Parts shelf life and cannibalising old devices...

[johnalexwarren]

Hello.

In the past I've cannibalised defunct audio/video (a DVD player, video player, old amp) for parts and used them in projects with 100% success (and saved myself some £ in the process).

I've read that some electronic components, especially caps, have limited shelf life. However, if one plugs in an electronic device, regardless of age, and it still operates as it should, does this not therefore indicate that all the components contained therein are OK?

Your opinion would be appreciated!

Thanks,

John.

[firechief]

Well, you are going to get some responses to the question. The answer is yes, no, and it depends. I think that electrolydic caps are subject to degrading over time. Factors that may effect this include brand, how they are implemented, and tempature. Semiconductors for the most part don't degrade. They either work or they don't. Tubes, of course can go down hill. That being said, saving parts from the dump is a time honored technique of the DIYer, and most componets can be re-used. Someone else jump in here, come on, it's ok.

[indianajo]

I'm an inveterate salvager, especially as now I am not working. That said, the worst to salvage are electrolytic caps over 15 years old, especially over 20 years old. E-caps are cans with water-borax solution inside. They are sealed with rubber. Rubber deteriorates due to ozone-oxygen, etc. Lots of people "reform" electrolytic caps by charging at low voltage, then ramping up to the design voltage. This is good for the aluminum foil and water inside, but doesn't do anything for the seals. If an electrolytic cap in a circuit with significant energy (power amps, motor drives) goes dry, it can explode, showering debris and conductive borax water over everything around.
That said, the ratio of seal area to volume on 5000 uf up electrolytic caps has tempted me to use them in experiments (they worked fine up to 30 years old). However, I would never leave them turned on 10 hours a day that way.
Vacuum tubes can leak air, but the getter turns white if they do.
Paper capacitors, it depends on the seal. Some were packaged in wax cylinders, if temp was below wax melting temp, they can be okay. Some were garbage 2 years after they were made.
Plastic caps, no discernable life limit, yet.
Fuses, resistors, same. Carbon PILE resistors can have water issues, but these were rarely used except in battery chargers, generator testers, etc.
Transformers have to have been kept dry.

[johnalexwarren]

So ceramic/metalised/polyester/polypropelene caps should be OK?.

I inherited a reasonably large collection (about 600-700) of components that my late father built up during the '80s so I know for a fact that some of them are over 20 years old. Think it's probably time to dump the electrolytics....!

There are also over 550 resistors ranging from 1R right up to 20M which have all been tested and catalogued (a long night that was!), a number of standard & schottky diodes and assorted trannies and ICs. So far I've built a couple of low power amps and a mini tester with only the cost of some new copper clad and a few chipamps God bless yer Dad!

Anyway, I digress...!

Anybody have any idea on the self life of various types of semicons?

[indianajo]

Some people that own farfisa organs from the fifties report the leads and shells of the germanium transistors are rotting off. I suspect acid solder flux was use, or else Italy is a country out to sea.
I built a 2 transistor phase shift oscillator last year from 2n2907's removed from DFSII's in a TI service bulletin in 1973. Probably 1969 transistors. Worked fine. I have a dozen more.
I repaired my ST120, 1966 design and probably 1972 construction, and found the modern 1985 NTE60 transistor had so much gain it was causing the power supply to cut off current at 40 watts. I had to find a really rusty RCA TO3 five digit transistor that had low enough gain for the circuit to work properly. Life on transistors, seems to be the steel can vs the sea air. By contrast, modern TO3's have better Ft (frequency) and improve the slew rate of the output, improving the sound and cutting distortion. See the thread on solid state about a Leak Delta ST70, a british amp. 2N3772, 2N3055, 2N3055a, TIP3055, each later generation TO3 transistor improves the spice simulated distortion. I thought my NTE60 transistor equipped amp sounded better than the horrible reviews for the ST120.
Ceramic/plastic caps started about 1966, I haven't seen any reason to replace any not melted by too much current or the soldering iron. Mostly the soldering iron. Mica, I've got a couple in the PAS2 circa 1961, I didn't replace them when I did all the paper caps, and it sounds "fine". A little bright with the new polyester caps, but I think that is because they are .022 uf instead of .02 uf. Some ceramic caps were poorly made for low price, John Curl has a lot of nasty things to say about them. The milspec ones we used at Ford Aerospace, I never had a problem. I probably specified 10000 of them to go in the JSC. But some ceramics, you can see the metal, which obviously won't have stable performance with moisture.
In the modern is better department, when I replaced the carbon comp resistors in the PAS2 preamp, with metal film ones (india) it really did have noticeably less hiss. Most noticeable on the 250000 ohm plate resistors. the higher the resistance, the more thermal noise, I believe the theory says. Modern Power transistors have better safe operating area. Old switches and relays have more cycles built into the contacts (bigger).

[dave_gerecke]

I would have to support most of the comments here. But I would add, some of the old germanium transistors do develop issues when they get old, at least the preamp I have seemed to. Haven't messed with it in years. Also, any old selenium rectifiers you find, dispose of them. If you read around, they have a reputation of going bad, and taking the rest of the power supply, etc. with them.

Dave

[indianajo]

Yes, if you have any selenium rectifiers, replace them with 1n4002 etc and the equivalent resistance. Think about what the resistance is good for before just installing the silicon diode. Just had a 1970 tractor battery charger toast the $110 (now) transformer because the silicon rectifier shorted out (lightning?). The old seleniums had some resistance, weren't quite so dangerous a short. I'm putting 4 ea 1 ohm 10 watt resistors in series with the battery charge leads in case I try to charge a shorted battery Also a "thermal PTC resettable fuse", which seems to be the modern replacement for the thermal shap action "interrupter" battery chargers used to have.

[johnalexwarren]

Nothing like that in my goody box, just electrolytics (now binned), polyester, metal film and ceramic. Talking of which, a lot of the ceramics are showing bare dielectric underneath - is this normal or a sign of ageing/dying capacitors?

[indianajo]

Originally Posted by johnalexwarren

Nothing like that in my goody box, just electrolytics (now binned), polyester, metal film and ceramic. Talking of which, a lot of the ceramics are showing bare dielectric underneath - is this normal or a sign of ageing/dying capacitors?

ceramics capacitors, all the mil specs ones I had installed in JSC had a nice, complete green coating all the way down to the legs. Kept the moisture out, I hope. JSC was about 1 mile from the Gulf of Mexico, after all. I don't know what moisture does to ceramic dielectric, but I know paint is better formulated to resist its attack. Bypass caps on computer circuits, maybe vendors let them slide with bad paint. John Curl had a lot of nasty things to say about ceramic cap vendors. (that try to sell to consumer goods companies) PC's are obsolete in 2 years anyway, if they don't blow up. Audio circuits, a little 20% change in the value is a real nuisance.

[EBM_dude]

My 2 Cents on capacitors: I've had several PC mainboards and PSUs blow up because of bad electrolytics. The same is true for several engine control units. Some I could repair, others were completely fried. I avoid reusing capacitors whereever possible. OTOH, I have a nice collection of really big electrolytic caps salvaged from oscilloscopes made by a famous oscilloscope manufacturer (the name starts with a "T" ) in the 1970ies. They still work flawlessly.