> I put the 2 diodes in series and fed the gate a 2n7000 > FET with a pullup resistor on the drain and the source > to ground.
The 2N7000 aka VN10KM device is an enhancement mode FET, which turns on around +3 vdc. You probably didn't need the two diodes in series. In a start from scratch circuit you might also consider adding a gate to source 15 V Zener Diode (cathode or banded end to the gate) in parallel with around a 470K resistor and a 220pf capacitor. The zener diode protects the gate from reverse & over-voltage, the resistor provides a gate to source potential bleed down path and the 220pf cap some RF bypassing. Some people (like myself) also like to put a low value resistance in series to the gate... say a 47 to 100 ohm resistor in place of one of your un-needed series diodes. You might also RF bypass the drain to ground using another 220pf cap and parallel a diode with the drain lead when controlling an external relay (for protection). A single common source FET circuit inverts the logic... low to high or high to low. If folks want/need same logic transition (change) in to out voltage swing they often follow the FET with another same (pretty much) exact device for another inverted logic circuit. Two series inverters equals same in to out logic transitions. If your repeater controller has a COR logic active high or active low choice... the second FET inverter follower is kind of silly unless you spec active low logic at each controller I/O locations. > I tied the gate of a second 2n7000 to the drain of the first, > the source to ground, and the drain through a pullup resistor > to 12v. For the life of me I could not figure out why I > wasn't getting the 1.4v drop across the two diodes. Then it > came to me, there is no current from the gate to source in a > FET. Yep, the FET can almost be considered an ideal device but you've got to pay attention to the details. An ideal gate lead path (high impedance) also makes a pretty darn good radio/RF antenna and (in the presence of radiated RF) you might find the circuit active for no obvious visual reason. > I put a resistor between the gate of the first FET and ground > (provided some current to pass through the diodes) and VOILA! > It worked like a champ! You might parallel a low value disc cap (say 220pf) and a diode with the resistor for added protection (as mentioned above). > I haven't played around much with FETs. I just got started > with them when a keyer I bought had a bad FET and I had to > troubleshoot it. Depending on what type of FET you buy... you might consider it a solid state triode tube less the heater supply and a different grid control voltage requirement. > Thanks for all of your help. The voltage at the COR input is > 0.00 volts when the correct pl is heard and the repeater > controller keys up just like it should. > 73, > Jerry Steele K8CMI Clap your hands up high and back away from the black-jack table with a smile on your face. Shift change... Congrads on getting your VXR-7000 to external repeater controller circuit working. Pretty much the same issues are involved with most all the other brands of equipment. cheers, s.
