09-27-2008, 10:21 AM
I don't know anything about your specific signal generator, but if it is an old-fashioned analog generator tuned by a large multi-scale dial, it should be similar to lots of other models. Elements of Radio Servicing has a couple of chapters on signal generator operation and set up. Chapters 5 and 6.
To peak the IF compensators by ear, I would use a strong station to get it close and then a weak station to finish it. Remember that this operation gets all of the IF cans tuned to the same frequency, but not necessarily to the correct frequency. If you can't get fair sensitivity on the broadcast band by this method, either you're way off the 455 KHz frequency or something else is wrong.
To use the signal generator to peak the IF compensators, first short the oscillator section of the ganged tuning capacitor as described on page 422 of Elements.
On your signal generator, find the IF frequency, 455 kilocycles (Kc or KHz), on one of the dial scales. Set the pointer on that frequency. There should be a band selector switch somewhere. Set that to the band with the IF frequency on it. There should be another switch with settings for RF, Modulated RF, External, and maybe Audio. Set it to Modulated RF. A potentiometer somewhere sets the level of the RF. Turn it to about 3/4 of the way up. Another potentiometer sets the audio modulation level. Turn it all the way up.
Connect the black lead (ground) from the signal generator to the radio chassis. Clip the red lead to a .1 uf capacitor, and attach the other capacitor lead to the broadcast antenna terminal (terminal 2, I think.) Do not connect the generator lead directly to the antenna without using a capacitor. Elements suggests connecting the capacitor directly to the mixer grid, but I use the antenna terminal with good results.
If you have a high impedance analog meter, connect the red lead to terminal 1 and black lead to the radio chassis.
Turn the radio on. Turn the signal generator on. Set the radio band switch to broadcast. You should hear a sustained beep in the speaker. If not, turn the RF controls on the generator all the way up. If you still don't hear anything, turn everything off and connect your .01 capacitor to pin 6 of the XXL mixer tube. Not sure which XXL that is in your radio, but I'll make a guess and say the one nearest the IF can #29. Underneath you should see a lead from the mixer tube plate going into that can.
If you get the audio signal (beep) then adjust the 34A, 31A, 29A, and 29B compensators in that order until you get the strongest audio (or the strongest reading on the meter.) Follow the instructions in Elements to lower the modulation level of the signal generator and repeat the adjustments until you can use the lowest modulation.
If you still don't get good sensitivity, there is something else wrong that IF alignment won't help you with. Try adjusting compensators 4 through 4E following the instructions in the John Rider page from Nostalgia Air. Make sure your band switch is clean. It can have a MAJOR effect on sensitivity.
If you don't get the beep, there might be something wrong with your signal generator.
The SW band these days does have a lot of background noise and distorted stations, but there are a few strong stations that you should be able to pick up clearly, especially at night. Radio Beijing is very clear most nights, near 10MHz if I remember right. Use the built-in loop antenna or an external long-wire for best sensitivity on this band.
To peak the IF compensators by ear, I would use a strong station to get it close and then a weak station to finish it. Remember that this operation gets all of the IF cans tuned to the same frequency, but not necessarily to the correct frequency. If you can't get fair sensitivity on the broadcast band by this method, either you're way off the 455 KHz frequency or something else is wrong.
To use the signal generator to peak the IF compensators, first short the oscillator section of the ganged tuning capacitor as described on page 422 of Elements.
On your signal generator, find the IF frequency, 455 kilocycles (Kc or KHz), on one of the dial scales. Set the pointer on that frequency. There should be a band selector switch somewhere. Set that to the band with the IF frequency on it. There should be another switch with settings for RF, Modulated RF, External, and maybe Audio. Set it to Modulated RF. A potentiometer somewhere sets the level of the RF. Turn it to about 3/4 of the way up. Another potentiometer sets the audio modulation level. Turn it all the way up.
Connect the black lead (ground) from the signal generator to the radio chassis. Clip the red lead to a .1 uf capacitor, and attach the other capacitor lead to the broadcast antenna terminal (terminal 2, I think.) Do not connect the generator lead directly to the antenna without using a capacitor. Elements suggests connecting the capacitor directly to the mixer grid, but I use the antenna terminal with good results.
If you have a high impedance analog meter, connect the red lead to terminal 1 and black lead to the radio chassis.
Turn the radio on. Turn the signal generator on. Set the radio band switch to broadcast. You should hear a sustained beep in the speaker. If not, turn the RF controls on the generator all the way up. If you still don't hear anything, turn everything off and connect your .01 capacitor to pin 6 of the XXL mixer tube. Not sure which XXL that is in your radio, but I'll make a guess and say the one nearest the IF can #29. Underneath you should see a lead from the mixer tube plate going into that can.
If you get the audio signal (beep) then adjust the 34A, 31A, 29A, and 29B compensators in that order until you get the strongest audio (or the strongest reading on the meter.) Follow the instructions in Elements to lower the modulation level of the signal generator and repeat the adjustments until you can use the lowest modulation.
If you still don't get good sensitivity, there is something else wrong that IF alignment won't help you with. Try adjusting compensators 4 through 4E following the instructions in the John Rider page from Nostalgia Air. Make sure your band switch is clean. It can have a MAJOR effect on sensitivity.
If you don't get the beep, there might be something wrong with your signal generator.
The SW band these days does have a lot of background noise and distorted stations, but there are a few strong stations that you should be able to pick up clearly, especially at night. Radio Beijing is very clear most nights, near 10MHz if I remember right. Use the built-in loop antenna or an external long-wire for best sensitivity on this band.
John Honeycutt
