Something's happening

 Yes! We have progress. 

I finally finished the garage workbench upgrade and very splendid it is too. With the nice new heavy duty vice and bench drill installed, it was time to drill large holes for the fans and finish off the mechanical engineering side of the project. This was, more or less, completed just before the new year and since then I have been testing the amplifier in its almost finished state.

I quickly discovered that the sensors are a problem. RF gets into the wiring and gives silly readings, especially on SSB where the RF output level is continuously changing. The temperature sensor is especially prone to difficulties and I spent some time trying to analyse what was going on. 

Eventually, after much faffing around, I attached the oscilloscope to the temperature sense line and found much RF on the line and, oddly, a fairly significant level shift, suggesting that the RF was getting rectified along the way. This is problematic, as the fans are supposed to be speed controlled by temperature and, of course, that really wasn't working. Worse, because the RF interference made the processor think that the temperature had gone down, the fan logic was working the wrong way round!

Well it turned out that it was my wiring. I had naively assumed that the very well filtered temperature sensor output from the amplifier module together with the high level of screening in the case would mean I could get away with unshielded wires. Nope.

Replacing the sensor connections with shielded wire made a big difference but the temperature display was still slightly unstable. So I decided to go the whole hog and instal a filter comprising a 100uH series inductance and 10nF capacitance to ground at the PCB. That fixed it!

With hindsight, I should have put the input filtering on the PCB at the outset but I was too trusting of the amplifier module filtering and simply didn't think the RF could get around my carefully designed screening quite so easily. As I've said before, I'm not very good at RF engineering, especially the fast wiggles of VHF and beyond.

So I've had a bit of a redesign of the control circuit, putting choke/capacitor filters on all input lines, as shown in the circuit fragment to the left. It won't be possible to modify the existing PCB to accommodate these changes and I also have a couple of  other issues that I commented on earlier, so I reckon I am heading for a Mk II PCB. It's so inexpensive to get PCBs made these days that it's hardly worth doing otherwise.

I'm working on the new PCB layout and will probably get it into production in the next week or so. 

Meanwhile the amplifier has been getting some use in the UKAC contests, which seem to be the only time that there is any actual activity on 2m.