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Flash TNC

Case

Flash TNC fitted to an inexpensive metal case from Dick Smith Electronics (H2806) with a custom Dynamark label to finish the case.

The artwork for the label was created in IGrafx Designer 2000 and printed to ordinary laser OHP film on a HP6MP printer. The positive was inverted using reversing film and contact printed to black imaging film which was applied to a white base.

Download the artwork in PDF format.

flashtnc.jpg (65682 bytes)

Update: It seems that Dynamark may have been discontinued by 3m. Try http://www.vpcinc.com/Products/dynamark.htm for an alternative.

Reliable initialisation / brownout reset

The initialisation circuit is unreliable, does not work if the power supply increases slowly, and does not force RESET when voltage is too low to ensure reliable operation. This causes corruption of the bb RAM, and clearing of the RAM on next startup, blowing away all the settings.

The modification involves cutting the track from U2-4 to U10-21, and inserting a Motorola MC34064 reset device with the reset pin connected to U2-5 and U10-21, and the +5 and ground wires connected to U2-14 and U2-7. This modification has proved reliable, and the unit works fine in UIMODE=ON for APRS in a mobile situation with interrupted and bumpy power.

Improved tolerance of vehicle power supply

The +5V regulator has a 12 ohm resistor in series with its input. This causes the regulator in my unit to drop out at 11.1V input to the box.

I have reduced the resistor to 8.2 ohms to bring the dropout voltage down to 9.5V. Do not make the resistor too small, it will increase the dissipation in the regulator IC.

27C512 EPROM

This mod adds an external switch, pull-up resistor and capacitor for controlling the high address bit of a 27C512 to bank switch the EPROM so that two different ROM images can be installed and selected prior to initialisation using the switch.

Modulation range switch

The modulation range switch is a two position switch controlled attenuator between the PCB and the DIN socket to provide a coarse range adjustment on the Tx Audio out of the TNC.

DIN pinout

DIN pins viewed under socket.

1- tx audio

2 - ground

3 - PTT

4 - rx audio

 

 

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