I have a 1960's Philips all transistor radio which I restored recently and replaced two of the AF11x PNP germanium transistors on the mixer/oscillator and IF amp stages. After these replacements, the set is picking up stations fine on all the SW 1/2/3 bands, but the sound is very low as if there is no amplification, but the audio stages can't be the culprit since the pick up is working fine. The second suspect appeared the antenna connections from the wave-changer but since I'm not having the circuit diagram of the radio I can't fiddle with them. What could be the reason for this almost inaudible sound?
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1\$\begingroup\$ What's the radio model number? \$\endgroup\$– Andy akaCommented Jun 21, 2014 at 10:45
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3\$\begingroup\$ Did you use germanium transistors for the replacements? If you used silicon, the biasing of the transistors will need to be adjusted. \$\endgroup\$– Dave TweedCommented Jun 21, 2014 at 11:06
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1\$\begingroup\$ Try this site for circuit diagram radio-workshop.co.uk/manuals.shtml \$\endgroup\$– Andy akaCommented Jun 21, 2014 at 18:37
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\$\begingroup\$ Nice site but I'm not sure it covers any radios new enough to have transistors! \$\endgroup\$– user16324Commented Jun 22, 2014 at 10:14
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2\$\begingroup\$ Put the old transistors back. If they ain't broke don't fix 'em. \$\endgroup\$– user207421Commented Jul 21, 2014 at 23:05
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2 Answers
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Check any electrolytic capacitors in the audio amp section. Could be they have dried up and are killing your audio.
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Since the replacement transistors aren't identical to the ones replaced, it's more than likely that the local oscillator tuning is now skewed and not tracking the RF properly, and/or that the IF is now misaligned, either occurrence resulting in a low IF amplitude out to the detector.
In any case, your troubleshooting procedure should start with a complete realignment of the radio.
answered Jun 21, 2014 at 11:20
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1\$\begingroup\$ If the Local osc is 10kHz out it will still tune signals perfectly normally thru the I F filters. I don't see this as causing a low freq noise problem. \$\endgroup\$– Andy akaCommented Jun 21, 2014 at 13:02
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\$\begingroup\$ I meant low volume.... On stupid android LOL \$\endgroup\$– Andy akaCommented Jun 21, 2014 at 13:06