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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.

https://elektrotanya.com/philips_90rl020_sch.pdf/download.html

https://www.diodes.com/part/view/ZTX690B

See also: http://www.dicks-website.eu/low_noise_amp_part3/part3.html

philips.jpg

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    \$\begingroup\$ Faster may not be the only issue. (Or even the issue.) What are the old parts? What are the new ones? (Datasheet links would be appreciated.) And can you supply a photograph of the salient area of the radio circuit board? \$\endgroup\$ – jonk Nov 16 '20 at 19:47
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    \$\begingroup\$ Repair questions are required to provide details and demonstrate understanding of the thing being repaired. You have not documented the circuits into which you have inserted these. \$\endgroup\$ – Chris Stratton Nov 16 '20 at 19:51
  • \$\begingroup\$ How old's old...not a Sinclair Micromatic by any chance? \$\endgroup\$ – TonyM Nov 16 '20 at 20:15
  • \$\begingroup\$ you could add 50pF to the collector and try that, but was it Germanium? like a 2T65 used by Sony? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 16 '20 at 21:37
  • \$\begingroup\$ Your problem could be too much gain or a higher speed transistor, (higher Ft). To much gain could cause an oscillation due to a feedback path that with lower gain devices would not oscillate. \$\endgroup\$ – SteveSh Nov 17 '20 at 1:03
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Hotter transistors can cause problems .There are cures that can be tried .You can remove the neutralisation caps on the IF stages .These caps were used to compensate for the CB capacitance which was higher in older transistors .Modern AM sets do not need these small caps due to better transistors .You should also check and replace any suspect bypass caps like the one on the AGC line . Now you can place damping resistors across the full primary of the IFTs to reduce gain and selectivity .Start at 47K going down until the set stops squealing .Bad cases may want 10K .An alternative to damping resistors is to place some unbypassed emitter resistance like 33ohm in the offending transistor .These tricks worked for me in the early 1970s in the absence of good test gear .

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