Restoration methods

Interstage Audio Transformers Repair

Author: Dave Gonshor


Even for the novice, 1920's battery sets can be easy to get going, once the power supply problem has been solved. Repair of battery sets generally consists of checking the wiring (many times these were tinkered with by a previous owner, probably to see if he could 'soup it up' to get performance like a newer set?), cleaning contacts, and repairing the filament rheostat.

By far the most common and troublesome problem with battery sets, in my experience, is the interstage audio transformers. In a typical five tube tuned rf set, there are two interstage audios: one between the detector and first audio stage, and the other between the first audio and the second audio stage. Usually, one of the two audios has an open winding. Most commonly, I have found the primary side, which has +B across it, to be open, but either the primary or secondary are subject to failure. The main failure mechanism seems to be corrosion of the connection to the very fine winding wire. How can these be fixed?

Some have advocated "zapping" the open winding with high voltage (on the order of 100 volts), but my experience with this is limited. I have one transformer I have successfully done this to and it is still working. Others say it is a temporary solution, as the original cause of the open circuit will work to undo the "repair". In any event, if this process fails, then what? It is possible to convert from a transformer coupled interstage to a resistor-capacitor coupled interstage.
However, I don't recommend this for two big reasons: originality and performance. Performance will suffer with an RC coupled stage, because the transformer actually provided "gain" due to the step-up of the turns ratio. This loss of gain can't be tolerated in a battery set, because performance is usually marginal anyway. The originality reason is self explanatory. The best thing to do is to restore the transformer to operating condition.

If you're lucky, you can find the break in the connection on the outer surface of the winding doughnut, if that happens to be the winding that has failed. Peel the paper overwrap back and follow the heavier lead wire until you see the splice to the fine winding wire. This is usually where the problem is. Using typical surgeon skills, which most of us have (not!), reconnect the wires by removing the varnish from the winding wire and re-solder.
If this doesn't work, or if the broken connection is on an inner connection, then the winding will have to be replaced. These can be rewound, but this is beyond most of us because you are working with 40 AWG wire (finer than human hair) at least a mile long.

A better way to go is to buy a replacement winding from Antique Electric Supply. These come with a center tapped secondary, so they can be used for a push pull output stage. For non push pull, just don't use the center tap.
The only trouble is, their one offering is limited to a one-half inch window. This happens to fit most, but not all, audio transformers.
Begin by removing the iron core slats one at a time. Then reverse the process with the new windings. It's really easier than you might think, just a little time consuming. What do you do if the window is larger than one-half inch? What works exceedingly well is to use a miniature transformer that can be concealed within the original one. This requires the center leg of the iron core to be cut out, one slat at a time. This leaves a hole in the center, where the new miniature transformer fits within the old iron core and clam shells of the original transformer. This miniature transformer, available only from Gary Schneider at Play Things of Past, has a high permeability core and gives excellent performance for a tiny transformer. It can fit inside most any original can, so the repair is invisible from the outside.