>I don't think the diode isolation will work in this case. > > Scott K9MA
If the Icom used an amp key line from an NPN transistor with an open collector and a high value (e.g., 10K) pull-up resistor to a positive supply rail, then all one needs is a steering diode added to the Icom key line configured with the cathode to the junction of the Icom's collector and pull-up resistor. The diode's anode points to the Alpha 9500. The K3 with its MOSFET output simply parallels the 9500 key line. It doesn't care. If the Icom is unpowered or powered, a Schottky steering diode will isolate the pull-up resistor while still allowing the driving transistor's junction to pull low enough to ground. An unbiased or unpowered NPN open-collector switching transistor should not pull to ground potential if the rig is powered down. The only remaining obstacle is that when the Icom is powered down, the Alpha 9500 key line would see 10K resistance through the pull-up resistor to the power supply positive rail. Powered down, that rail may be close to ground potential. So, whether a steering diode alone will work is really a matter of: (1) the value of the pull-up resistor; and (2) whether the Alpha 9500's key line circuitry is sensitive to high-impedance switching. No problem if the Alpha uses a photo-transistor on the input key line, for example. But if say a pair of both pull-up and pull-down resistors are used ahead of a CMOS gate, then the value of the resistors need to be known. If a steering diode still doesn't work, then a separate switching transistor can be used, using an open-collector without a pull-up resistor -- or a MOSFET without a pull up on the drain. Paul, W9AC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
