01-28-2017, 02:31 PM
Rod, where are you connecting your frequency counter? If you are connecting across the LO tuned circuit at the tuning cap, the loading effect will be much greater, especially at the high end of the band. You might try connecting the counter to the cathode pin 6 of the 12SA7, where the loading effect will be much less and you can get a more accurate reading.
I don't think the osc coil is the problem. It is practically impossible for the coil to increase in inductance, which would be required to make the LO run low in freq. More likely there is some additional capacitance in parallel with the tuning cap, causing it to run low
From your original description, you mentioned that the sound is garbled and you received 1500 at 700 on the dial. It might be there is a problem with the antenna loop (open?) and it is not actually tuned to the dial freq. You could be receiving the 1500 signal as an image (1500 - LO freq = 455 IF) or by mixing with the second harmonic of the LO if the antenna circuit is not actually resonant at 700 KHz.
What is the resistance and voltage reading you get from pin 8 of the 12SA7 to ground?
I don't think the osc coil is the problem. It is practically impossible for the coil to increase in inductance, which would be required to make the LO run low in freq. More likely there is some additional capacitance in parallel with the tuning cap, causing it to run low
From your original description, you mentioned that the sound is garbled and you received 1500 at 700 on the dial. It might be there is a problem with the antenna loop (open?) and it is not actually tuned to the dial freq. You could be receiving the 1500 signal as an image (1500 - LO freq = 455 IF) or by mixing with the second harmonic of the LO if the antenna circuit is not actually resonant at 700 KHz.
What is the resistance and voltage reading you get from pin 8 of the 12SA7 to ground?
