Author Topic: vintage tantalum capacitor replacement  (Read 1737 times)

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Offline mongo

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vintage tantalum capacitor replacement
« on: December 11, 2016, 10:20:05 am »
I am having a hard time finding reliable information on the real value of replacing these.  I know that the 70's versions were fragile and would fail as a short but I am planning a major bit of maintenance on some vintage poly-synths and I would prefer to avoid replacing these if possible.

I am already going to replace all of the electrolytic as well as replacing all of the 4000-Series Chips with socketed modern versions but that is a far smaller number of items than the massive number of tantalum caps.

I will probably document all of the values as the schematics for these large vintage synths is quite sparse but as the number of VCOs, VCFs and VCAs is massive I am mostly worried that they binned these in matching values when they were built and that I will not have the budget to do the same.

But if there is a real concern I will do the work just so that I don't have to dig into them again.

As a side node only Vishay and Lelon seem to make axial electrolytic capacitors in the 10% tolerance range, but only for 500 hours in the sizes I need.  I have never used those brand and I think my normal axial brand Nichicon at 20% will be more stable over time but I would appreciate opinions.

I have never really had to plan for multi decade survivability before but I am doing both a Octave Plateau Voyetra 8 and an Oberheim ob-8 and would like to preserve them for as long as possible.

Thanks in advance.
 

Offline stj

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Re: vintage tantalum capacitor replacement
« Reply #1 on: December 11, 2016, 12:21:23 pm »
there is no value, failure is rare and usually caused by poor psu's effecting tants on a power rail.
i have never seen one fail in an audio path.
 

Offline mongo

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Re: vintage tantalum capacitor replacement
« Reply #2 on: December 11, 2016, 01:17:54 pm »
Thanks, it had a woo smell to it.
 

Offline stj

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Re: vintage tantalum capacitor replacement
« Reply #3 on: December 11, 2016, 02:22:01 pm »
they are dipped in resin. unless it shows signs of cooking i would ignore it.

 

Offline SeanB

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Re: vintage tantalum capacitor replacement
« Reply #4 on: December 11, 2016, 04:33:08 pm »
Why replace the working 4000 series devices, if they still work no need. Only change the dead ones, and if there are 2 or more with the same batch code that died then do all of those date code. Your replacement devices have much higher probability of either being dead from ESD damage or of being recycled and remarked fakes.

Only dipped tantalum capacitors on power rails had issues, and only there where they were used close to rated voltage. A 35V capacitor on a 5V rail almost never fails, but a 16V device on a 15V rail has a fair chance of popping, Replace with a regular 105C electrolytic, 20% tolerance is fine, and for a power rail you can go to the next higher capacitance and a higher voltage with very little issue, provided it will fit there. In any case a 100uF 63V 105C capacitor will replace any 10,22,47uF tantalum capacitor on a power rail with no problems, it will fit, and will have similar low ESR and a long life.
 

Offline mongo

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Re: vintage tantalum capacitor replacement
« Reply #5 on: December 12, 2016, 09:46:51 am »
There are a few reasons.

The 6 micron RCA CD4000 and Motorola MC14000 series from the era have a reported metal migration issue so getting them into sockets will allow easier repairs in the future.

There are no known public schematics for the Voyetra 8 control lines and because they are simple two layer boards it will be easy to document when putting in sockets.

And while I am in there I can just replace these older chips with modern units for a very small amount of money and avoid the issues all together.

 

Offline dan3460

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Re: vintage tantalum capacitor replacement
« Reply #6 on: December 12, 2016, 11:46:16 pm »
In my experience tantalums depend on the year, I have had Tektronix equipment where almost all Tantalums are bad and I have had others, even older equipment, with all tantalums in top shape.
A couple of years ago I would do a shotgun replace of all tantalums, since then I have only been replacing what is bad. If I see that I had to replace two or three tantalums, I will do a shotgun replacement of all that I see.
 
 

Offline David Hess

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Re: vintage tantalum capacitor replacement
« Reply #7 on: December 13, 2016, 10:17:58 am »
In my experience tantalums depend on the year, I have had Tektronix equipment where almost all Tantalums are bad and I have had others, even older equipment, with all tantalums in top shape.

A couple of years ago I would do a shotgun replace of all tantalums, since then I have only been replacing what is bad. If I see that I had to replace two or three tantalums, I will do a shotgun replacement of all that I see.

The early marketing and specifications for dipped solid tantalum capacitors touted them as not requiring voltage derating which Tektronix perhaps foolishly followed for a time.  All of the failures I have seen involved lack of voltage derating and I do the same thing; when I replace one, I check the others and replace them if they were not sufficiently voltage derated.

I have tracked down three different possible causes for the failures but they all involve thermal runaway of the self healing characteristic of these capacitors which is ameliorated by appropriate voltage derating.

Only dipped tantalum capacitors on power rails had issues, and only there where they were used close to rated voltage. A 35V capacitor on a 5V rail almost never fails, but a 16V device on a 15V rail has a fair chance of popping, Replace with a regular 105C electrolytic, 20% tolerance is fine, and for a power rail you can go to the next higher capacitance and a higher voltage with very little issue, provided it will fit there. In any case a 100uF 63V 105C capacitor will replace any 10,22,47uF tantalum capacitor on a power rail with no problems, it will fit, and will have similar low ESR and a long life.

When replacing solid tantalums I use roughly the same rule of thumb.  To get an equivalent ESR, an aluminum electrolytic capacitor will need about 2 to 4 times the capacitance.  This still will not yield as good of a performance at high frequencies but I have yet to see it be a problem.

Usually though I hit my local surplus electronics store and go through their bins for inexpensive NOS solid tantalum capacitors to use as replacements with appropriate voltage derating of course.  Since their origin is somewhat nebulous, I conduct my own burn in test as shown.
« Last Edit: December 13, 2016, 10:19:45 am by David Hess »
 
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