Yaesu FT-901DM Maintenance Service Manual page 95

SERVICING
defective
If no CW semi-break-in, check Qo; —
Q607 + Qeo9 Qer0
(5) No return to RX
from TX
+e eH
PTT line grounded
Defective Q¢o7
Defective Qso2 —Qoos Qeoe
Defective keyer unit (CW)