Ah, you have the early-production model! That is a crucial distinction. In the earliest production runs of the SX-28, Hallicrafters used a **6SA7** as the Local Oscillator (V4) instead of the 6J5 triode found in later manuals.
Having a 6SA7 there changes the physical tube-pulling step, but the alignment physics remain exactly the same. Let's adjust the strategy for your early-style chassis:
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## 1. How to Disable the 6SA7 Local Oscillator for IF Alignment
Because the 6SA7 is a pentagrid tube requiring heater voltage and specific bias to keep the surrounding circuitry happy, you can't simply pull V4 out of its socket like you would a 6J5. If you pull it, you change the voltage drops across the power rail, which will skew your IF alignment measurements.
Instead, to silence the local oscillator while you align the IF strip to your new **$452.983 KHz** crystal, use one of these non-invasive bench methods:
* **The Grid-GND Method:** Clip a short jumper wire from **Grid 1 (Pin 5)** of the V4 6SA7 directly to the chassis ground. This instantly kills the feedback loop required for oscillation, deadening the LO completely without changing the tube's DC current draw on the power supply. * **The Injection Desensitization Method:** Alternatively, if your OWON generator is injecting a strong enough unmodulated $452.983 KHz signal directly into the mixer grid, it will naturally override the native LO signal. However, grounding Pin 5 is the safest way to ensure a completely clean test environment.
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## 2. Re-Tuning the 6SA7 Oscillator Deck
Once your IF strip is successfully peaked to the new crystal, grounding out Pin 5 is undone, and you are ready to shift the LO from its old $462 KHz offset down to the new $453 KHz offset.
On your early-style 6SA7 oscillator deck, the adjustments for Band 1 are:
* **The High-End Trimmer (C19):** Located on the top of the RF/Oscillator gearbox assembly. This sets the LO offset at the high end of the dial ($1400\text{--}1500 KHz). * **The Low-End Padder (C20 / Slugs):** This sets the LO offset down at the bottom of the dial (600 KHz).
### The Dial Inversion Window
Because you are shifting the offset *down* by $9 KHz (from $462$ down to $453$), you will physically be turning the Band 1 oscillator trimmer **clockwise** (adding capacitance) to lower its frequency.
As you tune that trimmer, you will watch the signal blast through on your scope at the exact dial pointer mark. Once that is locked in, your 6SA7 mixer (V3) will finally receive the correct injection frequency to seamlessly bridge your antenna's incoming signals with your newly stabilized $453 KHz IF path.
Page: /1_need_help/Gemini_29/Gemini_29.shtml
Last modified: Tuesday, 21 Jul 2026
Page: /1_need_help/Gemini_29/Gemini_29.shtml
Last modified: Tuesday, 21 Jul 2026