That is an incredible plot twist. Finding out that the old crystal *also* peaks right around $453 KHz when isolated in the test jig completely changes the diagnosis.
The fact that it measured **$462 KHz in-circuit** but drops down to **$453.621 KHz out-of-circuit** means your original crystal isn't actually bad or cut to the wrong frequency. Instead, it is being violently **"pulled" (frequency-shifted) by the heavy stray capacitance and mistuning of the radio's internal crystal filter bridge.**
Here is the exact engineering physics behind why a crystal will shift its resonant frequency by a massive $9 KHz when dropped into a misaligned or modified SX-28 circuit.
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## 1. The Physics of Crystal "Pulling"
A quartz crystal does not have just one single resonant frequency; it has two that sit incredibly close together:
1. **Series Resonance ($f_s$):** The true physical mechanical resonance of the quartz slab where its impedance drops to a minimum. This is the $453.621 KHz you just measured on the bench. 2. **Parallel Resonance ($f_p$):** A slightly higher frequency where the crystal's internal electrical capacitance cancels out its mechanical inductance, causing its impedance to skyrocket to a maximum.
When a crystal is placed into a real-world circuit like the SX-28's phasing bridge, **external stray capacitance acts in parallel with the crystal.** If the external circuit introduces excessive capacitance (due to a leaky phasing capacitor, a misaligned crystal input transformer, or stray wiring capacitance), it forces the crystal to operate away from its low-impedance *series* peak and pushes it up toward its high-impedance *parallel* anti-resonance point. You weren't measuring a $462 KHz crystal; you were measuring a $453 KHz crystal being forcefully skewed up to $462 KHz by the radio itself.
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## 2. The Smoking Gun: The Phasing Condenser and T1
On the SX-28, the crystal sits inside a balanced bridge network balanced by the **Phasing Control (C40)** and the center-tapped secondary of the **1st IF Transformer (T1)**.
``` ΓöîΓöÇΓöÇΓöÇ[ Crystal (453 kHz) ]ΓöÇΓöÇΓöÇΓöÉ From T1 Secondary ΓöÇΓöÇΓöñ Γö£ΓöÇΓöÇΓöÇ To Next IF Grid ΓööΓöÇΓöÇΓöÇ[ Phasing Cap (C40) ]ΓöÇΓöÇΓöÇΓöÿ
```
The purpose of the Phasing Control is to inject an out-of-phase signal to cancel out the static capacitance of the crystal holder, allowing you to "notch" out interfering heterodynes.
* If the Phasing Capacitor (C40) is physically broken, leaky, or completely misadjusted, it fails to balance the bridge. * If the secondary of T1 is severely misaligned, the voltage fed to the crystal is unbalanced. * This unbalance loads the crystal with massive external capacitance, which "pulls" its apparent resonant peak drastically up the dial to $462 KHz.
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## 3. What This Means for Your Restoration
This is actually the best possible news for your restoration and for **sx28.info** because it proves you have **two perfectly good $453 KHz crystals.** * Crystal 1 (New): $452.983 KHz
* Crystal 2 (Old): $453.621 KHz
They are only $638\text{ Hz}$ apart from each other! This means Hallicrafters absolutely intended for your specific radio to run on an intermediate frequency of **$453 KHz**, not 455 KHz.
### The Diagnostic Verdict
The reason your radio was performing so poorly ("not good, but better") isn't because the crystal was cut to $462 KHz. It's because **the 1st IF transformer (T1) or the phasing bridge circuitry is so badly misaligned or electrically leaky that it was actively distorting the crystal's behavior.** When you drop either crystal back into the radio, if you don't thoroughly clean/rebuild that phasing bridge and properly align T1 to match the $453 KHz center point, the circuit will just pull the new crystal up to $462 KHz as well.
Since both crystals are excellent, which one are you going to use as your baseline to begin tracking down the stray capacitance in the radio's phasing bridge?
Page: /1_need_help/Gemini_32/Gemini_32.shtml
Last modified: Tuesday, 21 Jul 2026