AMT1.5 I USED A SPST SWITCH TO MERELY SHORT THE INCOMING RECEIVE AUDIO TO THE OUTGOING TX AUDIO. YOU MAY WANT TO USE MORE CONTACTS TO FURTHER ISOLATE THE AMT FROM THE XCVR. MOUNT THE SWITCH ON THE REAR PANEL.10), 11) THE TTL/RS232 SWITCH IS MORE INVOLVED THAN THE OTHER MODS THUS FAR. AS STATED BEFORE, YOU NEED TO DECIDE WHETHER YOU WANT TO HAVE THE CAPABILITY OF HAVING BOTH AN RS232 AND TTL CABLE PLUGGED IN AT THE SAME TIME BEFORE YOU PROCEED. THE FIRST PART OF THIS WILL ASSUME THAT YOU WANT TO USE ONE COMMON JACK AND JUST SWITCH VOLTAGE LEVELS. THE SECOND PART WILL DEAL WITH A SEPARATE INPUT/OUPUT FOR TTL. A DPDT SWITCH IS NEEDED FOR THE SINGLE JACK VERSION AND A 4PDT SWITCH IS NEEDED FOR THE TWO JACK VERSION. ONE JACK VERSION... LOCATE IC21/22 ON ShEET 1 OF THE SCHEMATIC. THE TRACE FROM P6 LABELED DIN SK2/1 RTTY IN TO IC21, PIN 1 SHOULD BE CUT. THE ARMATURE OF ONE SECTION OF SWITCH SHOULD GO TO THE P6 SIDE OF THE CUT. THE N.C. CONTACT SHOULD GO TO IC21, PIN 3 AND THE N.O. CONTACT TO IC21, PIN 1. NOW LOCATE IC3 ON SHEET 3 OF THE SCHEMATIC. REMOVE THE LINK JUMPERS FROM LK1 AT IC3, PIN 11. PUT THE ARMATURE OF A SWITCH SECTION TO THE B HOLE OF LK1. THEN HOOK THE N.C. CONTACT TO THE A HOLE AND THE N.O. CONTACT TO THE C HOLE OF LK1. THAT SOUNDS SIMPLE BUT BE CAREFUL OF TRACES THAT GO TO OTHER PLACES. BE SURE WHEN YOU ISOLATE A TRACE THAT IT ONLY GOES WHERE YOU WANT IT TO. THAT IS ESPECIALLY TRUE AROUND IC22 IF I REMEMBER CORRECTLY.