I have an old pocket radio which uses silicon NPN transistors for the mixer and IF stages. I replaced 2 of the IF transistors and the unit functions, but the new transistors are faster than the old vintage parts. I’d like to hear any recommendations for reducing these spurious signals in the IF stages, which sound like squealing when tuning the radio. Should I build shields for the stages?
EDIT: Radio is similar to a Philips 90 RL 020, twin band LW/MW. 1st IF amp, 2nd IF amp, and audio preamplifier (Q2, Q3, and Q4) were all replaced with low noise ZTX690B transistors ordered through Farnell. Small signal hFE was measured to be around 600 for the new parts. Staggering the IF yields good performance on MW sacrificing adjacent station rejection. Unit functions exceptionally well on LW with the IFs peaked so it’s close to working. Experimenting with shielding seemed to reduce oscillation but not enough. All electrolytic capacitors have been replaced. The purpose of this project is to learn how to improve RF circuit stability.