Tiny 2 Mods

This modification affects only the transmit audio circutry of the Paccom Tiny 2. it’s purpose is to bring the deviation levels of the low tone and the high tones closer together and to make the deviation level adjustment easier.

In the unmodified circuit of two Tiny 2 TNCs tested and modified so far, the audio output levels resulted in an approximate FM deviation of 1.5kHz for the low tone and 4.7kHz for the high tone, the deviation difference being 3.2kHz.

After modification, the low and high tone transmit audio output levels are such that the FM deviation levels are within 1kHz of each other. This is particularly critical when connecting to tnc’s that use the exar 2206/2211 chipset for the modem, as the demod chip of the set will NOT decode if the recovered low and high tone signal levels are too different.

The output circuitry of the Tiny 2 has two 0.1uF monolithic capacitors in series in the signal path; c27 decouples the transmit audio output of the tcm3105 modem chip from r12, the output level pot, while c8 blocks any DC coming from the radio.

The four components (c8, r11, r12 & c27) of the tx audio path form a pi section filter whose response is frequency dependent. Changing the capacitor values changes the filter’s effect on the low and high tones.

The output deviation level mod as shown on is simply to add two 1uf monolithic capacitors, one each in parallel with c8 and c27, on the solder side of the board.

Making the deviation level adjustment easier is effected by replacing the normal 270 degree preset pot with a 15 turn preset pot of the same value (10k) on the component side of the PCB. This is best achieved by clipping the leads of the preset pot r12 close to the pcb, removing the lead tails and cleaning out the PCB holes before adding the new pot.

After modification the difference between the low and the high tone levels is approx 1kHz and the output level setting is much easier.

Warren Stirling, VK3XSW 22-10-2002

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