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/// The same transistors are not used for both AM and FM in some radios. It is possible to get FM but not AM on almost any type of radio. If for instance you were sitting next to a car with a highpower transmitter when it was keyed down you could have the input transistor for AM blown. In addition the wiring for some of these input circuits is small. It can be blown by a large EMF pulse such as lighting.
The ground for both AM and FM is the metal case of the radio. It doesn't matter where it is grounded. If it has power it is grounded.
Unless this is a super expensive radio it is not worth fixing if the problem is in the main radio board. You can open the case on a bench and run the input from the antenna. Clip any capacitors that are between the antenna input and ground. (failed or shorted capacitors on that antenna line would kill AM reception.) Look for burned small wiring at that input.
Except for point to point checking of the input the repairs are beyond the average mechanics training. That KC4IAI on my signature is an amature radio call sign.
I would double check everything you did. If for instance the line from the new antenna was kinked and you pulled it out rather than untwisting it the solid center can puncture the insulation and short to the braid shield. This can be checked with an ohm meter. If you failed to push the Motorola antenna plug all the way into the radio that could give these symptoms.
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