Steve, The first thing I would do is to connect the TNC-S PIC modem to the FT8100, command the TNC to activate transmit and then measure the voltage at the base and collector of the 2N2222.
The 2N2222 should be conducting, so there should be only a small voltage at the collector - the base should measure about 0.7 volts. Then with the FT8100 alone, measure the current in the circuit that you ground for transmit. The 2N2222 should be able to handle up to 800 ma in the collector, but the Hfe parameter may be limiting it. The metal can 2N2222a has an Hfe of 35 or more which requires at least 1/35th of the collector current to be driven into the base for saturation (i.e. if the collector must stay saturated with a 35 ma collector current, you must drive 1 ma available into the base) - your TNC's 10 k resistor provides a current limiting function for the base current - reducing the 10k value should increase the base current and allow the collector to handle more current without coming out of saturation. It is possible that the 2N2222 does not have adequate base current to sink whatever current your FT8100 requires to activate transmit - you could reduce that 10k resistor in the 2N2222 base circuit to allow a greater current to flow in the collector. You could swap a 2N7000 for the 2N2222, but that may also require an adjustemnt of the current base resistor and may require rearrangement of the circuit currently driving your 2N2222 - any problems with the 2N7000 operation are likely to be with the OFF state (it may be turned on all the time) - the real answer depends on the rest of the circuit in your TNC - in the ON condition, the 2N7000 will conduct easily, just put a voltage on its gate. 73, Don W3FPR > -----Original Message----- > > Hi all, > I know this is not directly related to Elecraft, but I think it is an > acceptable OT topic per the admins, and I think I remember Wayne even > talking about this subject long ago, so here goes. > > I have a radio (FT8100) that I want to use with a TNC-X PIC > modem. However, > the tnc will not put the FT8100 into transmit. The tnc uses a simple > 2n2222a switch with a 10K resistor in series with the PIC output and the > base of the 2n2222a. > > 1) If I disconnect the radio cable from the tnc and ground the PKS (ground > to transmit) wire, the FT8100 goes into transmit. > > 2) If I connect the same cable to a new Icom IC-208H, the TNC-X works fine > and keys the radio. > > John Hansen the designer of the TNC-X says that there are many hams using > the FT8100 with the TNC-X and there are no currently known problems with > using the TNC-X and FT-8100. > > I am thinking that the 2n2222a might be a little weak or have too much on > resistance. I will be checking the 2n2222a with my analyzer. > > I think I remember Wayne, N6KR, saying how great the 2n7000 was over a > bipolar transistor for use as a switch. Can anyone recommend a circuit > change using a 2n7000 or perhaps some other simple mod to the tnc > other than > using a reed relay to let the FT8100 do its thing? I relize that I should > probably go into the FT8100, but I am not looking forward to that. > > > Thanks, > Steve Pituch, W2MY > Corpus Christi > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
