having had just one rifa explode, I would strongly recommend replacing them. 
If one explodes:
1. it really stinks the room out for days. 
2. it will make a mess of the PCB
3. it could damage the equipment. 

[arneh]

Thanks all of you! I really appreciate the feedback!

I've managed to get the PCB out :-)! But... I did break some small parts of the clips - but I think it's ok nevertheless. To get this thing out without breaking anything is hard! I noticed a trick to use some cable tie to get a grip at the PCB. So with some swearing and sweating I managed to pull it out!

Anyway, the RIFA looked quite ok. Pictures below. But for now, I've just removed it. The scope seems to work fine without it. I plan to see if I can get a new one later.

Thanks for all advice!

[Gyro]

I'd say that one has already been replaced with a modern Rifa replacement. Its appearance (especially the cross-hatch patterning) and zero cracking, isn't consistent with the age of the scope.

It probably means that the rest of the scope, including the electrolytics, has been preventively maintained. It's a shame you had to pull the board out though.

[tkamiya]

Yes, the scope will work fine without them.  They are there to reduce EMI. 

I would have left them alone.  To do a good PM, you'll need to check all the electrolytic caps, as long as Rifa caps.  Dust the inside, deOx iffy connections, etc.  I wouldn't bother until something fails.  I have room full of stuff.  I only had one incident of Rifa failing and few on electrolytic caps failing.  Just because they are Rifa caps, it doesn't mean they will definitely fail.

[james_s]

As I mentioned earlier, I think they are much more likely to fail on 240V mains. I've only ever had one of them fail ever, and it was not very spectacular. I have others that I've left alone and figure I'll replace them if they ever cause problems.