Saturday, 31 October 2015

Chinese Airband Radio nears completion

Two issues with the LCD frequency counter module needed resolving before I could finish the electronics on this project. First of course was the detuning affect of body proximity. The second was a desire to ensure the radios voltage regulator was not working too hard.

The second issue was the easiest fix. Rather than draw an 8v supply for the LCD module from the airband radios own 78L08 regulator, I modified the LCD module by removing the power connector, and soldering in another 78L08 onto the board, its output pin going to the +ve connection pad and direct to the counters input diode and 5v regulator (which, it seems is only spec'd to 9 or 10v anyway!), its GND pin to the -Ve pad, along with the black 0v wire, and the red +ve wire direct to the new regulators input pin, with a bit of heat-shrink sleeving. The black blob of the regulator and yellow sleeve can just about be seen in the photo below, above the 'SQL' pot connections. The whole LCD unit now gets a direct 12v supply from the fuseholder.

Issue number one, the loading of the oscillator, was a bit trickier. To help solve this, I knocked up a little emitter follower buffer amp, using a 2N3904, on a bit of spare PCB material, with pads milled out of the copper using a Dremel. This was mounted directly to the LCD module by removing the signal input connector and replacing it with PCB pins, the buffer PCB soldering to these. It can be seen above, sitting above the main PCB at a slight diagonal. The pink wire is the input, which taps the oscillator of the NE602 IC from a pad of one of the capacitors on pin 7.

Body proximity effects are now reduced to about 20kHz shift when close, which I think could only have been improved by using a metal case. This is still well within the IF passband! So even when detuned slightly, a selected station is still receivable.

The LCD frequency counter module was then secured in place with hot melt glue. The final tasks are to drill a grid of speaker holes in the lid and mount the loudspeaker, and to find a 10k switched pot to replace the volume control, so I have an on/off/volume function. In fact, it took a while to find a speaker that didnt cause too much detuning when in close proximity to the VFO!

Another task I need to get on with is a controller for the dew heater strap for my camera when taking astro photos. I got this cheap LED dimmer unit from the far east

 which claims to be a PWM circuit. I have to say I had my doubts, so opened it up. Inside, as well as a potentiometer, there is a small PCB, but no visible components!

But, on removing the PCB it does turn out there is some electronics on the other side. Ive tried to see the thing in action on the oscilloscope, but its playing up again (must be and earth thing!?) so instead I proved it does indeed dim an LED by putting one and a 4k7 resistor across its output.

Next step with this is to attach the heating strap, and monitor the temperature.

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