Oscillator Feedback Adjustment (C7 - C12)

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. Place the radio on its side, keeping the heavy power transformer side down for a stable center of gravity.
• Test Equipment: A modern Oscilloscope with a 10X passive probe (e.g., 200 MHz TL803b). Ensure your scope channel is set to 10X attenuation so the voltage readings are scaled correctly.
  • Note: You do not need the vintage 1-megohm isolation resistor at the probe tip. A modern 10X probe naturally provides a 10-megohm input impedance with ultra-low capacitance (10–15 pF), which safely mimics the old VTVM isolation.
• Connect the oscilloscope probe to the oscillator grid (Pin 5 of the V4 6SA7 tube) or the ungrounded side of the oscillator tank circuit. Connect the probe's ground clip to the chassis using the shortest possible lead to prevent AC ground loops.
• Set the oscilloscope to DC Coupling. You will be looking at a large AC sine wave riding on a deep negative DC baseline (often -50VDC to -70VDC).

The Alignment Procedure (Bands 1 through 5)

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
   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.

The Band 6 Probe Trap

Do not attempt to use the oscilloscope probe directly on the oscillator grid for Band 6 (21 to 43 MHz).

At these extreme frequencies, the 10 to 15 pF of parasitic capacitance introduced by your 10X scope probe acts like an AC short circuit. Touching the probe to the grid will instantly drag the Q of the tank circuit down and kill the oscillation entirely (or cause it to start and stop erratically).

To adjust C7 (Band 6 Feedback): Remove the scope probe entirely. Instead, use a Digital Multimeter (DMM) to monitor the isolated DC return line (the AVC/bias bus for the oscillator). Because the receiver's internal resistors isolate this point from the raw RF tank, the DMM leads will not detune the circuit. Sweep the dial from the low end to the high end, and adjust C7 so the negative DC bias voltage remains as flat as possible.

Comments

This was exceptionally good up until it got to "the isolated DC return line (the AVC/bias bus for the oscillator)." See the next exchange.

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Last modified: Monday, 27 Jul 2026