04-02-2021, 09:07 PM
Since others are explaining their methods for doing a rough tune in of the IF transformers on a radio where they have been tweaked badly out of alignment, here is a method I have found to work on radios which use a 6K7 IF amp and a 6A8 mixer oscillator. It can be modified to use on most any radio.
Start by turning the tuning condenser until the plates are meshed. Next remove the 6A8 tube from the radio. Turn on the signal generator. Set it for INTERNAL modulation, so the AM frequency has a tone modulating it. Let it sit for at least 20 minutes to stabilize well. Using a good, accurate frequency counter make sure of the accuracy of your signal generator's setting to the appropriate IF frequency. You can alternately use a good digital readout radio to check this using the first harmonic, 910KC for 455KC IF, 920KC for 460KC IF, 940KC for 470KC, etc. Tune the signal generator until you hear the loudest signal coming through the radio. Now, with the signal generator set for the highest output, connect the output THROUGH A 0.01 MFD CAP to the grid of the 6K7 tube, which will inject the modulated IF frequency into the IF amplifier. Now you can tune the 2nd IF transformer until you hear the loudest signal pass through.
Next put back the 6A8 and connect the same output from the signal generator through the .01 MFD cap to the grid of the 6A8 tube, and repeat the same procedure on the 1st IF transformer.
This will get you into the close ball park, and from there you can follow whatever the instructions are given in the radio's documentation for final IF alignment.
Start by turning the tuning condenser until the plates are meshed. Next remove the 6A8 tube from the radio. Turn on the signal generator. Set it for INTERNAL modulation, so the AM frequency has a tone modulating it. Let it sit for at least 20 minutes to stabilize well. Using a good, accurate frequency counter make sure of the accuracy of your signal generator's setting to the appropriate IF frequency. You can alternately use a good digital readout radio to check this using the first harmonic, 910KC for 455KC IF, 920KC for 460KC IF, 940KC for 470KC, etc. Tune the signal generator until you hear the loudest signal coming through the radio. Now, with the signal generator set for the highest output, connect the output THROUGH A 0.01 MFD CAP to the grid of the 6K7 tube, which will inject the modulated IF frequency into the IF amplifier. Now you can tune the 2nd IF transformer until you hear the loudest signal pass through.
Next put back the 6A8 and connect the same output from the signal generator through the .01 MFD cap to the grid of the 6A8 tube, and repeat the same procedure on the 1st IF transformer.
This will get you into the close ball park, and from there you can follow whatever the instructions are given in the radio's documentation for final IF alignment.
