I am restoring this radio which has the electric pushbutton tuning and the pushbuttons are so messed up that I cannot figure out how it works. Clearly, there is a tuned tank circuit for each pushbutton that is tuned to a particular station, but I cannot figure out how the pushbuttons function mechanically or electrically to select each tank circuit. Is there a document which describes this? I am going to look for a patent, but I was hoping someone here might know of something else.
Of course, I would like to find replacement pushbuttons, but that is unlikely. Therefore, I will have to make my own and I can only do that if I understand how they operate the circuitry.
Thanks,
barry
If the 39-25 buttons work the same as the 40-150, then this is how it works.
Inside the chassis, there should be a mechanical switch assembly block that is actuated by the push buttons, maybe 4 or 5 inches long and an inch or so wide, pretty much in the middle of the chassis. Bolted to the back of the chassis is a second assembly that holds coils and compensators. The two assemblies are connected by fabric covered wires and braid.
For normal dial tuning, the tuning condenser has two separate sets of plates that move together in a gang. One is for tuning the oscillator frequency, and the other is for tuning the frequency of the antenna circuit. When you select the "Dial" button, the gang condenser (and associated coil) is in the circuit and you tune with the tuning knob.
If you select any of the other buttons, you cut the gang condenser and coil(s) out of the circuit and substitute a manual tuning coil with a movable slug for the oscillator and a manual compensator (adjustable mica cap) for the antenna. Each push button selects a different pair of these. For tuning the push buttons, you access the coils and compensators from the back of the chassis.
In addition to these adjustable components, there are some fixed mica condensers that are used because they are stable with temperature. As long as they are in spec, and they usually are, you don't need to worry about them.
Each push button coil and compensator has a limited tuning range. The ones on the right as you face the back of the chassis are for the lower part of the AM band. The frequency range of each push button is probably etched on the back of the chassis, but it will certainly be part of the service data.
For tuning, you push in a button (except the "Dial" button), and then find the corresponding coil and compensator. Try moving the coil slug first. Usually, you zero in on a station with the coil, and then adjust the antenna compensator for greatest volume. Then you go back and forth a few times to make sure you have both adjusted optimally.
Good luck.
Thanks Raleigh,
I see the components you describe and the problem seems to be the mechanical switches - they are frozen. I can force some of them to move, so I now know what needs to be fixed first. But how should the pushbutton switches work? When you push a new button, is the currently pushed button supposed to "un-push"? Or, do you push the current button to un-push it and then push a new button?
Barry
You push a new button, and the old one "un-pushes."
I don't have a lot of experience with this, because I've never seen a frozen one, but I'd bet if you sprayed deoxit or similar in and around the push button assembly you'd loosen some of the corrosion that is causing them to be stiff, and add some needed lubrication as well. The push button assembly does come out if you unsolder the wires and remove the two screws in the front, but I've never had to remove one.
Those pushbutton assemblies on 1939-42 Philcos always need to be cleaned and relubricated, else they will not work properly.