The pre-amps for each audio source ( incl cassette ) need to standardize and normalize the gain to produce 1V ( either rms or pp) to the source button selection so that the audio levels are uniform. It "sounds" like your power amp gain is too high and cassette pre-amp gain is too low or the heads are warn out or misaligned. or ? ... since this is a portable and not a rack mount with line inputs, your "source: selector signal level with high gain power amp may be "normalized to some value like xx mV p-p sine at 1KHz.
Otherwise you cannot expect the same volume control position to be same for each source , which is your situation but you should expect the same audio power out of the speakers for each input source. So adjust the volume control to max power , regardless if it is 80% on cassete or 20% on FM and then traceback to see where the 1kHz sine starts to clip. Ideally the clipping should occur at the output stage being limited by the final stage Power Supply DC Voltage and NOT the pre-amp or intermediate stages.
If Bass is a problem , that can be power supply, speakers or EQ filter that needs tuning for each source. or even Short wave filters not getting enough high frequency by being uncalibrated ( LO/IF) double tuned tank coils
For the final stage output to speakers Vce across the final transistors may start to get some harmonic distortion when Vce ( collector emitter voltage using two probes to see difference voltage) or single ended probes.. such that Vce>1V and < V+ when balanced. Also there should be no crossover distortion when tuned by that pot which also affects DC current drain and heat. This can be tested at various levels from lowest to high. Remember to calibrate your scope probes if 10:1 using the test waveform on scope....( assuming the designers included that)
Otherwise you cannot expect the same volume control position to be same for each source , which is your situation but you should expect the same audio power out of the speakers for each input source. So adjust the volume control to max power , regardless if it is 80% on cassete or 20% on FM and then traceback to see where the 1kHz sine starts to clip. Ideally the clipping should occur at the output stage being limited by the final stage Power Supply DC Voltage and NOT the pre-amp or intermediate stages.
If Bass is a problem , that can be power supply, speakers or EQ filter that needs tuning for each source. or even Short wave filters not getting enough high frequency by being uncalibrated ( LO/IF) double tuned tank coils
For the final stage output to speakers Vce across the final transistors may start to get some harmonic distortion when Vce ( collector emitter voltage using two probes to see difference voltage) or single ended probes.. such that Vce>1V and < V+ when balanced. Also there should be no crossover distortion when tuned by that pot which also affects DC current drain and heat. This can be tested at various levels from lowest to high. Remember to calibrate your scope probes if 10:1 using the test waveform on scope....( assuming the designers included that)
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