After 4 iterations of my Wadley loop receiver, all my hard work was a disaster. So, ripped it all apart and learned from my mistakes. It now has more in common with the original Pogson Deltahet than my interpretation of it, hence the new topic.
Here is a link to the very first signals received, here on 9 megs, some weird god station, always good for strong signals during a bad time of day, but no miracles were performed despite the theological BS.
It was still a battle to get this far, and birdies remain a problem, especially at the 1 megacycle boundaries. This is not suprising as the harmonic comb generator is as yet unsheilded. Another issue was getting valve mixers to have enough conversion gain at these frequencies, I had long forgotten just how hard it can be.
Ideally I need an extra amplifying stage in the 37.5 meg IF, I found that using a hi slope pentode with grid leak injection worked fine and saved another current hungry valve, the mains transformer is working close to its limits. ... we have 15 valves!! I tried cathode injection on the Wadley loop input, here the comb feeds into the cathode, the VFO into the grid, this too works very well. Another 300mA saving in heater current was using a ECF80 , the triode portion as the VHF oscillator, the pentode section as the third mixer. The first mixer remains problematic, this one converts the RF from the preselector to the 40 meg first IF. Currently this is using a 12AT7 twin triode, but efficiency seems low. The RF amp is a 6BA6, as are the 2 455kc IFs. I stuck with the ECH81 for the tuneable IF, as I am very familiar with these. The detector is an anode bend for AM, and will see the BFO injected into its cathode for SSB/CW. There are still problems with the 40 meg bandpass filter, and am struggling to find a good coupling to prevent double humping, and signal loss.
There is no preselection yet, other than a random tuned circuit temporarily hooked up to the first mixer.
AGC seems to work fine, this uses a voltage doubler using germanium diodes, giving a negative 10 volts on s9 signsls. The voltages to the BFO, VHF oscillator and associated mixers are regulated by an OA2 neon., giving a stabilised 110 volts.
The s meter uses a bridge circuit, one side is the cathode of the third IF valve, the other side derived from a voltage doubler from the heater supply. This needs improving, it drifts all over the place as the set heats up.
Another two quick videos graphically show the workings of the Wadley loop.
The next few days I will tidy up what I have so far and try to pinpoint the weaknesses. At least with sound, it helps to track these issues.
And a trap for us young players.....
I had a pulsing effect on my secondary HT rail, this one feeds the tuneable IF section and demodulators with regulation performed by an OB2 regulator neon. These stike at around 150 volts and then shunt regulate at around 105 volts. The rail was pulsing between the above figures, cycling at around 2 cycles/second. Turns out, I had mistakenly put a 50 mFd cap across the neon to decouple that rail to ground. This turned the neon into a relaxation oscillator, it would strike at 150v, then pull the rail down where it would cease conduction, the rail then returning to 150v, then striking once again......the time constant being the series HT feed resistor, and capacitor. We rememeber something every day.....