Modification to FT290 mkII AF amplifier for packet use ------------------------------------------------------ The FT290R Mk II (and possibly the FT690R Mk II) take quite a time to unmute their RX audio amplifiers when the TX is un-keyed at the end of a packet. This may cause users a problem when working a station whose turn-around time (AXDELAY, TXDELAY, DWAIT etc) is set short. Acknowledgement packets (RR's etc) returned by the other station arrive before the RX audio amplifier is unmuted and are missed. This may be overcome by modifying the AF amplifier muting such that the AF amplifier is permanently unmuted (even while transmitting). This sets the TX/RX turnaround time to about 100ms (as opposed to about 450ms), which is more than adequate for packet use. Component/connector designations are taken from the circuit diagrams of the FT290R Mk II. Procedure --------- 1). Remove both lids and side brackets (the ones the shoulder strap fastens on to). The loudspeaker may be disconnected by unplugging P9 from J05 on the "main" PCB. 2). Unplug multi-way cable assemblies from PLL/PA board (P11 from J03, P1004 from J05 and P1001 from J07). Also unplug the flying lead from the PLL/PA board to the control board (J09). 3). Unplug the three coaxial connections that resemble miniature car radio aerial plugs. There are two that are in the PA compartment and mate with sockets in the "main" PCB (PJ01 & PJ03) and one on the "main" unit that mates with a socket in the PLL/PA board (PJ01). All other plugs, sockets and wires may be left connected. 4). Ease the whole front panel assembly forward away from the chassis of the radio until the connectors between the control board and the PLL/PA and "main" PCBs are disengaged. This may take some force - the procedure I adopted was to pull the front panel forward from each end in turn and use finger pressure to ease the PCB connectors apart. 5). Remove all the screws holding the PLL/PA board to the chassis (this includes the one through the tab of the PA transistor). With the radio in front of you, PLL/PA board uppermost and the front panel assembly away from you, lift the PLL/PA board up to disengage it from the pins of connector J08 of the "main" PCB and then hinge the PLL/PA board out and over to the left hand side of the radio. 6). Locate components C34 (47uF 16V electrolytic) and Q09 (uPC575C2, 8 pin DIL IC with a small heatsink sticking out of one end). On the underside of the PLL/PA board, between the positive end of C34 and Q09 pin 8 will be seen two surface-mount resistors. One of them (R48, 150 ohm) connects directly between the positive end of C34 and Q09 pin 8. The second resistor (R118, 10K) is alongside and immediately to the right of the first one and connects between the positive end of C34 and the emitter of surface-mounted transistor Q21 (located alongside and just to the right of the AF amplifier IC Q09. 7). Carefully remove the 10K resistor R118 from the board. The resistor is glued and then vapour-phase soldered in place so the best tactic is probably to remove the solder from its ends with fine desoldering braid and then to apply the bit of the soldering iron to the side of R118 and gently "scrape" it off the board. If done skilfully this will remove R118 from the board leaving the pads on the PCB intact so that a resistor may be re-fitted later if desired. 8). Re-assemble the radio, following the reverse of the dismantling procedure. Ensure that when pushing the front panel assembly home onto its mating connectors that the plastic push rod that actuates the lamp switch is correctly located on the switch. That completes the modification. The AF amplifier is now permanently enabled. Some breakthrough in the loudspeaker may be noticed, particularly on SSB transmit or if excessive level is applied to the microphone socket. I do not find the level of breakthrough I experience to be troublesome (indeed it serves as a useful indication that something is being transmitted!) 73 Richard, G4GND @ GB7HXA (Downloaded from GB7SYP)