Replacing low value electrolytics with ceramic caps?

[TERRA Operative]

I'm recapping an oscilloscope (Tektronix 2235) and I've found a number of low value electrolytic capacitors in the unit.
With the advancement of capacitor technology, I was wondering about the possibility of replacing them with equivalent value ceramic capacitors.

They are 1uf, 2.2uf and 3.3uf caps I'm thinking of swapping. Used for decoupling and a few other bits a few examples as shown highlighted in the schematics below.
Also two 2.2uf Tantalums used for decoupling too, can these be swapped for ceramic caps as well?

Are there any general guidelines or rule-of-thumb on this sort of thing or is it even possible?

[JanJansen]

I use Wima film capacitors as replacement, only i dont build power supplys,
they also might be expensive.

[T3sl4co1l]

You can try, the downside is the electrical characteristic: tantalum is typically chosen, not for low ESR, but for modest and stable ESR.

For example, ceramic on that reset line, will probably resonate significantly when the button is pressed, triggering the logic IC's ESD diodes and potentially damaging it.  A few ohms of added ESR dampens this.

The same admonishment goes for film capacitors.

Aluminum and tantalum, polymer and hybrid types, are available in a range of ESRs.  Do not drop one in willy-nilly.  Look for an appropriate ESR for replacement.

Timing capacitors can be replaced without much worry about ESR, as long as a stable value is used.  Mind the three things which affect ceramic value: voltage, temperature and age.  If you can't find characteristic graphs in the datasheet, don't buy it, keep shopping.  It may seem unbelievable to worry about voltage change in a logic circuit, but that's how advanced ceramics are these days.  Physically larger caps can be (but aren't guaranteed to be) better than small ones.  Mind that leaded ceramics are nothing more than an SMT chip with leads attached, and dipped in epoxy; the size of the chip inside is what matters.

Tantalum shouldn't be failing in a timing application, so I would just as well recommend leaving those alone.  Or if they are failed, replace them with the same type.

Tim

[SiliconWizard]

As said - what will change significantly is the ESR.
For caps used for timings such as those that seem to be highlighted on your schematics, it should be no problem. For caps used in power supply sections, that could be a whole other matter.

Failure modes can also be quite different. And although ceramic caps in a "stable" environment *can* last much longer than electrolytics, they are prone to cracking under stress and that can lead to shorts. Not necessarily pretty.

[TERRA Operative]

Good information. Thanks all.

I assume the Tek engineers chose what they did for good reasons.
Maybe for now, I'll replace like for like to ensure I don't mess anything up... I kind of want this scope to have a slight chance of working after I've finished poking it in the nether regions.

[David Hess]

The last two, C750 and C944, will cause problems if replaced by film or ceramic capacitors because of their low ESR.  The three shown on the first schematic would be fine but I think it would be a waste of effort.

[Wolfgang]

Hi,

when doing this you could run into the following problems:

- High-value ceramic dielectric materials are microphonic. El. caps are not.
- Their capacitance is strongly dependent on voltage (loss of capacitance in several 10 percents as voltage goes up)
- They age (capacitance decreases)
- their ESR is very low. This could reduce loop stability in regulators or may cause ringing effects.

You need to try. I would only do it where I cannot get an electrolytic replacement. Thats the minimum risk.