KPETERSEN@SIMTEL20.ARPA (08/31/85)
From: Keith Petersen <W8SDZ@SIMTEL20.ARPA> This is relayed from a local packet machine: Posted: Sun Aug 25, 1985 6:00 AM GMT Msg: OGIF-2241-3677 From: TCLARK To: docs Subj: TAPR TNC2 Hints 'n Kinks Now that many of you have TNC2's coming, here are a few minor things you might want to watch out for: (1) I'm not sure if the artwork got fixed for the "production" run, but on the "beta" test boards, there is one trace error. On the DB25 RS-232 connector, there is a trace on the back side going to pin 16. It should be going to pin 6. An easy way to fix it is to cut the trace to pin 16 on the back side, and then run a small jumper wire from the plated-thru hole near the DIP switch to pin 6. (2) If you have trouble in the demod calibration exercise, the problem may be with noisy transitions from U8 [U8 is use ONLY in the calibration steps, not in normal operation]. I found that a 0.1 bypass between pins 3 & 4 helped a lot. (3) Unless you use all CMOS parts, the 7805 regulator runs a bit warmer than I'd like to see. In later versions of the TNC2 board, the regulator will bolt to one of the top rails in the cabinet. For the first batch of boards, I'd suggest the following modification to improve the heat conduction from the 7805 to the aluminum cabinet: a. Drill a small hole (to clear the 4-40 or 6-32 screw in step b.) in the bottom of the case to line up with the mounting screw for the 7805 regulator. this should be 1.32" from the front lip of the case and 0.55" from the side of the case. b. Find some 4-40 or 6-32 threaded (preferably aluminum) spacers and cut off a 0.30" piece. Use this threaded spacer as the mounting nut for the 7805 on the back side of the board with a short screw going thru the tab on the top side of the board. After you slide the board into the cabinet, use a second short screw to "bond" the spacer to the cabinet. (4) Also on the topic of heat control, I'd advise mounting R80 "in the air" about 0.3" above the board. On my board, the leads are at about the same height above the board as are the leads on the adjacent C12 electrolytic capacitor. (5) Since N2WX added in the time-of-day clock software, the frequency accuracy of the xtal oscillator is more critical. I'd suggest putting a small 10-50 pf trimmer capacitor in C47 in lieu of the 22 pf "blue" capacitor provided with the kit. (6) Keep the leads to the electrolytic capacitors C3, C6, C7, C8, C10, C16 and C18 as short as possible, or you may have trouble with them hitting the rails on the top of the aluminum cabinet. (7) Although headers are (probably) not provided, JMP1 will be necessary if you try to use hardware handshaking (e.g. on a W0RLI BBS). I'd also suggest installing JMP3 header -- the plated-thru hole on the trace going to the + end of C12 carries a lot of current and is a potential "fuse"; the JMP3 header will insure a positive jumper between the top and bottom of the board. (8) Be very careful to trim the tabs on the top of the push-button switch and power connector or else they can short to the rails on the top of the case. (9) I have a mod that gives a PTT indication by changing the intensity of the power LED. Is anybody interested? 73, Tom