Oscillator Feedback Adjustment (C7 - C12)

While the Gemini response for the theory section was quite remarkable, the adjust part didn't fare so well. See the source here. It's a sequence of questions and responses that eventually lead to making what follows below. Each page has a link to the next at the bottom.

The goal of this procedure is to ensure the Local Oscillator (V4, 6SA7) provides a uniform output amplitude across the entire tuning range of each band. We are not tuning for a specific maximum voltage; rather, we are tuning for flatness. We want the difference in amplitude between the low end of the dial and the high end of the dial to be as small as possible.

Equipment and Radio Setup

• Power off. Radio upside down.
• Connect the oscilloscope probe to the junction of R65 and C68. Connect the probe's ground clip to the chassis using the shortest possible lead to prevent AC ground loops.
• Set the oscilloscope to AC Coupling. You will be measuring differences in p-p amplitude.

The Alignment Procedure

1. Power on. Allow enough time for the receiver to reach thermal stability.
2. Set the Band Switch to Band 1.
3. Tune the main dial to the extreme low-frequency end of the band. Observe the peak-to-peak height of the sine wave on the oscilloscope.
4. Tune the main dial to the extreme high-frequency end of the band. Observe the peak-to-peak height of the sine wave again. You will likely notice a significant difference in amplitude.
5. Adjust the Band 1 Feedback Trimmer (C12). Your objective is to minimize the difference in the wave's amplitude between the high end and the low end. Adjust the trimmer, sweep the dial back and forth, and find the "sweet spot" where the wave height remains as constant as possible across the sweep.
6. Repeat this exact process for the next four bands using their corresponding trimmers:
   • Band 2: Adjust C11
   • Band 3: Adjust C10
   • Band 4: Adjust C9
   • Band 5: Adjust C8
   • Band 6: Adjust C7
   Note: As you move to higher bands, the overall peak-to-peak voltage will naturally decrease due to lower tank circuit impedance. This is normal. Focus only on the flatness within that specific band.

Page: /Alignment/Osc/Adjust/Early_osc_feedback_adjust.shtml

Last modified: Friday, 24 Jul 2026