Capacitor question

Mods: I’m not sure if this is an IMHO question 'cause it seems like simple facts that I just don’t happen to know. Feel free to move it if necessary.

I got brave enough to pull apart my ancient stereo receiver (Yamaha RX469) and blow out the dust and squirt electronics cleaner into the holes in the potentiometers. I was quite pleased to feel them loosen up just like the guys on the YouTube videos said they would. While the solvent is drying I started looking around and noticed this white stuff at the base of the giant capacitors in there. There’s a couple photos of them in this Google Photo album I’ll share for a couple months. (These are safe for work; just a couple boring electronic items

All the YouTube videos I’ve been watching very strongly warn against touching those capacitors. Nevertheless, I’m wondering

  1. is that white stuff is a safety coating, some kind of adhesive, or something from the capacitors leaking out?
  2. is the life expectancy of those capacitors (and therefore my receiver) in jeopardy?
  3. should that white stuff be cleaned off? With what?
  4. Can the capacitors be replaced easily (I can solder, but don’t do it frequently), or should I seek a repair shop, or should I just be ready to throw the receiver out once it dies?

Thanks in advance!


looks like silicone adhesive, I wouldn’t worry about it

If the capacitors did ever die, look at the rating printed on them and replace them with identical ones. Don’t reverse the polarity…

Admit it, you’ve been waiting for a thread where you could use this phrase in all sincerity.

Yea, that’s just glue. The caps are relatively heavy components so they added a few drops of glue to hold them down.

Concur that it looks like a silicone RTV (room-temperature vulcanizing) adhesive. I don’t think it is holding the capacitor down, I think something happened to the crimp connector on the end of the black wire and the RTV is holding the wire in place. Have you always owned the unit and if so was it ever repaired? Just curiosity, nothing to worry about.

Manufacturers will often use a staking compound to “glue” a high-profile component to a PCB. This makes the component more rigid, and less susceptible to failure due to mechanical shock and vibration.

Those capacitors are aluminum electrolytic. They contain a liquid electrolyte. (A more accurate description is that it’s a paste.) The electrolyte dries out over the years, causing a decrease in capacitance and an increase in ESR. When refurbishing old equipment, a lot of people don’t even test these kind of caps - they simply replace them.

There’s a lot of junk parts out there, and lots of counterfeit parts. So if you replace them, spend a bit more money and buy them from Digi Key or Mouser.

Not familiar with old stereo models, but in ham radio this is a situation we are careful about. Caps go bad . If you just power something up (particularly tube stuff), sometimes the magic smoke is released. Bad.

Maybe look everything over very carefully and look for corroded stuff, charred stuff, lose connections. Blow out dust and accumulated grock first- be gentle. And yes, capacitors can bite. They hold charge for longer than you might think.

Good luck!

Wow! Less than 5 hours and I’ve got a half-dozen answers.
I love the way Dopers fight ignorance!

The unit is used – bought third-hand and cheap, actually, and I didn’t bother to look inside four years ago when I bought it. My worry was that the capacitors were (and had long been) leaking and would eventually fail. Glad to know that’s not occurring; very glad to know I don’t need to dig my hands in there to mess with them.

My thanks to you all!
I’ll consider this case closed and go close up that case.


There was an episode about 15 years ago where a lot of computer boards were failing. The story I heard variously was that one company “stole” the formula from another company for the electrolyte (goop) in the capacitors - and either the first company got wind and fed them a phony formula, or the second company got cheap and mixed the batch wrong. In any case, capacitors would start to seriously bulge, especially on top (that cross-hatch on the top might even burst) from internal pressure. When they did, they of course stopped working and the electronics failed. With the number of capacitors, it was usually cheaper to replace the circuit board.

Your capacitors don’t seem to have any indication of bulging.

Most of the time, old electrolytic caps look perfectly fine (no bulging, no liquid leakage, etc.) but are still bad. The electrolytic inside the cap is a paste, and simply dries up. There are ways of testing caps, but you’ll need access to special test equipment. I would replace them.

Probably overkill. There are plenty of devices of that age that are fully functioning without the need to replace all the capacitors. I have an even older Yamaha set that works fully and I’ve never had to open up.

If you have a device that works fully, that’s fine. I wouldn’t touch it.

But if you have something that is acting squirrelly, often it is faster and easier to just replace all of the electrolytic caps at once. You don’t waste time troubleshooting and testing, plus you get all of the caps that were close to failing since they were produced at the same time as the ones that did already dry out and fail.

We call it the “shotgun” approach at work. Yes, it is overkill, but it’s overkill with a purpose. The repair ends up being faster and more reliable.

As above. If is working, no real value in messing with it.
IMHO and experience, capacitor degradation is often over emphasised. Those Elna 63v 6800uF power supply caps are near bulletproof. I have 40 year old ones still working perfectly. They are not exactly cheap to replace either. The killer is heat. Modern switching power supplies are much harder on their capacitors - lots of high frequency switching energy to turn into heat. Nice old conventional transformer power supplies tend to be much easier, and if the caps don’t get hot (and you don’t leave the amp on all the time) they can last nearly forever.

That said, low value signal caps do tend to dry out and an old amp can be resurrected by replacing all the coupling and bypass caps.

Most problems in old amps can be sheeted home to bad caps, with a small residual being icky switches and potentiometers. I cleaned what I felt sure was a few decades of cigar smoke residue from one magnificent 70’s era monster amp. Fixed all manner of intermittent problems.

I have recaped some of the fabulous monster amps from the 70’s and brought them back to as new spec from basically dead. Some have some rather poor design when it comes to managing heat.

Aren’t capacitors those thingies that hold electricity even after the device is unplugged and that can electrocute you if you mess around with them?

Yes. But the danger is massively exaggerated.

There are a lot of people out there who haven’t touched old electronics in their lives who recite the danger from capacitors like an 18th C landlubber talking about the danger from sea monsters.

Oh, you mean like me. LOL

Um … what’s an “18th C landlubber”? Does “lubber” mean “lover”, as in a “land lover”? If so, how come I’ve never heard phrases like, “sealubber” or “mountainlubber”?

I’m assuming the point is that, if one wants to survive as a seafarer, one cannot be a “stupid fellow who lives in idleness” because one would sink and drown. Thus we hear “landlubber” but never “sealubber”. I can see the point, but I still consider the comment prejudicial in nature.

“Landlubber” does not seem to be quite as offensive as the definition
of “Lubber” suggests …

This one’s even better !

Yes you are right. Old CRT TVs and some other electronics did have capacitors that would give you a nasty shock. But nowadays most electronics do not use have that kind of voltages.

Besides, electrolytic capacitors like the ones in OP are like very small rechargeable batteries and they do not go to high voltages. Very unlikely to get a shock from an electrolytic capacitor.

The point about retaining charge after plugging off is valid too but again not with this kind of capacitor. Electrolytic capacitors have a leakage resistance that discharges them fairly quickly