That is an incredibly noble and valuable project for the vintage radio community. Documenting a truly comprehensive, modern alignment procedure for the SX-28 including those forgotten "factory set" trimmers is exactly what keeps these historic pieces of hardware alive and performing as they were engineered to, rather than just functioning as static shelf queens.
Since you are writing this as a definitive guide for **sx28.info**, let's formalize the exact methodology for aligning the **Oscillator Injection/Feedback Trimmers (C7 through C12)** so that future restorers can safely adjust them from scratch using modern test bench equipment.
---
## Section Template for your Guide: "Aligning the Factory-Set Injection Trimmers (C7ΓÇôC12)"
To optimize the Local Oscillator (LO) injection amplitude across all six bands. This ensures maximum conversion gain in the mixer stage, minimizes internal receiver noise, and prevents the oscillator from dropping out at band edges or breaking into parasitic oscillations.
1. Turn off the receiver and flip the chassis upside down to access the RF compartments.
2. Locate the 'Local Oscillator grid leak resistor', R66 (50 K Ω), inside the frontmost chassis compartment.
3. Connect the negative lead of your DMM/VTVM to the chassis ground.
4. Connect your DC voltage probe (with the $1\,\text{M}\Omega$ isolating resistor inline at
the tip) to the grid side of R66 (Pin 5 of the 6SA7 Local Oscillator tube).
> **Critical Warning Note for the Guide:** If C12 is adjusted too tight (excessive feedback), the negative voltage will spike violently toward -15 V or -20 V at the high end of the band. This indicates the tube is over-driven and generating heavy harmonics, which will cause severe front-end tracking errors. If adjusted too loose, the voltage will drop below -3 V or stall out completely at the band edges, making the radio deaf.
Once Band 1 is flat and stable, keep the meter connected to R66 and repeat the process for the remaining bands by switching to the band and adjusting its respective injection trimmer:
| Band | Frequency Range | Injection Trimmer | Target DC Range (Across R66) |
|---|---|---|---|
| Band 1 | 0.55 ‐ 1.6 MHz | C12 | -5 V to -8 V |
| Band 2 | 1.6 ‐ 3.0 MHz | C11 | -5 V to -8 V |
| Band 3 | 3.0 ‐ 5.8 MHz | C10 | -5 V to -8 V |
| Band 4 | 5.8 ‐ 11.0 MHz | C9 | -4 V to -7 V |
| Band 5 | 11.0 ‐ 21.0 MHz | C8 | -4 V to -7 V |
| Band 6 | 21.0 ‐ 43.0 MHz | C7 | -3 V to -6 V |
**Note on Bands 5 and 6: Due to natural losses at higher frequencies, the injection voltage will inherently drop. Adjust C8 and C7 primarily to guarantee that the oscillator does not "drop out" (voltage drops to 0V) at the highest frequency limits of those bands.*
---
## Why This Matters For Your Site's Readers
By publishing this, you are giving bench technicians a concrete metric (Negative DC grid bias voltage) to measure and set, rather than forcing them to guess based on an audio peak. Once they lock these six trimmers into their optimum injection windows, they can perform the standard RF coil and padder alignment knowing the mixer is receiving the perfect amount of drive.
Do you plan to include a section on decoding the actual physics of why Hallicrafters separated the oscillator and mixer into two distinct 6SA7 tubes, or are you keeping the site focused strictly on the hands-on alignment steps?