Saturday, 10 May 2008
The 160m transmitter is now delivering 800mW of nonsense to the dummy load, and this diminishes when I modulate. I added another gain stage between the second mixer and the pre-driver, with the idea of increasing the output at the driver's collector to around one watt. However, the thing is taking off, and is not responding to extra decoupling, so I will have to remove the extra stage and just add the PA. I will see if the output is usable (I'll accept anything from two watts and up), and clean.
I've always had 'trouble' with regenerative TRF radios. Some work, but poorly; others scream their heads off at the slightest sniff of a carrier, others flatly refuse to do anything. I try all the traditional remedies (including the usual beginner's error of an upside-down regen coil), but they simply suck.
So, I'm having another go. This time, a 'Heard All Continents' Twin-Transistor Receiver. It's a mature design, which everyone who has experience of has only positive things to say. I'm sure I can break the canon. I've started the job by using stripboard instead of tagstrip; I have a lump of tagstrip somewhere, but modern components lend themselves to the smaller pitch of the 0.1" stripboard. I've made track cuts to simulate the layout of the tagstrip, and made the board no wide than necessary to avoid long lengths of strip giving stray capacitance. The strip board is today's image, at top-left. I've just wound the first coil, a 50-turn monster for the high-MF and low-HF bands (around 1.6 - 5MHz).
The coils in the original were Denco Green Range, but I've not seen these since my childhood. Nowadays, you have to make your own. Go to http://www.arar93.dsl.pipex.com/mds975/content/trfradios02.html for the details. You will find the circuit for the H.A.C. (and others) on the same page.
I've got a lovely old epicyclic-geared knob to go on the tuning condenser (this old tinned brass device can't be called a capacitor), made of Bakelite. It has a slipper-ring attached to an outer knob, so you can defeat the gearing for fast tuning. I also have the main tuning gear from an R1132A RN VHF set, but that would be wasted on such a radio as this.
I want to try a frame aerial with this radio, or perhaps an 'active' antenna. Let's get the thing running, first!