I Need Help - AI Part 12

Prompt: The procedure I used to align the oscillator at 600 KHz was to adjust the tuning slug in T25. If it was not possible to find a peak (the 462 KHz crystal is at the limit of specified tolerance and makes this difficult), I adjusted C12 which seems to vary the amount of feedback to the 6SA7 first grid rather than be part of the L-C tank. C12 is marked as 'factory set, don't adjust' in the Hallicrafters alignment procedure. This worked for band 1 but did not for some of the higher bands.

That is an incredibly deep dive into the SX-28's oscillator circuit, and your observation about **C12** is spot-on.

You are completely correct about its function: **C12 is a feedback injection capacitor** (specifically, a small gimmick or trimmer capacitor providing feedback from the oscillator anode/screen to the injector grid of the 6SA7 mixer). Adjusting it doesn't change the $LC$ resonant frequency of the tank circuit; instead, it changes the *amplitude* of the local oscillator signal being fed into the mixer.

Here is exactly why tweaking C12 gave you a false "peak" on Band 1, and why that trick completely fell apart when you moved up to the higher shortwave bands.

--- ## 1. Why Adjusting C12 Appeared to Work on Band 1

When you tuned the slug in **T25** (the Band 1 oscillator transformer) and couldn't find a true tracking peak because of the $462 KHz crystal shift, the oscillator was running highly inefficiently. It was delivering a very weak local oscillator (LO) voltage to the 6SA7 mixer grid.

When a mixer tube doesn't get enough LO voltage, its conversion gain drops off a cliff, making the receiver incredibly deaf.

By twisting **C12** (against Hallicrafters' explicit warnings!), you cranked up the feedback loop.

* This forced the 6SA7 to accept a much hotter, high-amplitude oscillator signal. * The sudden surge in LO voltage artificially boosted the mixer's efficiency, making the radio louder. * **The Illusion:** It *felt* like you found an alignment peak, but you didn't actually fix the frequency tracking mismatch. You just forced a misaligned mixer to pump out more volume by over-driving it. ## 2. Why the C12 Trick Fails on Higher Bands On the higher shortwave bands (Bands 3 through 6), the physics of the SX-28 change drastically. Stray capacitance increases, coil inductances get tiny, and the 6SA7 oscillator circuit naturally struggles to maintain stable oscillation at high frequencies. When you try to use that same technique on higher bands: * **Parasitic Oscillation / Squegging:** Cranking up the feedback on higher bands doesn't just boost the signal—it pushes the circuit into wild instability. The oscillator will either stall out completely, lock onto a unwanted harmonic frequency, or start "squegging" (motorboating). * **Severe Tracking Error:** At 600 KHz (Band 1), a $7 KHz tracking error means your front-end coils are off by about **$1\%$**. But on a higher band—say, around $20\text{ MHz}$—the mechanical geometry of the tuning condenser plates means that a $7 KHz shift at the IF forces a massive, multi-megahertz tracking error at the front end. No amount of feedback tweaking can overcome the fact that the antenna coils are looking at an entirely different slice of the sky than the oscillator. ---

The Path Forward: Resetting C12 and Solving the Real Issue

To get your sensitivity back across the whole radio, you will unfortunately need to undo the adjustment to C12 and return it to its nominal factory setting (enough to ensure stable oscillation without flooding the grid).

Because your T25 slug adjustment couldn't reach a true peak at 600 KHz to satisfy the $462 KHz IF, the radio is telling you that **the $7 KHz shift is mathematically outside the tuning range of the factory inductors.**

Since you have an accurate OWON generator, you have two real choices to fix this permanently:

Option A: Force a "Parallel Shift" Realignment (The Dial Offset)

Instead of forcing the oscillator coils to stretch to 462 KHz while keeping the front panel dial perfectly on the 600 KHz mark, shift your alignment points.

  1. If your IF is high by 7 KHz, the oscillator needs to be high by 7 KHz.
  2. Deliberately misalign the dial markings during front-end tracking. Align the oscillator so the radio picks up 607 KHz when the generator is at 600 KHz.
  3. If you map out this 7 KHz offset across the bands, the factory trimmers and slugs will suddenly fall back into their mechanical comfort zones, and your sensitivity will return—though your front panel dial pointer will always be off by a hair.
Option B: Address the Crystal

If you want the radio to be highly sensitive *and* have the dial markings read true across all bands, you have to get the IF back to 455 KHz. Since modern crystals won't work in the SX-28's low-impedance bridge, the definitive solution is to carefully slide the quartz slab out of its vintage holder, clean it with isopropyl alcohol to remove the microscopic oxidation layers causing the frequency drift, and drop it back in.

Did you notice if the 6SA7 oscillator hiss changes drastically or cuts out entirely on the higher bands when you select them?

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Last modified: Tuesday, 21 Jul 2026