Excellent work getting the IF and oscillator locked in. With the AVC verified, the heart of the receiver is officially beating again.
To answer your first question: **The Mixer comes first.**
In any superheterodyne alignment, you always work backward against the signal flow from the speaker toward the antenna. Since you just finished the Oscillator, the next stage upstream is the Mixer, followed by the 2nd RF, and finally the 1st RF (Antenna) stage.
### The Source of the "Buzz"
What you are hearing is the modern curse of the AM band: Radio Frequency Interference (RFI) from switch-mode power supplies, LED lights, and routers.
Right now, because your Mixer and RF stages are untuned, the receiver's front-end selectivity is wide open. It is acting like a broadband garbage collector, passing every harmonic and hash spike straight into your beautifully tuned IF strip. As you align the Mixer and RF stages, you will sharply narrow the bandpass (increasing the Q of the tuned circuits). As those circuits peak on the actual signal, the broad, out-of-band buzz will drop away significantly.
Here is the definitive procedure for bringing the front end of Band 1 into alignment. Since your AVC is working, monitoring the DC voltage on the AVC bus with your DMM is the most accurate way to find the absolute peaks.
1. **Set the High-End Alignment Point:** Target: 1400 kHz. Set your RF signal generator to output a steady, unmodulated carrier at exactly **1400 kHz**. Connect the generator output to the $A_1$ antenna terminal (with $A_2$ grounded to the chassis, or using your custom matching box). Tune the SX-28 main dial exactly to the 1400 kHz mark on Band 1.
2. **Peak the Mixer Trimmer:** Capacitance adjustment. Locate the Band 1 Mixer trimmer capacitor. Slowly adjust it while watching your DMM on the AVC line. Tune for the maximum negative DC voltage. *Note: If the signal gets so strong that the AVC voltage climbs massively (e.g., past -10V or -15V), turn down the output level on your signal generator to keep the receiver sensitive to small peaks.*
3. **Peak the 2nd RF and 1st RF Trimmers:** Working toward the antenna. Move upstream to the Band 1 2nd RF trimmer capacitor and peak it for maximum negative AVC voltage. Finally, do the exact same thing for the Band 1 1st RF (Antenna) trimmer. Your high-end tracking is now set.
4. **Set the Low-End Alignment Point:** Target: 600 kHz. Change your signal generator to **600 kHz**. Tune the SX-28 main dial down to exactly 600 kHz.
5. **Peak the Inductors:** Slug adjustment. At the low end of the band, the trimmer capacitors have very little effect; the tracking is controlled by the coil inductance. Working in the exact same orderΓÇöMixer, 2nd RF, then 1st RFΓÇöadjust the **iron dust slugs** (the adjustable cores inside the coils) for maximum negative AVC voltage.
6. **Iterate and Lock:** The rocking process. Because adjusting the inductance (slugs) slightly affects the high-end capacitance (trimmers), and vice-versa, you must repeat the process. Go back to 1400 kHz and touch up the trimmers. Then return to 600 kHz and touch up the slugs. Repeat this 1400/600 cycle until adjusting one end no longer detunes the other.
Once you finish that rocking process, Band 1 will be as hot and selective as the day it left Chicago.
Page: /Alignment/Mix/Adjust/Gemini_1101.shtml
Last modified: Thursday, 23 Jul 2026