Mixed Dielectric Capacitors - UK Vintage Radio Repair and Restoration Discussion Forum

Philips210 · 13th Jan 2017, 11:33 pm

Hi.

Thinking about the well known issue with those Rifa class X safety capacitors that suffer cracks to their cases, I have also been experiencing cracked cases on those mixed dielectric capacitors used extensively in 1960's and 1970's equipment. These were often made with a two tone case typically blue and white. I think latterly they were usually just white in colour. The cracking seems to occur around the welded seam and it appears to have happened due the innards expanding. I have found several unused examples suffering the same problem. The mixed dielectric, does it contain paper?

I suppose the dielectric must be absorbent to some degree for the swelling to happen. Paper as a dielectric can't re regarded as long lived unless the quality of construction prevents any ingress of moisture.

Regards
Symon.

Diabolical Artificer · 14th Jan 2017, 6:13 am

I have some in the shed, not sure why I kept them ( pot's and pot's of cap's and you never have the one you need) think mine are called "duomold'" or something similar. I'll check them out as havn't checked for a while.

Andy.

dazzlevision · 14th Jan 2017, 8:10 am

Have you seen this thread: http://www.-radio.net/forum/showthread.php?t=98964

The capacitor type you mention was made by Dubilier (the British company, not Cornell-Dubilier). The various coloured versions are shown in the pictures. They were called "mixed dielectric", which were paper and polyester, with a moulded polypropylene case.

The yellow coloured "Duomold" capacitors were made by TCC (later branded Plessey TCC).

Boater Sam · 14th Jan 2017, 8:11 am

I am coming round to thinking that the cracking is due to internal pressure due to degrading of the dielectrics.
Any paper has an acid content from the pulping process, even acid proof tissue has a minute trace element.

I know that the perceived wisdom is that moisture ingress is the cause but I find that hard to believe in a component that is sealed in a modern polymer.
There must also be some physical stress caused by changing voltage.

Further, the lead out wires are a possible cause either due to moisture or chemical or galvanic changes. A lot of later components have wires with a high iron content, they are magnetic, and lead tinning and silver tinning must cause some galvanic corrosion.

m0cemdave · 14th Jan 2017, 11:32 pm

I have seen unused, old stock, ones with the casings cracked along the seams between the blue and yellow (or blue and white) plastic. Maybe it shrinks with age, or temperature cycling?

13th Jan 2017, 11:33 pm	#1
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,580	Mixed Dielectric Capacitors Hi. Thinking about the well known issue with those Rifa class X safety capacitors that suffer cracks to their cases, I have also been experiencing cracked cases on those mixed dielectric capacitors used extensively in 1960's and 1970's equipment. These were often made with a two tone case typically blue and white. I think latterly they were usually just white in colour. The cracking seems to occur around the welded seam and it appears to have happened due the innards expanding. I have found several unused examples suffering the same problem. The mixed dielectric, does it contain paper? I suppose the dielectric must be absorbent to some degree for the swelling to happen. Paper as a dielectric can't re regarded as long lived unless the quality of construction prevents any ingress of moisture. Regards Symon.

14th Jan 2017, 6:13 am	#2
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,670	Re: Mixed Dielectric Capacitors I have some in the shed, not sure why I kept them ( pot's and pot's of cap's and you never have the one you need) think mine are called "duomold'" or something similar. I'll check them out as havn't checked for a while. Andy. __________________ Curiosity hasn't killed this cat...so far.

14th Jan 2017, 8:10 am	#3
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,623	Re: Mixed Dielectric Capacitors Have you seen this thread: http://www.-radio.net/forum/showthread.php?t=98964 The capacitor type you mention was made by Dubilier (the British company, not Cornell-Dubilier). The various coloured versions are shown in the pictures. They were called "mixed dielectric", which were paper and polyester, with a moulded polypropylene case. The yellow coloured "Duomold" capacitors were made by TCC (later branded Plessey TCC). Last edited by dazzlevision; 14th Jan 2017 at 8:15 am. Reason: Correction.

14th Jan 2017, 8:11 am	#4
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Mixed Dielectric Capacitors I am coming round to thinking that the cracking is due to internal pressure due to degrading of the dielectrics. Any paper has an acid content from the pulping process, even acid proof tissue has a minute trace element. I know that the perceived wisdom is that moisture ingress is the cause but I find that hard to believe in a component that is sealed in a modern polymer. There must also be some physical stress caused by changing voltage. Further, the lead out wires are a possible cause either due to moisture or chemical or galvanic changes. A lot of later components have wires with a high iron content, they are magnetic, and lead tinning and silver tinning must cause some galvanic corrosion.

14th Jan 2017, 11:32 pm	#5
m0cemdave Octode Join Date: Feb 2009 Location: Bletchley, Buckinghamshire, UK. Posts: 1,223	Re: Mixed Dielectric Capacitors I have seen unused, old stock, ones with the casings cracked along the seams between the blue and yellow (or blue and white) plastic. Maybe it shrinks with age, or temperature cycling?