In , Liam Ness
wrote:

I was wondering if anyone has an info about converting an AM radio to
a SW one without down converting the signal? I have a defunct old
valve superheterodyne SW reciever and an old transister AM/FM
superheterodyne. Is it possible to replace the AM radio's tuning
variable capacitors with the SW ones and get SW, or is it more
involved than that?

It's much, much easier to build a converter for the band you want. But if you
are really ambitious....

Ok, it's not THAT hard, provided you don't plan on tuning a wide bandwidth.
First you need to replace the original RF tuning section with a new front-end, a
bandpass filter with steep slopes that covers the band you want to tune. Toss
the RF coil and disconnect the appropriate section on the tuning capacitor, both
of which will not be needed anymore. Then you must add a padder cap in series
with the local oscillator section of the tuning cap; and depending on how high
in frequency you want to go, you will need to either remove turns from the
oscillator coil or replace it completely. That's the fundamentals, but as always
there is going to be some fine tuning involved, like changing the values of
bypass or coupling caps, or modifying the wiring of the local oscillator to
handle the higher frequencies. Oh, and don't expect spectacular performance.

It's much more involved if you want to keep that tuned RF front-end. First you
need to determine what kind of tuning capacitor you have -- if it is designed
for AM BC superhet (one section smaller than the other) then you need to replace
it with one where both sections are the same. Then you will need to add some
padder caps in both series and parallel with the local oscillator section, and
modify or replace both the RF and local oscillator coils. All the values need to
be calculated depending upon what band you wish to tune -- a job not for the
timid as it takes numerous iterations to match the curves so the RF section will
track with the local oscillator to get a constant IF. Assuming you manage to get
all that done, you then need to align the padder caps so the two sections track.

BTW, I wouldn't even TRY this with the transistor radio, but with the tube radio
it might make an interesting project. You might also consider building a new
tuner (RF section, local oscillator and mixer) on a seperate chassis, then send
the output to the IF and AF sections of the original radio. That's how I did a
couple projects. Modification to the original radio involved only the addition
of a switch and a phono jack to the input of the 1st IF section. Just flip the
switch and the radio still works as before.







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