how long do various capacitors last?

[maark6000]

i'm about to dig in to repairing an old synthesizer (Arp Omni) from 1977, and I know I need to replace the tantalum capacitors, and I'm going to replace all the caps in the power supply, but on the lower voltage boards (+/- 15v) I'm wondering whether I need to replace the ceramic disk caps, and the plastic ones as well.

thoughts?

thanks.

 

[davenn]

The tants will probably be ok they last a very very long time unless mistreated ...eg excess voltage etc. disc ceramics the same pretty much for even. Its the electrolytic type... the ones in the aluminium cans that have a short lifespan. Gosh they will dry out just sitting as stock on a shelf for 15 years, let alone in use.

plastic ... polyester, polypropylene, mylar etc will last a lifetime like the ceramics

electrolytic caps in use wil last varying times .... really depends on...

....manufacturer quality ... there are junky ones out there
....whether they are normal or low ESR types
....surrounding thermal conditions ... if the equip they are in runs hot, their lifespan can be very short only a year or 2, compared to a cool running bit of gear

so in short..... just look at the electrolytic ones

cheers
Dave

 

[(*steve*)]

Why do you need to replace the tantalum capacitors? I would have thought that electrolytics would be the first target.

My second target would be... nothing. Unless there is something to indicate failure, total replacement is likely to cause more problems than it fixes.

 

[maark6000]

yeah, i've heard tantalums can last a while, but if you look at any of the documented recaps of the Arp Omni (mk. 1) on the web, they all say that the tants start leaking and causing stuck notes and other mischief.

This particular keyboard was notorious for having a poorly designed power section, and would produce transient spikes when powered on/off. Also, the caps were subject to high reverse voltages, that may have caused their early demise.

I'm definitely not going to touch the ceramic disk caps. And for now I'll leave the plastic ones, and see where I end up.

Thank you all, I'll be posting more questions about this project as I progress.

 

[davenn]

I actually doubt that there are tantalum caps in something from 1977
would be extremely rare in any equip from that era
would love to see a photo of the board(s) to confirm that
If you have your one opened up ... please take a couple of pics and post then on here

one guy in one forum commented about replacing "tantalum" caps with non-polarised ones, if the cap in there is polarised, then it should be replaced with a polarised one

Dave

 

[maark6000]

here are the requested photos. the first one is of the 22 uF caps. The next is just a larger zoom out of the lower voicing board. I think this synth was manufactured between 1977 - 1979. So could be a little later.

 

[davenn]

well, well would ya lookie at that !! ... lots of them
maybe I'm getting that old I forgot how long ago they started becoming prolific

I still stick with my earlier comments that they wont easily fail.
unless they look physically damaged eg ... cracked, heated by other hot components around them say hi wattage resistors etc ( which those ones dont) they will have a long life. Thats not to say ... like all things in electronics, failures do occur randomly

I would suggest... if it aint broke, dont fix it ... aka ... if its working ok, dont screw around with it

cheers
Dave

 

[maark6000]

Thanks for your help Dave.

and... well, that's just it. it isn't working. there's two sections to this keyboard, a "string" section, and a "synthesizer" section. The synth part isn't working at all, but that's probably due to a fried CMOS somewhere, probably an op amp. The keyboard as a whole though is supposed to not make any sound when not being played, but that's not the case. if you can imagine what it would sound like if every key was played at once, but then turned down low, that's the latent sound this keyboard emits when not being played. Then when you play a key, the keyed note is louder for sure, but i'm guessing that base noise is due to leaking capacitors.

the real question i'm faced with is: if i have to replace them... what to replace them with? The values of the tantalums are fairly low... from 1 to 22 uF, never more than 35v rating, mostly 20% tolerance (a few 10%). replacing them with electrolytic caps seems a little ridiculous as those seem to be the most prone to not aging well. Supposing the tantalums have to go, what would you replace them with?

 

[davenn]

replace them with new tantalums, same ratings

there's an old trick when checking polarised caps. Get a good one and nip the ends of the 2 leads with your side cutters to get sharp points. Then hold the cap ( correct polarity) across each other cap in turn and see if you find the one that is faulty.
will often work as a test before you even use the soldering iron

Dave

 

[maark6000]

ha, so obvious!

thanks!

 

[shrtrnd]

Your caps are epoxy-dipped solid tantalums. When I first started reading this thread
I thought you were talking about the wet-tantalums that were popular in the 1970's (they
look like aluminum electrolytics, and are still being sold today).
Anyway, It's up to you on replacing them if it makes you feel better, but I wouldn't.
Electrolytics, as mentioned above, are the first to go.
Inspect your solid tantalums VERY closely for physical damage if you think you have
a problem there. Usually, you can see cracks in the epoxy at the leads, or burn or
crack marks on the epoxy if they go bad.

 

[maark6000]

As I understand it, the issue with this keyboard is not so much that the tantalums are a bad capacitor medium, it's that the circuit design was abusive to them. They experienced high reverse voltages, and also transient spikes from a poorly designed power supply, and I think over the years it created holes in the dielectric. You can actually hear the effect of the leaking 22 uF caps, and I'm not sure how to gauge the effect of the decoupling caps, but I imagine replacing them might resolve other issues. I'm going to replace them with the same thing... although I am tempted to up the capacity.