Bill,

Transistor amplifiers with inputs and outputs tuned to the same frequency
turn out to be oscillators, especially if the the tuned frequency is very
low compared to the transistor's Ft. If you're using a modern transistor
with an Ft of say, 800 Mhz, and you're tuning it to say, 500KHz, then you
could have available gain of over 60 dB in one stage. This is a sure-bet
oscillator waiting to happen.

You can lower the gain by 'loading' the input and output tuned circuits with
parallel resistors (try 1K for starters and work up from there if it stops
the oscillation. Just remember that this technique, in addition to lowering
the gain, will also load the tuned circuit and degrade selectivity.

You can also try a small resistor (less than a couple hundred ohms) in
series with the collector lead of the transistor, or in series with the base
lead. This technique will lower the stage gain and actually increase the
impedance seen by the tuned circuits, improving selectivity.

Keep in mind that raising the selectivity too much can cause tuned circuit
losses, just as parallel loading it will lower selectivity and increase
losses.

Joe
W3JDR


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