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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Jan 2019, 10:30 am   #21
John Caswell
Hexode
 
Join Date: Jul 2009
Location: Wokingham, Berkshire, UK.
Posts: 437
Default Re: Can a capacitor's value increase with time?

Martin asked "Can a capacitor's value increase with time?"
As far as I am concerned that answer is an unequivocal yes.
I have just overhauled a Quad 22 control unit and every small brown tubular capacitor, Dubilier, I think, in the tone and phono equalisation circuitry had changed value upwards. eg a 5000pF - 5nF had changed to 33nF. Most were replaced by polystyrene and/or polyester types.
Interestingly the "notorious" black Hunts capacitors were all perfectly OK as far as value and leakage were concerned. All the electrolytics had changed both in value and ESR so of course were replaced, and the switch suppressors were both s/c, replaced with modern R/C combinations from CPC/Farnell.

John
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Old 12th Jan 2019, 12:18 pm   #22
SteveCG
Nonode
 
Join Date: Jun 2009
Location: Herefordshire, UK.
Posts: 2,495
Default Re: Can a capacitor's value increase with time?

Re post no. 21:

Yes John, I had the same experience with a Quad 22 control unit a few years ago.

Since Acoustical (Quad) gave treble/bass cut and lift plots in their documentation I thought I'd check them out on this unit. And to cut a long story short I found that those capacitors had gone up in value. I replaced them and the treble/bass lift then was as Quad had said.

I must admit though that I never checked to see if the originals were electrically leaky - At the time I knew about older paper capacitors going leaky but I had not learnt about the extent of the problem until joining this forum.
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Old 13th Jan 2019, 7:42 pm   #23
Maarten
Dekatron
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
Default Re: Can a capacitor's value increase with time?

Quote:
Originally Posted by G8HQP Dave View Post
Yes, a mixed dielectric capacitor could go high in value without going leaky. However, what was the point of using two different dielectrics? Maybe nobody trusted this new-fangled plastic film stuff?
The new fangled plastic stuff had already proven itself since the late 1950's. The problem was in the applications that required low loss or high dV/dt capability. Polyester film was worse at that than paper, so they sought a way to combine the strengths of both dielectrics without the weaknesses.

Later, polypropylene was the material of choice, having better specs than polyester and paper. However it was much harder to get the sputtered plate materials to stick to polypropylene so they were often foil wound and/or also mixed dielectric. Only in the mid to late 1970's polypropylene capacitor technology had matured sufficiently to replace all paper capacitors, but not every component manuacturer chose to do so (Philips/Mullard was probably one of the first in Europe, as they had been with the introduction of polyester film).

Last edited by Maarten; 13th Jan 2019 at 7:49 pm.
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