NOS electrolytic capacitors?

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Hi redrooster,
Yes, but first you have to reform the capacitor by charging them up slowly through a resistance. Easiest way to do this is with a variable power supply and a 4K7 resistor, or anything near that value. At some point in time, the capacitor will stop drawing current and have a terminal voltage that equals the supply. If you remove the voltage the capacitor should hold a charge, it will slowly discharge.

Never discharge a capacitor with a short circuit, like a screwdriver. This can really damage an otherwise good capacitor. Once installed, you can check the health of the capacitor by looking at the wave form with an oscilloscope.

-Chris
 
The elastomer seal has been deteriorating due to oxygen in the air since the manufacture date. Reforming does nothing about the pressure integrity of the seal. If run dead cold, might not be a problem, but if they ever warm up one or more is likely to outgas the water.
Middle of the road capacitors, not known for epoxy seal, not known as complete ****, so I'd give them 20 years before the seal dries up. ****y caps used in TV's and computer parts, about 5 years only. Epoxy seals, like green CDE's from the 70's, forever.
I use parts like this for one off experiments that won't accrue any hours. don't stress with a lot of expensive parts you can't afford to replace if one or more shorts out after a few hours warm. Or a hundred hours. My entertainment amps can burn up 100 hours life in a month.
I write the 2 digit year code on the end so I know what I am using, and keep the used **** in a special unsorted bin.
Post germanium NOS transistors and diodes, I see no problem. Resistors, post 1961? allen-bradley sprague with mil-spec paint, also not much age problem.
Film and ceramic caps, depends on the brand and original quality point.
 
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Hi Joe,
I guess you could always apply an epoxy seal to some of the more expensive capacitors. Also, don't forget the orange electrolytic capacitors with the black epoxy seal. Commonly found in Marantz and a few others in the audio path.

-Chris
 
Not sure painting a little epoxy over a crumbling elastomer disk can help, if at all.

The main point is not keeping moisture out, all you could achieve with such surface treatment, but pressure in when they heat up.
They are also designed to leak pressure out at a defined point; a random user applied layer will have unknown properties.

That´s why sealing rubber disks are crimped around the edge.

Epoxy factory sealed ones were built taking account of that, of course.
 
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Hi Joe,
The pressure relief areas are normally on the top of the larger electrolytic capacitors. On the small ones, there is a weak spot designed into the rubber seal, but they usually explode leaving that spot intact, the can traveling outwards at great velocities.

The small orange electrolytic capacitors that I find in 70's to 80's Marantz units (Sansui and a few other brands) do have an applied epoxy seal at the bottom. I think they feel comfortable with that because the caps are used in the audio path as coupling capacitors only. You never see these in positions for decoupling.

The factories take this into account, and I can see that. They explode exactly like their non-sealed cousins. An explosion followed by a cloud of fine paper bits. The seal intact holding the now pointless legs in place in the PCB.

I think the seals fail because the leads are not formed, but rather spread due to being inserted into hole spacing that don't match their lead spacing. I actually do form the leads, taking care not to stress the area where the leads exit the capacitor.

I think you might be a little over excited about this circumstance. As long as there are weak spots in the capacitor casing, there won't be a problem. If it is a small capacitor, then they don't typically blow through their vents to begin with, and the epoxy will form an effective moisture blocking seal. Epoxy does bond to metal and plastic, therefore creating an effective seal against the electrolyte escaping. In the situation where you have an over-pressure event (bang), it will fail easily as the thickness won't be great, and the attach area to the sides is very small. But the internal rubber seal would have to fail first, and we already know they don't unless it is a slow buildup of high pressure.

Been doing electronics service for a long time too.

-Chris
 
I've always wondered if anything special was done for epoxy seals. I've seen some ARC gear where it looked like they added a seal using RTV. IMO, the deterioration of rubber seals is less of an issue than one might think, as is the drying out of electrolyte. It's glycol based and doesn't evaporate nearly as readily as water. OTOH, spreading the leads of a stiff seal is a recipe for trouble, in fact it shouldn't be done with new caps either. What will really get caps, regardless of seals and good handling, is contamination at the factory. An internal fingerprint (chlorides) will kill them. So will the use of the wrong cleaning products after the board is built. Solvent-proof seals weren't that common on older caps. IMO, when you see capacitance over the rating, it's often a sign of internal problems. Hardly any new caps ship at over the rating, usually 10% under in a zero bias test. I'd do a DC leakage test at 80 volts, as that's often the first thing to degrade. If the leakage doesn't come down quickly, and stay down after the cap is formed, save 'em for R&D projects. New caps of that value/voltage are near a buck, at least here, but it drops to half that if you buy 100.
 
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Hi Conrad,
I'll agree with you completely on that.

I should add that if people spray the board (or clean them in) WD-40, all the capacitors and some transistors are garbage. I know from a fellow tech that cleaned all the strips in an old mixing console in WD-40. That poor fellow was fixing those strips for a while. It was a Neve, so well worth keeping.

-Chris
 
You can air condition your room to rediculous, or live outdoors in Norway, and old caps with **** for rubber won't feel much vapor pressure. In southern/midwest churches not air conditioned 150 hour per week and homes like mine where the A/C thermostat is 80 deg F in the summer, new caps are a lot cheaper than all that energy.
I've put > 200 e-caps in old organs since I quit working in 2008. In coupling and filter service, old ones don't explode, they just sound bad. I checked after every 2 of the 71 I put in H100 #9574; nearly every pair improved a function that didn't work, increased volume, or increased the frequency breadth of the sound. Before and after power out on the bass channel of 9574 was 7 vac and 16 vac, with original rectifier & output tubes.
A 1980 Allen organ that went silent in a service in 2015, before power amp e-cap change speaker voltage was 2 VAC. After cap change speaker voltage (S100 amp) was 18.
An Allen USPS-3 switcher PS board from 1986, didn't explode the mains caps, but the series NTCR surge protector was vaporized. After e-cap replacement I cooked it at 11 amps 5 v and 100 ohms on +-15 v for 90 minutes before shipping it.
The ST70 amp has needed re-e-cap 5 times since 1970, due to low speaker voltage out, typically 7 instead of 17. Plus output tubes and rectifier tubes needed replacing about half as often.
Due to the popcorn noise I heard coming out of the tantalum caps in my ST120 amp, I change all those too when I repair an old amp. 3 of 4 were changed in #9574 by 1973, (build date 1968) and the tantalums have all been bad in the spare H100 perc/pedal chassis I received internet order.
Do what you want, I like to restore them and play trouble free for another 7-20 years. Been 6 years on #9574 already.
 
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