kk9w@pur-ee.UUCP (David Andersen) (09/03/85)
UB 40 Well, my TNC2 arrived last wednesday, and over the long weekend, I spent some time putting it together. Copies the packet traffic over our local digipeaters and bulletin boards just fine. However, it won't key my transmitter, so I can't talk back. The transmitter (IC 290H) works fine when you short the PTT line (pin 5 of mic connector) to ground with the hand held mic, screwdriver, or whatever. However, when I hook up the tnc2 to the mic connector, it doesn't work. Q10 of the tnc2 is the switching fet that is supposed to key the transmitter. The directions said to make sure that is switching properly. The voltage in receive mode is about 13.4V (just down from the ps voltage) and in transmit mode is less than 0.25V, but definitely not zero. What actually happens is that the transmit led lights up on the radio when it is keyed by the tnc2, but no output power is forthcoming. My radio manual says to check the PTT connections when that happens. Anyway, my conjecture is that Q10 is not saturating hard enough to pull the PTT voltage down to a sufficient level. Somebody around here suggested I substitute a 2n3904 for the fet and see if that wont saturate better. I though that was logical, but lots of 2n3904 transistors are used in the tnc2, so why did they get a special one for this switch? Am I still missing something crucial? Did I screw the thing up when I built it or is the 290H just a particularly finicky radio when it comes to PTT switching (it works fine with a tnc1)? Any help you guys would have to offer (gals too, dont want to be sexist) would be much appreciated. I understand some TAPR guys are on this net. Do you have any good reason why a 2n3904 wouldnt work? Do you have any suggestions? Has TNC2 worked successfully with a 290H before? Thanks in advance for your suggestions. Dave, kk9w