Hello Hans:

Using a power transformer from a set that consumes less power can be an
invitation to failure -but it can be done if there is no other option.

In most receivers, the voltage out is similar. The problem for a
transformer is that when the load increase beyond design, heat will rise.
Most receivers are already pretty warm inside.

Compare the total power consumed by the two transformers in their respective
receivers. If the replacement power transformer is something like 15% less
power, it can be a candidate for swapping. But, there are a couple of
things I would do. First, replace the rectifier tube with a solid state
version (including resistors to drop voltage equal to the drop by the
rectifier tube. Secondly, use a variac to keep the line voltage down to 110
vac (or 220 in Europe). Third, ad a small muffin fan to remove heat from
the radio.

Of course using a proper transformer is the best solution - but sometimes it
is necessary to improvise. The SX-100 is a much more valuable receiver,
however, and inserting a transformer from an S-40 will decrease the value.
You might be able to find a transformer from a SX100 or SX-96, which would
be a better solution.

Many years ago, I oten did things like that because that was all that was
available.

Good luck, Colin K7FM