Philips PM3215 Scope Tantalum Capacitor Upgrade

[techbiker]

Since I'm always curious about these things, I went ahead and opened up the working Philips PM 3215 oscilloscope that I just purchased last month. Unfortunately, I noticed after doing this that tens of old, blue electrolytic capacitors litter its main board and power supply.

About 15 of the capacitors on the main board and 16v, 15uf electrolytic capacitors. Would I be able to replace these with 16v, 15uf tantalums safely? It would only cost me $7 more to do so and unlike electrolytics, tantalums are supposed to last forever.

Another problem is that there is a 4700uf, 40v electrolytic capacitor that is an old-style snap-in with 4 leads. Where could I find a suitable replacement? I realize that I might have to move up in voltage rating.

Either way, I feel that I need to replace all of the electrolytic capacitors in this scope as it turns 31 this year! I'm sure that all of the electrolytics are all dried up. 0.o

Thanks for the help.

 

[tomchr]

If it ain't broken, don't fix it.

 

[Rundmaus]

If it ain't broken, don't fix it.

That's right, basically...

Concerning old electrolytics in any electronic device, I wouldn't stick to that rule. If I buy any old equipment, I usually check for old electrolytics and try to replace them before any (possibly expensive) failure occurs.

Regarding the OP's question: I would replace the old electrolytics by new ones, not by (different) tantalum caps. Especially oscilloscopes are quite delicate circuits - I would stick to the original specs unless I know very well what I am doing.

Regards,
Andreas

 

[techbiker]

Andreas, thanks for the tip. I'll stick with electrolytics for the smaller capacitors.

The 4700uf electrolytic capacitor is giving me trouble though because it is actually an electrolytic/ tantalum capacitor. I checked online and new ones from Mouser go for $130-$250 (not even in the same package). I could call up Philips but I have no idea if they are even still supporting this scope with parts. I didn't even know that electrolytic/tantalum capacitors even existed.

Do you think that I could just leave this capacitor alone or would it wear out as quickly as other electrolytics?

Thanks,

Dallas

 

[techbiker]

Update:

Electrolytic capacitors of the 15uf, 16, 25, or 35 volt variety are very hard to come by. The ones that I can find are all $2.50+ a piece. I'd actually save money if I go with a set of high quality niobium or tantalum capacitors with a much smaller tolerance. Do you think it's worth the risk?

 

[macboy]

All Tantalum capacitors are "electrolytic". They are just another type of electrolytic. Aluminum electrolytic is another type, and is usually what people mean when they just say "electrolytic".

Tantalums fail more than, or at least as much as, aluminum electrolyitcs. They are a common source of failure in older test equipment, and often fail in a spectacular fashion, complete with smoke and fire. Aluminum electrolytics tend to fail when operated hot, since that dries out the electrolyte.

Also, I can't understand why you can't find cheap 15uF 16V electrolytics.

 

[Mooly]

It's all a bit confusing this...

Tants are very very specific in there application. Do not just replace any electroylitic caps with tants on the basis that they might be better. Tants fail (as macboy points out) in spectacular fasion and the main reason is innapropriate application of them.

This 'ere 4700uF 40 volt cap. Are you sure it's not just a normal electrolytic (even if it is a snap in). Sometimes snap in can type caps can be multiple caps in one package... one common lead and say 100uf and 220uf and 470uf to the other pins as an example.

And finally... this is a scope. So you can use it to measure the ripple on it's own rails

 

[techbiker]

All Tantalum capacitors are "electrolytic". They are just another type of electrolytic. Aluminum electrolytic is another type, and is usually what people mean when they just say "electrolytic".

Tantalums fail more than, or at least as much as, aluminum electrolyitcs. They are a common source of failure in older test equipment, and often fail in a spectacular fashion, complete with smoke and fire. Aluminum electrolytics tend to fail when operated hot, since that dries out the electrolyte.

Also, I can't understand why you can't find cheap 15uF 16V electrolytics.

The problem is that virtually all cheap 15uf 16v electrolytics are in the radial variety. Axial electrolytics are usually more than $2 a piece. I suppose it doesn't matter, however I would like to use the best replacement for the job.

The electrolytic/tantalum 4700uf capacitor I am referring to appears to be a tantalum/ aluminum electrolytic hybrid. Most of these cost upwards of $200, probably due to the amount of tantalum they use. Could I replace this with a pure electrolytic? If so, how could I find one with 4 pins?

Thanks again for the help.

Edit:

Mooly, what should I be looking for if I measure the scope with its own probe? I've never checked rails for ripple.

Also, I double checked and the 4700uf 40v capacitor is listed as an electrolytic tantalum in the service manual. These do exist, but they are very expensive.

I'll go ahead and stick to electrolytics, even if their properties are not as good as tantalums. Thanks for the tip.

 

[Mooly]

A little light reading,
https://nepp.nasa.gov/docuploads/26...5cb887e955bd/nepp report ta caps 2007 v1.pdf#

The main reason tants fail is because the rate of rise of voltage with respect to time isn't considered (DV/DT).

 

[macboy]

The problem is that virtually all cheap 15uf 16v electrolytics are in the radial variety. Axial electrolytics are usually more than $2 a piece. I suppose it doesn't matter, however I would like to use the best replacement for the job.

Sorry, I missed the part about them being radial.
As for the big snap in cap, I agree with the others, "if it ain't broke don't fix it".

 

[Mooly]

Mooly, what should I be looking for if I measure the scope with its own probe? I've never checked rails for ripple.

Also, I double checked and the 4700uf 40v capacitor is listed as an electrolytic tantalum in the service manual. These do exist, but they are very expensive.

Any regulated (stabilised) rails should be virtually pure DC.

So with the scope AC coupled and the probe on the appropriate rail you should see only a minimal AC component present. Perhaps as low as a couple of millivolts. Somrthing like the 4700uF cap (and I'm guessing without seeing the circuit) sounds like a main reservoir cap before any regulators and so a couple of volts of ripple may be present here.

All test like this assume that the negative end of the cap is at ground potential (or positive end of cap in the case of negative rails).

Don't connect the ground of the probe to any point internally in the scope. If a cap seems to have a large ripple component then measure on the other end as well. It may not be at ground potential.

I think tbh the scope will be performing to spec... so don't break it

 

[techbiker]

Thanks for the recommendations. I will test the scope and see if I could benefit from new capacitors.

The build quality on this scope is actually pretty good. It's amazing that working test equipment from the 60's-80's is still floating around!

I called up Philips and they had no idea what the "Scientific and Industrial" division was lol. I then called up Fluke and the VERY helpful guy on the phone flat-out refused to support their older equipment.

It's a good thing this forum exists, that's for sure!