It is excellent news that your S-meter is working smoothly. A healthy S-meter on an SX-28 is a great indicator that your AVC bus and the plate currents of your IF amplifier tubes are all operating exactly as designed.
To answer your question directly: **The S-meter is incredibly convenient for rough alignment, but it is inferior to the Digital Multimeter (DMM) on the AVC line for the absolute final peak.**
Since you already have your DMM monitoring the AVC bus, here is the technical breakdown of how the S-meter compares and how you should (and shouldn't) use it during your Mixer and RF alignment.
### How the SX-28 S-Meter Actually Works
The S-meter is not directly measuring RF energy. It is a bridge circuit measuring the DC plate current of the AVC-controlled IF amplifier tubes.
* When a signal enters the receiver, the AVC system generates a negative DC voltage. * This negative voltage is applied to the grids of the IF tubes, reducing their gain. * As their gain drops, they draw less plate current. * The S-meter bridge detects this drop in plate current and swings the needle upward to indicate a stronger signal.
### S-Meter vs. DMM on the AVC Line
**1. Sensitivity and Resolution (The DMM Wins)** Because the AVC system is inherently logarithmic (designed to compress massive signal changes into a manageable audio output), the S-meter needle movement compresses at the top end. If you are turning trimmer C92 to find the exact peak of a resonance curve, a 50-millivolt change on the AVC bus will be glaringly obvious on your digital DMM display. On the S-meter, that same 50-millivolt change might not move the physical mechanical needle enough for your eye to perceive it.
**2. Mechanical Inertia (The DMM Wins)** Physical meter movements have mass and inertia. When you are "rocking" a trimmer capacitor or an inductor, a digital meter will instantly show you the rise and fall. The S-meter needle lags slightly behind your hand movements, which can cause you to overshoot the true mathematical peak.
**3. The Convenience Factor (The S-Meter Wins)** The S-meter is brilliant because it requires zero test leads. You don't have to worry about a slip of a probe shorting a high-voltage plate rail to the chassis.
### The Best Way to Use It on Your Bench
Since your goal is to build a masterclass set of alignment procedures for `sx28.info`, here is the optimal way to blend the two tools:
1. **The Rough-In:** If a stage is wildly out of alignment, inject a stronger signal and watch the S-meter. It is fantastic for getting you into the right neighborhood quickly without taking your eyes off the front panel. 2. **The Final Polish:** Once the S-meter peaks, lower your signal generator output (using your attenuator board to get down near that 4 ┬╡V threshold) and transition your eyes to the DMM on the AVC line. Use the DMM to dial in the absolute, microscopic peak.
Bottom line: Feel free to use the S-meter to speed up your workflow on Bands 2 through 6, but always let the DMM have the final say on where the trimmer gets locked down.
Page: /Alignment/Mix/Adjust/Gemini_1102.shtml
Last modified: Thursday, 23 Jul 2026