A ground plane should make the system more stable. The radios that I have
been designing over the years have gains in the I.F. systems that range from
90 to 120dB, depending on the topology. You are definitely getting signal
feedback through your power supply rails. 100 to 120 ohms is about as high
as you want to go as far as decoupling resistor values. .05uF is too high of
a value for a decoupling cap..............01uF is what you should use for
local decoupling.
You should also have at least one bulk decoupling cap that has a value in
the range of 10 to 47uF. The larger decoupling cap cancels out the
inductance of your power supply wiring while the smaller local decoupling
caps cancel out the inductance of the bulk cap.

Pete

wrote in message
oups.com...
I don't know what you mean by "small resistor" in series with the V+
line.
Each amplifier stage should have a series resistor of 100 ohms and a
bypass
capacitor of .01uF.

I've tried several values, 220,330,510, ect. with 0.05uF bypass.
Doesn't make much difference.

As far as physical distance for each stage, you should allow 1/2 inch for
every 40dB of gain. You shouldn't have this type of problem. A ground
plane
is always a good idea,

The IF cans are spaced 1 inch apart and I disconnected the 2nd IF stage
so I only have an oscillator, mixer and single IF amp stage. Same
problem, it always oscillates when the voltage is raised to obtain
reasonable gain.

Although it is possible if you have quite a bit of copper foil tape
around the
board, you can build this circuit on perfboard but you have to be very
careful,
as you have already discovered.

I took a look inside a AM/FM clock radio I have and checked the
ground connections. There are 3 RF transformers, oscillator, mixer and
single IF stage, and all of the metal shields of the transformers are
isolated. There are no physical connections between the shields of the
3 transformers.

Obviously, I am missing something. Grounding all the transformer
housings on a massive ground plane does not seem to be the answer.

-Bill