Test the 455 KHz Crystal

This procedure is derived from a Google Gemini conversation that included quite a few prior questions and answers about the SX-28 crystal.

You can see a sumation response here.

Additional1 question/answer sequences were used to clarify some issues.

Remove the crystal from the radio

Power off, radio upside down.

You may find it useful to take a few cell phone pictures first.

It may be possible to avoid removing the crystal by completely disconnecting all wiring from one terminal but this method was not tried and it's not clear what effect having the entire radio connected to the other terminal might have.

The crystal is crimped and soldered to a two terminal strip. Remove the nut holding the terminal strip and unsolder the wires from the terminal strip. Don't disturb the crimps. It's a lot less bother to remove the terminal strip with the crystal attached than to mess with the crimps.

Measure the crystal on the bench2

  1. Procure two standard 100 Ω resistors (¼ watt or ½ watt carbon or metal film)
  2. Connect the output of your RF signal generator to the first 100 Ω resistor. Connect the other end of that resistor to one pin of the crystal. Connect the second crystal pin to the second 100 Ω resistor. Connect the other end of the second resistor to the signal generator's ground.
  3. Connect a 10X oscilloscope probe across the second 100 Ω resistor (probe tip between the crystal and the resistor, ground clip to the generator ground).
  4. 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. Be aware that it is possible to sweep too fast or with too large a step and therefore miss the output you are looking for.
  6. 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. Record3 this frequency down to the hertz. This is your absolute IF center-line target.

Reinstall the crystal.

In-Circuit Testing (Identifying the Pull)4

Power off, radio upside down.

If the radio was recently turned on, grounding the points mentioned below to remove residual DC is a good idea.

  1. Set the Selectivity Switch to XTAL BROAD
  2. Set signal generator amplitude to 10 V p-p.
  3. Connect your RF signal generator to the plate of the 1st IF tube (V5, 6L7, pin 3). V5 is located between T1 and T2. The key points in the direction of the oscillator module.
  4. Connect your 10X oscilloscope probe to the grid of the 2nd IF tube (V6, 6SK7, pin 4). V6 is immediatly behind the bandspread flywheel. The key points in the same direction as V5 above. Keep the scope ground lead extremely short and connected to the chassis near the tube base.
  5. As before, be aware that it is very possible to sweep too fast or with too large a step and therefore miss the output you are looking for.
  6. Sweep your signal generator slowly from 450 KHz up through 465 KHz. Try increments of 10 KHz. If you see a change with one step, set increment to 1 KHz and try varying the frequency.
  7. Keep decrementing the step frequency until you are moving my single Hz.
  8. Note the frequency where the maximum voltage appears on the scope3. 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.

Balancing the Bridge (Correcting the Masking)6

This procedure is only required if the last step of the in ‐ circuit test indicated a problem.

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.

Power off to begin, radio right side up.

Review Crystal Phasing Knob Adjustment if there is a possibility the radio's knobs are incorrectly installed

Method A ‐ Right Side Up, Measure with Speaker
  1. To prevent the SX-28's native local oscillator from interfering with the alignment: Remove V4 and place on the tube extender. Set switch to 5. Connect test point to ground with a clip lead.
    This cleanly kills the oscillation without altering the DC current rails by removing the tube.
  2. Set RF signal generator exactly to the out-of-circuit series resonant frequency recorded in Section 1 (e.g., 453.621 kHz).
  3. Set signal generator modulation to 1 KHz.
  4. Connect signal generator output to the 1st IF tube mixer grid (V5, 6L7, top-cap).
  5. Monitor the alignment by listening to the speaker
  6. Set the front panel Crystal Phasing control (C32) to the zero (center) position.
    7. Power on. Allow time to warm up properly
  7. 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
A ‐ On Side, Measure With Scope
  1. To prevent the SX-28's native local oscillator from interfering with the alignment: Remove V4 and place on the tube extender. Set switch to 5. Connect test point to ground with a clip lead. short Grid 1 of the oscillator tube (V4, 6SA7, pin 5) directly to the chassis. This cleanly kills the oscillation without altering the DC current rails by removing the tube.
  2. 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 1st IF tube (V5, 6L7, top-cap) mixer grid.
  3. 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. Set the front panel Crystal Phasing control (C32) to its physical top-dead-center position.
  5. Rotate the radio on to it's side. Transformer‐side‐ down lowers the center of gravity and makes it less likely to tip over and also puts the three capacitor adjustments in an easy-to-reach position.
    6. Power on. Allow time to warm up properly
  6. 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

Footnotes:

  1. After creating this page and uploading to the website, I asked Gemini to judge what I wrote against what it wrote. You can see the response here. I guess it likes me.
  2. This procedure was sucessfully performed on 12 Jul 2026
  3. During testing for the development of this procedure, my crystal measured 453.621 KHz and was used in some of the AI prompts. You will see this number show up in AI generated text as a result.
  4. This procedure was sucessfully performed on 14 Jul 2026
  5. In my #7 radio, the one I was using to develop this procedure, the in‐circuit frequency more or less matched my bench frequency. The pass band was wider by a few Hz.
  6. The test radio did not need to have this step performed so the test was postponed because getting the procedure for aligning the IF stages seems more important.

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Last modified: Monday, 20 Jul 2026