09-15-2017, 08:22 PM
22 ohms would not be unreasonable for a 455 kHz IF transformer coil, but a 9.1 MHz coil should read much lower than that.
Don't be afraid of trying the alignment, as the FM IF will almost certainly need adjustment. Sometimes it is the best way to determine if a problem exists in any of the tuned circuits of an IF stage. If you can't get a definite peak on any particular adjustment, then a problem exists with that related circuit.
I have a 48-482 and found the FM IF to be way out of alignment even though the trimmer adjustments were seemingly previously untouched. The mica compression trimmers have a tendency to drift in capacitance, especially after 70 years.
As to enough signal at 9.1 MHz, 5 mV should be more than enough if injected into the 7F8 grid at test point A. Try the full 5 mV first and see if you get any negative AVC voltage at the point I described previously. Use the minimum signal level into test point A that results in an increase in AVC voltage over the reading with no signal applied. Decrease the signal generator output as each tuned circuit comes into alignment.
Don't be afraid of trying the alignment, as the FM IF will almost certainly need adjustment. Sometimes it is the best way to determine if a problem exists in any of the tuned circuits of an IF stage. If you can't get a definite peak on any particular adjustment, then a problem exists with that related circuit.
I have a 48-482 and found the FM IF to be way out of alignment even though the trimmer adjustments were seemingly previously untouched. The mica compression trimmers have a tendency to drift in capacitance, especially after 70 years.
As to enough signal at 9.1 MHz, 5 mV should be more than enough if injected into the 7F8 grid at test point A. Try the full 5 mV first and see if you get any negative AVC voltage at the point I described previously. Use the minimum signal level into test point A that results in an increase in AVC voltage over the reading with no signal applied. Decrease the signal generator output as each tuned circuit comes into alignment.
