09-18-2015, 12:34 PM
After reviewing the schematic, I don't think I would jumper as you suggest. If your objective is to test the band switch, I would instead jumper across the switch itself, at each of the points that are connected for the broadcast band. You've already inspected it thoroughly and measured contacts with your ohm meter, so I doubt if that is your problem. If you suspect an intermittent, jumpers won't hurt.
To test the dial/push button switch, your suggested jumper from the antenna side of the tuning capacitor to the switch side of the 250 uuF mica looks like it would do what you want. But since you are receiving a station on the broadcast dial setting, I doubt that switch is your problem.
You need to be sure that there aren't any shorts in your tuning capacitor. You can do this by connecting your ohm meter across the capacitor to ground, setting the meter to beep, and then very slowly turn the capacitor through its range. You need to do this for both sides of the capacitor. If you get any beeps, then inspect to see where any bent vanes are touching or if you have any solder balls or other crud that might be making contact. Carefully bend the vanes so they don't touch or clean away the crud. Blow everything out with canned air if you have it. For a while it seemed like every radio I touched had bent vanes in the tuning cap. It is painstaking and a little frustrating to fix them reliably.
I don't know what you are using for a test antenna. If you are using a random wire for test purposes, I think I'd try to connect the original loop antenna in your cabinet to your chassis to see if that makes a difference. Jumper wires should be fine, or you could take the antenna out of the cabinet. My experience is that you get weird results if you use a random wire with the series 40-1xx radios, but I never had any symptoms quite like yours. The loop antenna (as I understand it) is not just a folded up wire antenna, but its inductance is an integral part of the tuned circuits, and it needs to be there for the thing to work right.
If there are resistors that are suspect, you can measure them in-circuit and be close enough. Most should measure correctly or a little low. If your reading is way off, especially way high, consider changing the resistor. You can check for open capacitors by bridging it with a cap of similar value. There aren't many that would cause this problem, as long as the push-buttons are working.
Always check to see that components are wired properly by tracing the connections through all the terminal strips until you find a connection that you recognize for sure and can't be anything else.
I'm not sure how to test your compensator caps, but you can at least test for leakage with your ohm meter. Each cap should read a complete open, unless possibly there are components that by-pass the cap on both sides. Then you'd probably need to disconnect one side of the cap.
I think I'd try the antenna first, look for tuning cap shorts second (both sides), components third, and only then re-test the switches. It seems like you've done that already, so best move on to something new before re-doing that work.
Keep baking the coil in the back of your mind if you don't get an answer doing these tests, or if one of the experts hanging around here don't suggest anything smarter than I can.
To test the dial/push button switch, your suggested jumper from the antenna side of the tuning capacitor to the switch side of the 250 uuF mica looks like it would do what you want. But since you are receiving a station on the broadcast dial setting, I doubt that switch is your problem.
You need to be sure that there aren't any shorts in your tuning capacitor. You can do this by connecting your ohm meter across the capacitor to ground, setting the meter to beep, and then very slowly turn the capacitor through its range. You need to do this for both sides of the capacitor. If you get any beeps, then inspect to see where any bent vanes are touching or if you have any solder balls or other crud that might be making contact. Carefully bend the vanes so they don't touch or clean away the crud. Blow everything out with canned air if you have it. For a while it seemed like every radio I touched had bent vanes in the tuning cap. It is painstaking and a little frustrating to fix them reliably.
I don't know what you are using for a test antenna. If you are using a random wire for test purposes, I think I'd try to connect the original loop antenna in your cabinet to your chassis to see if that makes a difference. Jumper wires should be fine, or you could take the antenna out of the cabinet. My experience is that you get weird results if you use a random wire with the series 40-1xx radios, but I never had any symptoms quite like yours. The loop antenna (as I understand it) is not just a folded up wire antenna, but its inductance is an integral part of the tuned circuits, and it needs to be there for the thing to work right.
If there are resistors that are suspect, you can measure them in-circuit and be close enough. Most should measure correctly or a little low. If your reading is way off, especially way high, consider changing the resistor. You can check for open capacitors by bridging it with a cap of similar value. There aren't many that would cause this problem, as long as the push-buttons are working.
Always check to see that components are wired properly by tracing the connections through all the terminal strips until you find a connection that you recognize for sure and can't be anything else.
I'm not sure how to test your compensator caps, but you can at least test for leakage with your ohm meter. Each cap should read a complete open, unless possibly there are components that by-pass the cap on both sides. Then you'd probably need to disconnect one side of the cap.
I think I'd try the antenna first, look for tuning cap shorts second (both sides), components third, and only then re-test the switches. It seems like you've done that already, so best move on to something new before re-doing that work.
Keep baking the coil in the back of your mind if you don't get an answer doing these tests, or if one of the experts hanging around here don't suggest anything smarter than I can.
John Honeycutt
