Measure continuity between pins C and A on J5 on the J-3513/U
If the meter indicates zero ohms, the lower portion of the system
keying circuit contains the short; that is, CX-4723, circuits
below K-1 in the J-3513/U, the KY-57, CX-13061/U, and CX-13062/U.
If the meter indicates infinite ohms, the short is in the upper
portion (A-RT remote keying circuit); that is, the circuits above
the K-1 in J-3513/U, RT, and MT-1029.
RT keying, continuous keying (test points 1 and 2) (continued).