I PRESENT HERE AGAIN THE MODS FOR AMTOR OPERATION FOR THE ICOM 720A TRANSCEIVER. ON MY ICOM 720A I DID THE FIRST SET OF MODS (BY G4ICM) AND NOW MY ICOM 720A WORKS WELL IN AMTOR AND PACKET ! I HOPE THAT THIS INFOS ARE USEFUL FOR SOMEONE! 73 DE IK3BVD OP.ENRICO ------------------------------------------------------------ MODS FOR THE ICOM 720A: MAIN BOARD: CHANGE C40 C42 C43 C54 TO 0.1 MF CHANGE C69 TO 0.01MF. I.F. BOARD: C87 MAY BE 33MF, BUT SHOWN AS 0.0047MF, WHICH IS ON PRINT SIDE. CHANGE C87 TO 2.2MF, LEAVE 0.0047 ON PRINT SIDE. R.F. BOARD: REMOVE C169 C127. CHANGE C165 TO 0.47MF. NOISE BLANKER UPDATE. (CHECK IF DONE) CHANGE C116 TO 10MF CHANGE C171 TO 0.01MF ============================================================================== DELETE C40 AND C43 ON MAIN UNIT CHANGE C42 AND C43 (10MF AND 2.2MF) TO 1MF. DISABLE A.F.MUTING- LINK SHORT-C69. DELETE CAP. CT14 AT PIN 6 OF IC3 ON I.F. UNIT. REARRANGE THE R9V TO TR Q22 ON I.F. UNIT SO THAT IT IS CONNECTED TO UNSWITCHED "9VOLTS SUPPLY. DO A SIMILAR MOD. TO THE "9 VOLT SUPPLY TO THE A.F. UNIT TO OVERCOME SURGE PULSE. ============================================================================== MAIN UNIT: DELETE C40,C43 IF UNIT: DELETE C87 AT PIN 6 OF IC 3 PICK UP AUDIO FROM ACCESSORY SOCKET (600 OHM) ============================================================================== CONNECTIONS TO ACC. JACK ON THE BACK OF THE TANSCEIVER: PIN 3 KEYLINE (NOTE THAT 200 MA CURRENT CAPABILITY IS REQUIRED SO A DIL RELAY MAY GIVE PROBLEMS) PIN 4 AF TO CONVERTOR PIN 9 FSK INPUT TTL LEVEL PIN 8 GROUND ON MAIN BOARD:REMOVE C40/42/43/54 AND REPLACE BY 0.1 UF REPLACE C69 BY 10 NF ON RF BOARD :REMOVE C169 AND 127 AND REPLACE C165 BY 0.68 UF WHEN DESIRED THE RELAY TO CONTROL AN EXTERNAL LINEAR AMP CAN BE DISCONNECTED TO REDUCE THE CLICKING NOISES ============================================================================== Subject: Modifying IC-720A to get rid of rotary relay I have just finished modifying an old IC-720A HF transceiver to replace the rotary relay filter control with a set of reed relays and a small digital circuit to select the appropriate relay and provide feedback to the existing CPU in the radio so it thinks it's still on the rotary relay. The cost for parts was about $25; the cost of my time was slightly more than that, but it was "fun". The modification is so far successful, and the radio is much quieter in band switching as well as being more reliable (the old rotary relay was impossible to get mechanically aligned correctly, leading to poor connectivity between the rx/tx and the antenna, as well as many missed stepping pulses). Summary: Use the step-pulse line to drive a small SPDT relay; use the SPDT relay to drive an R/S flip-flop (built from two nand gates) to provide a clean step signal (the existing step signal has some kind of very high speed clock train superimposed on it and therefore was not usable directly). Feed the pulse train into a CMOS decade counter type 4017. Take the appropriate decade outputs (active high) and recreate the switch-position feedback signals using a small diode matrix ala the original design. Also use the decade outputs through 33K resistors to turn on 2N3904 NPN transistors, which are used to select the appropriate (1 of 7) DPST reed relays. The CMOS chips (the nand gates and the decade counter) can be had at Radio Shack and must be powered from 9V (or so) because the radio does not use 5V logic (of course...) I mounted the reed relays inside the original low-pass filter subassembly, and the rest of the circuitry on a small perf-board near the SWR module (connected by ribbon cable). This is not a detailed design description but if you are interested I will provide more details. This project did ssalvage an otherwise-worthless IC-720A, but I don't think I'd do it again (rather time consuming, but perhaps you can benefit from my experience and save some time).