Anyone that collects and plays the early radios dating back to the 1920's eventually learns that the "Achille's Heel" of such sets are the interstage audio transformers - they are almost invariably bad. Why? One person on the Internet commented on a common construction technique of the time - embedding the transformer in a tar, or "potting" compound, and wondered whether this construction method served to extend component life -

"... in fact, I think that black tar they call "potting" compound is actually a mild acid - did this stuff keep one component alive ever?"

Randy Guttery, a member of The Mississippi Historical Radio and Broadcasting Society, offered this explanation why tar was used, and why the transformers often failed -

It does seem like potting was more of a pain than cure most of the time - but there were several reasons for potting in the early days - and the potting isn't the source of the problems (I'll come back to that subject in a moment). In the early days of transformers - the construction techniques left a lot to be desired - and a lot of problems. One was loose core materials - notice I didn't say laminations... One only has to ponder (nightmare!) about the audio interstages of say an AK 20 big box to understand "loose" core material. Even when laminations were used - manufacturing tolerances were so "loose" that power transformers would buzz (audibly) quite loudly - potting was the only way (then) known to silence them. Later, of course, better tolerance in manufacturing, "gluing" the laminations together, and even running a weld across the laminations at a null point in the magnetic field all serve to keep modern ones quiet... and of course electrical insulation has enjoyed tremendous improvements along the way as well.

The major "failure" mode of most potted items usually was caused by one of two conditions not caused by - but exacerbated by the fact it was potted.

Probably the #1 failure mode is that the wire was too thin - had too many impurities in it -- resulting in it being mechanically brittle. Since metal and tar expand and contract at different rates - over the years of temperature changes - the thin wire lost the "shoving match".

The Second major cause of failure was improper soldering and cleaning / sealing the joint. Again, in the early days, they thought nothing of stripping the enamel, or whatever the insulation on the magnet wire, soldering it to the take-off flying lead, and leaving it at that. Three problems here: 1) the flux back then was more corrosive than it should have been; 2) left on the bare wires - it would continue to "chew" through them - and the thinnest wire, the winding, will eventually "rot" off. And, 3) even if the flux were cleaned pretty well, moisture and other contaminants, now sealed in by the potting, will attack the bare portion of the magnet wire - chewing away at it over the years.

Today modern transformers have the joints super cleaned. Then the bare leads are re-sealed - usually in some type of inert confomal coating - the joint then "sleeved" - and mechanically anchored. Even when potted, the joint is protected inside the "sleeve" which allows some buffering from mechanical, thermal and even electrical "shocks".

So to answer your question: "did this stuff keep one component alive ever?" Probably not - that isn't what it was for... potting's major role was mechanical "securing" (check out the power transformer and filter caps in most early AC sets - like the Bosch 28 - nothing holds those parts "in the can" - except tar), as a further aid to "mounting", insulation and sound deadening. I can't imagine "early" potting being considered a preservative"... it was a mechanical means to an end.

While the old regenerative and TRF battery radios employed interstage transformers whose ratios ranged from 3:1 to 10:1, those ratios are usually not critical - the radio will certainly play using modern 3:1 transformers, and probably with less distortion, due to lower ratios and better steel laminations. Modern replacement 3:1 ratio interstage transformers are readily available and are small enough to be hidden inside the original transformer shell, or tucked away out of sight.

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