06-26-2013, 12:48 PM
I think Brenda has a good point regarding possible overloading of the IF when injecting a 460KHz signal. You may be injecting 230 KHz but the IF could be actually tuned to the second harmonic and is resonating at 460 KHz.
When you looked at the resonance peak, you had the frequency counter on the output of the generator, but not on the output of the IF. Can you connect the counter to the plate circuit of the detector and see what is the frequency found there when injecting 260 KHz into the grid of the converter? Alternately you could just connect the scope across the IF transformer and measure the period one complete cycle of the IF waveform at resonance. Taking the reciprocal of the period in uS will give the actual resonant frequency in MHz.
What type of signal generator are you using and what is the level in microvolts of the injected signal? Some generators have an asymmetrical waveform rich in harmonics, which also may cause a counter to read at a submultiple of the actual frequency. When doing the alignment what are you using as an indicator to measure peak signal levels?
I think it is highly unlikely that the IF is actually tuned to anything else than 460 KHz. This is the difference frequency that the antenna and oscillator coils were designed to maintain across the broadcast band tuning range. With a 260 KHz IF (and difference freq), the tracking would be way off and the sensitivity would vary greatly across the band. The dial calibration would also be way off.
When you looked at the resonance peak, you had the frequency counter on the output of the generator, but not on the output of the IF. Can you connect the counter to the plate circuit of the detector and see what is the frequency found there when injecting 260 KHz into the grid of the converter? Alternately you could just connect the scope across the IF transformer and measure the period one complete cycle of the IF waveform at resonance. Taking the reciprocal of the period in uS will give the actual resonant frequency in MHz.
What type of signal generator are you using and what is the level in microvolts of the injected signal? Some generators have an asymmetrical waveform rich in harmonics, which also may cause a counter to read at a submultiple of the actual frequency. When doing the alignment what are you using as an indicator to measure peak signal levels?
I think it is highly unlikely that the IF is actually tuned to anything else than 460 KHz. This is the difference frequency that the antenna and oscillator coils were designed to maintain across the broadcast band tuning range. With a 260 KHz IF (and difference freq), the tracking would be way off and the sensitivity would vary greatly across the band. The dial calibration would also be way off.
