Hello all AM,ers. I did some experimenting on the AGC circuits, along with the low distortion High Fidelity AM detector last night.
80 meters has been a real mess around my house lately, I have a regular intense impulse noise source, a street lamp that has crapped out no doubt, and a lack of strong signals to compare things against. I was copying some weak southern signals, and on comparison to the 1st home brew receiver, the second seems to work as well, but may need a bit more gain. I did not check any of the tubes before plugging them in, that may be the next step. I lowered the IF amp cathode resistors to 100 ohms, and got more gain without problems, and may change the 1st stage screen resistor to a lower value. The B+ is on the low side, running around 150 volts, and I may boost that a little. I did some experiments with AGC sources, and injecting the agc voltage into 1, 2, and 3 stages, both IF amp grids, and the mixer antenna grid. Without a strong Hi Fi signal, its hard to tell what is best. The short wave broadcast stations on 40 have loads of fading and distortion on them, and cant be used to tweak things. I may add some choices in AGC time constants, to the power switch (off, on agc fast, on agc slow). So far, so good, no serious problems, need to build the BFO, manual gain control, S meter and muting circuits after the AGC setup is finalized. I did some checks on startup drift, and got GREAT results! Once the LO starts, it drifts about 3Kc in about 10 seconds, then seems totally stable, staying on 3880.0 for an hour. Much better than the first home brew receiver, which drifts for 5 minutes on startup. The stability comes from B+W coil stock LO coil, and the smaller 6C4 LO tube I guess. Brett N2DTS
