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By-Passing Defective Audio and IF Transformers
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.
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