BY-PASSING DEFECTIVE AUDIO AND IF TRANSFORMERS
by Norman R. Leal
A radio with defective Audio Interstage or Intermediate Frequency Transformer can often be repaired without replacing or rewinding the transformer. For many of us, it is difficult to find a replacement that fits and we may not be set up to rewind transformer. In the case of IF Transformers, the early radios did not standardize on a frequency and an exact replacement may be nearly impossible to find.
The transformer coupling can usually be replaced by capacitor/resistor coupling often without any loss of performance.
In the case of Audio Interstage Transformers, connect a capacitor from the plate of the driving tube to the driven tube grid, The value is not critical. I suggest 0.1 mfd at 400 volts.
Then bypass the open winding with a resistor. Again the value is not critical. If the plate to B+ side is open, try using a 22K at 1 watt. If the grid winding is open, a 100K 1/2 watt will work.
I think you will be surprised at the results. The frequency response will probably be better than the original circuit.
You may do the same with Intermediate Frequency Transformers. With IF Transformers use small capacitors between the plate of the driving tube to the grid of the driven tube. This will allow the peaking of the one side of the transformer which is still working. In this case, capacitor values around 200 pf will usually work.
Again, the resistor value used to bypass the open winding is not critical. The same values used for Audio Frequency circuits will work. I have suggested rather low resistance values so almost any circuit will work. In the case of tetrode tubes like the 24A, it is important to keep the plate voltage above the screen and tubes like the Triode 27 should also be used with fairly low plate resistors.
Copyright 1995 California Historical Radio Society, all rights reserved.
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