UBITX Build Part 4 – Display and Rx Audio

UBITX wiring
The photo shows the transceiver with the ‘bonnet’ open. Usually the display would be forward-facing, fitted to the front of the Raduino board. Because I wanted the display facing upwards I had to connect the display via a ribbon cable. I used a 20cm ribbon cable supplied with so-called Dupont connectors, male on one end, female on the other. Note that the connections between the Raduino and the main PCB should not be extended as the Raduino board contains the frequency synthesizer and hence has high frequency signals on it.
The audio amplifier on the V4 UBITX PCB does not have sufficient output to drive a low impedance speaker. I tried it with the 4 Ohm speaker from the donor record player and the output was feeble. I have not tried it with a high impedance speaker. I therefore made use of the amplifier that came out of the record player. The UBITX headphone/speaker output is fed to the 3.5mm headphone jack and the switched contact on the jack connects to the input of the record player amplifier. Therefore the input to the record player amplifier is disconnected when the headphones are plugged in. If you wire the headphone jack in accordance with the instructions on the HF Signals website it must only be used with stereo jack plugs! A mono plug will short circuit the output and may damage the amplifier. The amplifier board can be seen in the photo below. I fixed it down using double-sided sticky pads. 
Audio amplifier

The original record player (hence the amplifier) was designed for a 9V supply. However, looking at the circuit, I see no issue with powering it from 12V instead. The amplifier uses a TDA2822 chip. Funnily enough, the original UBITX board used this chip, but it was replaced with a discrete transistor circuit in version 4. I only used one channel of the stereo amplifier as I was only using one of the speakers. I removed the other speaker to create a ventilation grille at the front of the case.
Wiring of the microphone and Morse keys will be covered in a future post.

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