03-04-2013, 12:39 AM
Try setting the push buttons. If they tune in high frequency stations or you can get them to tune with your signal generator, that'll tell you if the local oscillator and mixer are working at all frequencies. Ditto if you can tune in SW.
Anything you try will eliminate one thing or another as the cause. If SW works, then something involved in AM only is the cause. If the push buttons work, it's something involved only in the dial tuning. If neither of those things work, then zero in on the oscillator or mixer. It shouldn't be anything but the oscillator or mixer if you are actually getting good reception from the lower part of the band and if you can align all the IF cans with your signal generator.
Check or re-check resistors in the oscillator circuit.
Are you testing with the loop antenna or a long piece of wire connected to the antenna terminals? I use a long wire for test, but these radios will not align properly without the loop. Place the cabinet and the chassis back to back and connect the loop antenna to the correct terminals. This way you have access to all the compensators you might not be able to reach with the chassis installed in the cabinet. Try to realign with the loop connected. (You SHOULD be able to get all AM frequencies with a wire, but when you connect the loop the dial alignment will change. It is just possible, though, that a wire might make it difficult to bring in all frequencies on the band.)
Mica caps aren't often bad, but they are sometimes. They are nearly impossible to test in the radio because meter leads have enough capacitance that it can be hard to get reliable readings from low-capacitance micas. In some of these 40s radios there are large, thin, rectangular wax covered mica caps in the oscillator circuits that could be suspect. The 42-355 has at least two of them, but I don't remember if they are in the AM tuning circuits. If they are, you might try to replace them with modern silver micas. Any of the micas between #9 and #18, say, might be suspect. The "domino" packaged micas could also be bad. They usually aren't, but I've seen enough that are.
If I have to remove a mica to test it, I just replace it. It is just easier that way, and more reliable.
Old triode oscillator tubes are funny. Sometimes one will work as a mixer but not oscillate properly. If neither push buttons or SW work, you might try to switch your two XXLs to see if it makes a difference. If you have NOS spares, plug them to test. (If no difference with NOS tubes, I'd switch back to the originals to keep the NOS tubes from being "used.")
If you have a bad trimmer (compensator) it might limit what frequency range you can align to. If your push buttons work, consider checking those out. Blow them out with canned air and checked for cracked or missing mica. Do any of them act funny, like make no difference until you crank them down hard or something like that? Once again, if your push buttons and SW work you can zero in on the trimmers or whatever other components involve only AM. (#6, #6E, #6F.)
One final suggestion is that the tuning condensers sometimes short between plates in some parts of their travel but not others. I don't think a shorted tuning condenser matches your symptoms, but it's one thing to try if all else fails. By the way, a bad tuning condenser would show up in SW as well as AM.
Summary: 1) Test with the actual loop antenna & realign. 2) Insert a modulated high frequency (1300 - 1500 KHz or so) into the antenna terminals (or, better, use a coil of wire held close to the loop) to see if you get an audio signal out. 3) Make sure the the push buttons work at your problem high frequencies and that the SW works. If they don't, something in the the oscillator or mixer circuit is suspect (probably the oscillator.) If they do work, then a coil, cap, compensator, or resistor that affects AM dial tuning only is probably bad.
Let me know your results & maybe they will spark new ideas.
Anything you try will eliminate one thing or another as the cause. If SW works, then something involved in AM only is the cause. If the push buttons work, it's something involved only in the dial tuning. If neither of those things work, then zero in on the oscillator or mixer. It shouldn't be anything but the oscillator or mixer if you are actually getting good reception from the lower part of the band and if you can align all the IF cans with your signal generator.
Check or re-check resistors in the oscillator circuit.
Are you testing with the loop antenna or a long piece of wire connected to the antenna terminals? I use a long wire for test, but these radios will not align properly without the loop. Place the cabinet and the chassis back to back and connect the loop antenna to the correct terminals. This way you have access to all the compensators you might not be able to reach with the chassis installed in the cabinet. Try to realign with the loop connected. (You SHOULD be able to get all AM frequencies with a wire, but when you connect the loop the dial alignment will change. It is just possible, though, that a wire might make it difficult to bring in all frequencies on the band.)
Mica caps aren't often bad, but they are sometimes. They are nearly impossible to test in the radio because meter leads have enough capacitance that it can be hard to get reliable readings from low-capacitance micas. In some of these 40s radios there are large, thin, rectangular wax covered mica caps in the oscillator circuits that could be suspect. The 42-355 has at least two of them, but I don't remember if they are in the AM tuning circuits. If they are, you might try to replace them with modern silver micas. Any of the micas between #9 and #18, say, might be suspect. The "domino" packaged micas could also be bad. They usually aren't, but I've seen enough that are.
If I have to remove a mica to test it, I just replace it. It is just easier that way, and more reliable.
Old triode oscillator tubes are funny. Sometimes one will work as a mixer but not oscillate properly. If neither push buttons or SW work, you might try to switch your two XXLs to see if it makes a difference. If you have NOS spares, plug them to test. (If no difference with NOS tubes, I'd switch back to the originals to keep the NOS tubes from being "used.")
If you have a bad trimmer (compensator) it might limit what frequency range you can align to. If your push buttons work, consider checking those out. Blow them out with canned air and checked for cracked or missing mica. Do any of them act funny, like make no difference until you crank them down hard or something like that? Once again, if your push buttons and SW work you can zero in on the trimmers or whatever other components involve only AM. (#6, #6E, #6F.)
One final suggestion is that the tuning condensers sometimes short between plates in some parts of their travel but not others. I don't think a shorted tuning condenser matches your symptoms, but it's one thing to try if all else fails. By the way, a bad tuning condenser would show up in SW as well as AM.
Summary: 1) Test with the actual loop antenna & realign. 2) Insert a modulated high frequency (1300 - 1500 KHz or so) into the antenna terminals (or, better, use a coil of wire held close to the loop) to see if you get an audio signal out. 3) Make sure the the push buttons work at your problem high frequencies and that the SW works. If they don't, something in the the oscillator or mixer circuit is suspect (probably the oscillator.) If they do work, then a coil, cap, compensator, or resistor that affects AM dial tuning only is probably bad.
Let me know your results & maybe they will spark new ideas.
John Honeycutt
