I have several TNC's now in my arsenal.
Mod/repair came about because this Pico had gone completely dead one day and I just threw it to the side. I was going to send it to PacComm to look at it but I feared a large repair ticket as well as, I had read many bad reveiws about PacComm loosing repaired items and not repairing items the first time. Since the TNC was WAY out of warranty and I did'nt have the extra cash to throw into a new one, I figured what do I have to loose, trying to fix it myself. I had first opened it up about a week after it had went dead and could not get any response from the unit. I right off the bat figured it was a power supply problem and began to check for voltage in the proper areas. I tried following the included schematic and checking test points that were pointed out in the manual. (There is one good thing I can say for PacComm, they make the mauals like service manuals telling you what readings you should get where.) Using a scope I was unable to get the sine and square waves at the specific points and determined that the unit was in fact "DEAD". I ended up just putting it back together and throwing back on the shelf.
About a year later I found the Pico sitting on my shelf still and figured I would break it back open and give it another shot. This time I went straight to the power section and started to probe. I found that the voltage was good all the way to the regulator. When testing for an output voltage at the regulator I had no reading 0 VDC. So I narrowed it down to the regulator or somthing before it. Sinec there were only about 5 components before the reg. I figured it would'nt be to hard to find the culprit. I was testing the unit with a 9 V battery. I removed the power socket from the board and soldered in a 9 V battery snap connector. I found a limiting resistor right before th input of the regulator was putting out about .2 VDC, bad resistor? When I shorted over this resistor, the unit powered right up. Seen above, you can see that there is a toggle switch and the 9 V battery snap connector soldered to the board. I noticed that the unit runs pretty well with the 9 V battery and I decided it was best to make this the prime source of power for the unit. I left the snap connector and soldered the lead from the toggle to the pads of the "bad" resistor. Now when I switch the toggle on, the unit powers up fine and seems to hold steady. My only real complaint of the unit is, it seems to reset it'self once in a while and "forget" everything. When the unit does this reset, the unit forgets it's call and anything else I programmed in.
Above, you can see another shot of the PCB and the metal case that the unit lives in. I just try deal with the RESET problem. As far as the toggle switch goes, I used the hole that was already on the front panel for the power socket, I just made it a little wider to accomodate. I used a nibbling tool to take a few notched off of the front panel's edge for the 9 V battery connector's wires to get out. Now I just use a rubber band and strap a 9 V battery to the TNC and go. Makes it nice and portable for use with my palmtop units for portable packet and APRS.