Tuesday, 19 June 2018

Fixing the 20m QCX radio


I've had my 20m QCX kit working now since just after Christmas. While the build was quite straightforward I did have a few problems.

Click to enlarge the photograph by the way.

The first was that I managed to blow the 7805 regulator while leaving the radio running on WSPR one morning. Having replaced that I found that the rig was quieter and there was no TX output any more. Thinking I had also blown the PA transistors I replaced all three of the BS170s, but that didn't help either.

Anyway, after a lot of help on the QCX reflector I decided that the fault had also taken out the 74ACT00N chip. This had been soldered onto the board directly so had to be cut out with a Dremel and a new IC socket installed.

And after finding a short somewhere near the PA transistors all was well - success! Output using a 12.6V Lithium battery pack was about 1.8W

I have been using it on and off ever since as it has a really nice receiver - even took it to North Norfolk as I detailed in another post.

But one thing always bugged me - why only 1.8W? When I built it originally I was getting more like 3W, so perhaps it was the replacement BS170 transistors, which came off Ebay on a slow boat from China.

So having a few hours free today I ignored my own "if it ain't broke don't fix it" mantra and pulled it apart. I replaced all three BS170s with new ones bought from the GQRP club and while I was at it I replaced C22 with a 0.1uF capacitor to try and soften the sidetone a little.

I managed to break one of the wires to one of the control buttons on the front in the process, but it is now all back together.

And the power output is now just under 3W with 12.6V - success! I have no doubt I could get more if I messed around with the band pass filter, but I don't think it is worth it. I tend not to run it on 13.8V as the regulator gets VERY hot indeed.

Anyway, it is a great little radio and will be going on some more journeys no doubt.

I can thoroughly recommend it as a kit project - see QRPLabs site. My case came from Banggood in China and only cost  a few pounds. It did take about 10 hours to machine (posh word for drilling out with a Black and Decker and attacking it with a file).

The label was produced in Photoshop and printed off by Photobox.co.uk. I ruined the first one by spraying it with varnish, which made the dye run. Luckily I have a few more spares and may eventually cover one in sticky-back plastic. But for now it looks great.

The red buttons were bought just before Maplin went bust so are a lasting tribute to what was one of my favourite shops and the result of much ribbing by the family every time I went there - RIP Maplin.

No comments:

Post a Comment