If the RT does not key, the RT is bad.
If the RT keys at the SPKR jack, check the CX-13064/U ground
Measure continuity at the J-3513/U J5 pin A to chassis ground on
If zero ohms is measured, the ground circuit is good.
If infinity is measured, remove
RETRANS jack and the J3513/U J6.
Measure continuity of the CX-13064/U P2 pin A to P1 pin J.
If infinity is measured, the cable is bad.
If zero ohms is measured, the cable is good and the J-3513/U is
A-RT keying circuit, no RT keying.