Tuna Tin 2 Inside View and Modifications

The photograph to the left shows the wiring side of the PC board and the chirp modification components. I installed a five lug solder terminal strip (from Radio Shack) by soldering the center ground lug to the underside of the board. On the four insulated lugs, I mounted R8, R9 and Q3. The large black cylindrical component is a 1N5822 Shottky reverse polarity protection diode.

The PC board is secured in the chassis can by a standoff made by stacking several computer motherboard standoff's and a nut on the bottom of the tuna can.

A larger image file can be viewed by clicking on the photograph.

 

Modification Schematic

I made the following modifications to the TT2 kit:

Chirp Modification:

This is described in the TT2 Application Note (included in the kit instructions) which consists of Q3, R8 and R9. These parts were installed on the five lug terminal strip as seen in the photograph and illustrated in the modification schematic above. Note that the collector lead of the 2N3906 transistor (Q3) is soldered to the trace on the PC board which sends 12 volts DC to the Oscillator stage.

This modification also permits the use of an electronic keyer that keys to ground.

Transmitter PA Standby Modification:

This mod consists of replacing SW1, the Transmit - Receive switch with a DPDT switch. One set of contacts (SW1-A) is wired to transfer the antenna as shown in the TT2 construction book. The additional set of contacts (SW1-B) is used to remove 12 volts from the PA transistor Q2 when the switch is in the Receive position.

To perform this modification, you will need to cut the PC board trace which connects the keyed 12 volt buss to the amplifier stage P2/C8 junction. The best place to cut this trace is about 1/8 inch from P2. Solder wires to both sides of the trace cut and run to SW1-B. Configure the switch so that 12 volts is applied to P2 only when the switch is placed in the transmit position.

This modification is needed since the simple transmitter design has no SWR protection for the PA transistor. It is not wise to key the transmitter without a load connected. In the receive position, the 12 volt buss is disconnected from Q2 (the 2N2222 final amplifier stage) and pressing the key only causes the oscillator stage to turn on, which is useful for spotting purposes.

Reverse Polarity Protection

The stock TT2 has no reverse polarity protection. If you accidentally reverse your power connection, you could possibly smoke the transistors. A simple cure for reversed polarity is to install a diode in series with the 12 volt DC input. A standard silicon diode can be used, although the voltage drop through the diode will be 0.7 volts. If a Shottky diode is used, the voltage drop will only be 0.3 volts. I used a 1N5822 Shottky diode (available from radioshack.com) in my modification.

Crystal Socket

I made a socket for the crystal by cutting up a 16 pin DIP integrated circuit socket (available from Radio Shack). By holding the crystal up to the socket, you can see that alternate pins (1 and 3 for example) will accept the crystal. Simply cut up the socket with a pair of side cutters and remove the dead pin in between the two desired pins. Solder the two pins into the PC board at the crystal position and trim the crystal leads to about 3/8 inch. There is no photograph of this modification.

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