Saturday, 24 October 2015

Boxing the Chinese Airband Receiver

Today was the annual G-QRP club Rishworth convention. Although I didnt have a particularly long list of things I wanted, I did have on the list a box of some sort to build the Chinese Airband receiver into. This, as it turned out, was one of the few items on my list I actually managed to get!

I got a fairly generic, basic black ABS box from Bowood's stall. Another stall furnished me with a little ABS box ready to build a regulator unit that Bob has asked me to make for him. I thought it felt a bit heavier than expected, but it was taped up so thought no more of it. Once home I found that as well as the screws for the lid, it also had a number of new connectors inside! Clearly someone had started acquiring bits for a project that never happened. I also found a small, ancient card box labelled as containing a 3.6MHz crystal. On opening, inside was a wad of tissue paper, within which I found a big square quartz crystal slab! But, it was marked as being 900kHz. I bought it anyway for 50p - a nice teaching aid I thought. On looking closer once home, I found two smaller bits of tissue underneath, each containing another crystal blank!

I also grabbed a pair of channel crystals on S16 (2m - 145.400MHz) marked as for Burndept. Ive no idea which Burndept radio, but couldnt resist for another 50p!

Once home, after a detour for myself and Sam to devour copious quantities of fried chicken, I set to on boxing up the airband receiver. This took some thinking about. Sam decided on the front panel layout, and I set to marking out and drilling

The big problem when doing these sorts of projects is the need for the square cutout for the display. This inevitably comes down to drilling lots of little holes, punching out the excess, and then spending a long time filing and checking, until the desired hole is created.

In the photo below, the front has been mocked-up to check everything looks right. The big knob on the right is the tuning control, on the left, the top knob is the volume, and the bottom one squelch.

Despite the space in the box, getting both the receiver board and the frequency counter module fitted took some doing. For a start, all the sockets and controls had to come off the board, and be replaced with panel mount parts. In most cases, the PCB mount component could be reused. Ive added a fuseholder as well, since its likely this will be powered off a 12v SLAB. In order to get the main PCB right to the back, I had to cut the base of two of the PCB mounts in half.

The frequency counter module was then taken out again to be modified (it needed some hex studs removing and sockets replacing with direct wiring) and the wiring added for the front panel controls.

With the various controls wired, and the wiring looms twisted in bundles, I clipped on a temporary 12v supply and powered the receiver up. The first problem to sort was that the volume pot AND the squelch pot were working backwards! I'd got them wired up wrong, and had to swap the wires over. With that done, I found I couldnt tune in a test signal on 125MHz. I guessed that the tuning pot wiper was wired wrong. Checking with my meter, this proved to be the case. That corrected, I could find the test signal, but on increasing it by ten MHz to 135, found I had to tune backwards to find it! I changed the wiring yet again, and now have all the controls working properly. Even finding a few planes whilst I was at it!

Final act for tonight was to add a fuse. I found a 2A fastblow 20mm, which will do for now. I'd rather it was smaller, maybe 500mA, but its all I have at present. Next task is to add a buffer amp for the counter, and wire that in. I will probably give it its own 5v regulator as well, so as not to draw too much on the receivers regulator.

No comments: