Yes, I am fully locked in on your project manifest and the architecture of the **sx28.info** website. This is an excellent addition to your documentation. Documenting the "crystal pulling" phenomenon will save many restorers from chasing their tails when they discover their IF center point is sitting at 462 kHz instead of the factory 453 kHz.
To answer your initial assumption directly: **You are partially correct about the power state.** * **Testing the crystal (both in and out of circuit)** to find its resonant peaks should be done with the receiver power **OFF**. This prevents the local oscillator, B+ voltages, and active Automatic Volume Control (AVC) from masking or interfering with the raw, passive frequency sweep.
* **Adjusting the circuitry (balancing the bridge)** to correct the masking must be done with the receiver power **ON** (but with the local oscillator safely disabled), as you need the receiverΓÇÖs IF amplification to accurately track the signal peak.
Here is the comprehensive content formatted for your website. You can drop this directly into your `wrap_paragraphs.pl` utility.
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## Testing the Crystal: Determining True Resonance and Correcting Bridge Unbalance
One of the most misunderstood stages of the Hallicrafters SX-28 alignment is the crystal filter. It is incredibly common for restorers to sweep the IF strip and find a sharp peak at 460 kHz to 462 kHz, assuming the crystal has naturally drifted over the decades. In almost all cases, the crystal is perfectly healthy. It has simply been forced into its parallel anti-resonance point by a massive capacitive unbalance in the surrounding trimmers.
To properly align the IF strip, you must determine the crystal's exact native series resonance out of the circuit, verify the in-circuit pulling, and finally balance the phasing bridge to restore the crystal to its native frequency.
### Section 1: Out-of-Circuit Testing (The 100-Ohm Jig)
To find the true series resonance of the crystal, you must remove it from the high-impedance environment of the receiver. Placing the crystal in a low-impedance test jig swamps its internal parallel capacitance and allows the pure series resonance to shine through clearly. **(Receiver Power: OFF / Crystal Removed)**
1. **Build the Jig:** Procure two standard 100-ohm resistors (1/4 watt or 1/2 watt carbon or metal film). 2. **The Connections:** Connect the output of your RF signal generator to the first 100-ohm resistor. Connect the other end of that resistor to one pin of the crystal. Connect the second crystal pin to the second 100-ohm resistor. Connect the other end of the second resistor to the signal generator's ground. 3. **Attach the Oscilloscope:** Connect a 10X oscilloscope probe across the second 100-ohm resistor (probe tip between the crystal and the resistor, ground clip to the generator ground). 4. **Sweep the Frequency:** Set your signal generator to output a pure, unmodulated CW sine wave at a high output level (e.g., 1 to 5 Volts p-p). Begin sweeping the frequency slowly between 450 kHz and 460 kHz. 5. **Record the Peak:** You are looking for the frequency that produces the maximum peak-to-peak voltage on the scope screen. Because series resonance is the point of lowest impedance, the crystal will pass the maximum amount of signal at this exact point. Record this frequency down to the hertz (e.g., 453.621 kHz). This is your absolute IF center-line target.
### Section 2: In-Circuit Testing (Identifying the Pull)
Now that you know the true frequency, you can reinstall the crystal and test it passively within the receiver to see how much the surrounding circuitry is detuning (pulling) it. **(Receiver Power: OFF / Crystal Installed)**
1. **Inject the Signal:** Connect your RF signal generator to the grid of the V3 (6SA7) mixer tube. Ensure you use a small blocking capacitor (e.g., 0.01 uF) in series with the generator lead to protect your equipment from residual chassis DC. 2. **Probe the Output:** Connect your 10X oscilloscope probe to the grid of the V5 (6L7) first IF amplifier tube. Keep the scope ground lead extremely short and connected to the chassis near the tube base. 3. **Sweep the Frequency:** Once again, sweep your signal generator slowly from 450 kHz up through 465 kHz. 4. **Observe the Shift:** Note the frequency where the maximum voltage appears on the scope. If this peak occurs significantly higher than your out-of-circuit baseline (for example, peaking at 462 kHz instead of 453 kHz), the crystal is experiencing severe capacitive pulling.
### Section 3: Balancing the Bridge (Correcting the Masking)
The 9 kHz upward shift is caused by asymmetrical loading from the crystal gate trimmer array (C29, C30, C31) and the front-panel phasing control (C32). To drop the crystal back into its native series notch, we must re-balance this capacitive bridge. **(Receiver Power: ON)**
1. **Isolate the Local Oscillator:** To prevent the SX-28's native local oscillator from interfering with the alignment, short Grid 1 (Pin 5) of the V4 (6SA7) oscillator tube directly to the chassis. This cleanly kills the oscillation without altering the DC current rails. 2. **Inject the True Baseline Frequency:** Set your RF signal generator exactly to the out-of-circuit series resonant frequency you recorded in Section 1 (e.g., 453.621 kHz). Inject this unmodulated CW carrier into the V3 mixer grid. 3. **Monitor the Output:** You can monitor the alignment by connecting a digital multimeter (set to DC Volts) to the AVC line, or by using an RF demodulator probe at the detector stage. You are tuning for maximum negative DC AVC voltage or maximum demodulated signal. 4. **Center the Phasing Control:** Set the front panel Crystal Phasing control (C32) to its physical top-dead-center position. 5. **Adjust the Trimmer Array:** Slowly adjust trimmers C29, C30, and C31. These trimmers form a voltage divider bridge that neutralizes the stray capacitance of the crystal holder. Your goal is to maximize the signal throughput at this exact frequency. As you tune these trimmers, you are eliminating the capacitive asymmetry and forcing the circuit to accept the crystal's native series resonance as the true IF center point. 6. **Final IF Unification:** With the bridge balanced and the crystal happily passing the signal, proceed to peak all downstream IF transformers (T2, T3, etc.) to this exact frequency.
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When you implement this on the site, do you plan to include photographs of the 100-ohm jig on the bench, or will this section rely purely on the text descriptions?[an error occurred while processing this directive]