Isocore output transformer design - UK Vintage Radio Repair and Restoration Discussion Forum

trobbins · 24th Nov 2021, 1:07 pm

Rola Australia introduced the 'Isocore' style of output transformer construction in circa 1938. The transformer is potted and insulated from a metal enclosure, and the core is connected to the primary B+ tap (PP CT) so as to avoid any risk from electrolytic corrosion. This became a very common style of line and output transformer fitted to a speaker, especially for PA and radio equipment in the 1950's.

I have just partially restored the lead in wires to a ~12W 1950's rectangular canned output transformer that had this isocore style arrangement (the transformer is larger than any standard Rola product and is unmarked). Due to the potting and a small single hole in the metal housing, all leads (two for each PP half-primary, two for speaker, and one for core) exited in a close bunch and I was quite concerned about the state of wire insulation at the point of exit. So I removed sufficient pitch to gain access to each individual wire and then added insulation tubing to each wire to allow the bunch to exit the metal covering without the risk of poor insulation.

I now have the option to either connect in original isocore configuration, or the typical earthed core configuration. Core to metal enclosure insulation resistance is circa 600Meg at 1kVdc, and as I can't foresee any good reason to retain an isocore configuration I think I will err on the side of caution.

I've had to 'repair' a few transformers (typically power types) where 1940's lead insulation has crumbled at the level of where it exits the potting pitch - that has meant a tarry excavation to where I can make a good connection with new insulated wire.

Any thoughts on this issue?

Tim

Ed_Dinning · 24th Nov 2021, 4:46 pm

Hi Tim, an interesting design approach and may have been used to reduce voltage stress to the core (possibly also reducing stray cap a little as well.

Not a product I've come across over here, I'll try a bit of research in Electronic Engineering.
Nothing in a 50's addition of Langford Smith.

One trick we used on pitch encapsulated 6.6KV busbars when changing sections was to pack dry ice round the outer case. It was then easy to chip off quickly

Cheers, Ed

joebog1 · 24th Nov 2021, 9:35 pm

I think this generally happens because the pitch rots the rubber insulation underneath the cotton binding. Probably because the pitch holds the wire too ridgidly. I have repaired a few in this condition, and you have done exactly what I do to repair them.
I use PVC wire for the new leads however. I really dont like that style of construction at all, but you/we are stuck with it if we want to continue using the traffo. Rola made thousands of transformers this way, and most last a lifetime mainly because they don't get moved around.

Joe

24th Nov 2021, 1:07 pm	#1
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Isocore output transformer design Rola Australia introduced the 'Isocore' style of output transformer construction in circa 1938. The transformer is potted and insulated from a metal enclosure, and the core is connected to the primary B+ tap (PP CT) so as to avoid any risk from electrolytic corrosion. This became a very common style of line and output transformer fitted to a speaker, especially for PA and radio equipment in the 1950's. I have just partially restored the lead in wires to a ~12W 1950's rectangular canned output transformer that had this isocore style arrangement (the transformer is larger than any standard Rola product and is unmarked). Due to the potting and a small single hole in the metal housing, all leads (two for each PP half-primary, two for speaker, and one for core) exited in a close bunch and I was quite concerned about the state of wire insulation at the point of exit. So I removed sufficient pitch to gain access to each individual wire and then added insulation tubing to each wire to allow the bunch to exit the metal covering without the risk of poor insulation. I now have the option to either connect in original isocore configuration, or the typical earthed core configuration. Core to metal enclosure insulation resistance is circa 600Meg at 1kVdc, and as I can't foresee any good reason to retain an isocore configuration I think I will err on the side of caution. I've had to 'repair' a few transformers (typically power types) where 1940's lead insulation has crumbled at the level of where it exits the potting pitch - that has meant a tarry excavation to where I can make a good connection with new insulated wire. Any thoughts on this issue? Tim

24th Nov 2021, 4:46 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Isocore output transformer design Hi Tim, an interesting design approach and may have been used to reduce voltage stress to the core (possibly also reducing stray cap a little as well. Not a product I've come across over here, I'll try a bit of research in Electronic Engineering. Nothing in a 50's addition of Langford Smith. One trick we used on pitch encapsulated 6.6KV busbars when changing sections was to pack dry ice round the outer case. It was then easy to chip off quickly Cheers, Ed

24th Nov 2021, 9:35 pm	#3
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Isocore output transformer design I think this generally happens because the pitch rots the rubber insulation underneath the cotton binding. Probably because the pitch holds the wire too ridgidly. I have repaired a few in this condition, and you have done exactly what I do to repair them. I use PVC wire for the new leads however. I really dont like that style of construction at all, but you/we are stuck with it if we want to continue using the traffo. Rola made thousands of transformers this way, and most last a lifetime mainly because they don't get moved around. Joe