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Old August 17th 05, 12:48 AM
tim gorman
 
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wrote:



I'm attempting to build a small AM broadcast superhet transistor radio
from scratch and having a problem with the three IF stages oscillating
at the IF frequency (455KHz). The IF coils are salvaged from other
radios and the ground connections are to a common bus wire that
connects all the IF coil shields. It works reasonably well around 5
volts, but breaks into oscillation (at the IF frequency) if the supply
voltage is increased 1/2 volt and the signal drops off significantly if
the supply voltage is reduced 1/2 volt. So, it only works within a very
small supply voltage range of around 5 to 5.5 volts. The IF stages are
decoupled from the supply with a small resistor in series and bypass
cap to ground which helps, but doesn't solve the problem. I'm wondering
what can be done to stop oscillations and increase gain?

How did they manage to avoid the oscillation problems in the old tube
radios that were hand wired without any PC board?

-Bill



If you are getting oscillations then you have a feedback loop somewhere that
you don't want. What are you using for IF amplifiers? Transistors, Fet's,
mmic's, or something else? How are you biasing the stages? Are all three
stages oscillating or only one of them? If you are using transistors or
mmic's you might be better off not even using IF cans.

tim ab0wr