"Marvin Rosen" ) writes:
I have a GE Superadio-1 & an idea of making
a mod that would improve the tuning capability
& remove any backlash problems

Go into the radio & unsolder the leads to the
variable capacitor. Solder RG-174U coax to
the circuit board where the var cap was
connected. Bring the coax out of the radio
& connect to a quality 0-365 pfd var cap.
This var cap can be enclosed in a metal or
plastic container with a vernier dial. As an
alternative, a hole can be drilled thru the radio
with a connector mounted flush to the radio.
This would allow one to experiment with different
variable capacitors. Is my idea valid and/or practical?
Any merit to it? I realize the above may decrease the
resale value of this radio but that is not a consideration
of mine..

So you're suggesting a bandspread control, ie fine tuning.
You will not be able to tune just with this added control,
because the variable in the radio is multi-section and tunes
the front end as well as the oscillator. Unless you add multiple
sections (and that likely will cause problems because of the multiple
leads) you will have to tune with this added control, and then use
the main tuning to peak the front end. And since the sections are
no longer tracking, you risk mistuning, and listening to an image
response rather than a station on the frequency you are expecting.
This is why the various sections are ganged together, so you only
need one tuning knob, and they will all tune together.

Now all that's based on your "365pf" variable. It would make much
more sense to put in a smaller variable, so you tune with the main
tuning of the radio, and then have a tiny amount of fine tuning. So
long as you don't move the oscillator too far from where the main tuning
is, you won't have problems with a mistuned front end.

It probably is better to keep this added variable close to the main
tuning capacitor. The longer the leads, the more the whole thing will
upset the main tuning and calibration, and you may make it prone to
detuning as your hand nears the cables attaching this extra variable
to the main tuning capacitor. If you mounted it externally, you'd
very much want it inside a metal box.

It might make more sense, given that you only want some fine tuning,
to make this added variable capacitor a varicap, ie a voltage variable
capacitor. It will be easy to tuck next to the main variable, but
since the leads to a potentiometer (read "volume control" will be carrying
only DC, there won't be the mistuning problem. You could even get a multiturn
potentiometer for very fine tuning.

Michael