It is where the Ground connection on your radio is used. Normally the AC filter caps have a positive sign and a negative sign. The negative sign is the same as B-, so hook up the ground side of the signal generator to this connection. BE WARNED however that you had better use an Isolation transformer. IF this radio is an AA5 radio. Or sparks may fly and blow out your equipment and radio. The reason I say this is just because the way you asked the question, it says you have no experiences in radio repairing. So be careful!
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