AVC Test - AI Part 1

Prompt: If the AVC line is to be used as a calibration and alignment device, shouldn't there be a preliminary step or procedure to guarantee that it is working properly?

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You are absolutely right. Trusting an unverified AVC (Automatic Volume Control) line as your sole measurement reference is flying blind. If there is a leaky paper capacitor dragging the line down, or a weak tube in the AVC chain, you will be chasing phantom peaks and tearing your hair out.

This is especially critical on the SX-28 because it does not use a simple, passive diode detector for its AVC. It features an **amplified AVC system** utilizing a dedicated 6AB7 AVC Amplifier (V8) and a 6H6 AVC Rectifier (V9).

Before you touch another trimmer, here is the definitive, two-part bench test to guarantee your AVC line is healthy, responsive, and trustworthy.

The Cold Resistance Test (Power OFF)

The single most common failure on a vintage receiver's AVC line is a leaky bypass capacitor shunting the control voltage to ground. The AVC bus is a high-impedance circuit; it should look almost like an open circuit to DC ground.

  1. Unplug the receiver and discharge the filter capacitors.
  2. Connect your DMM (set to Ohms) between the chassis ground and the main AVC bus. (A great place to probe this is at the "bottom" of the grid-leak resistor for the 1st IF or 2nd RF tube, or directly at the AVC switch contacts).
  3. Read the resistance. You should see a minimum of 2 to 3 Megohms (the sum of the isolation resistors in the network).
  4. If you read anything under 1 Megohm ‐ or worse, just a few thousand ohms ‐ you have a leaky bypass capacitor on the AVC line. If the line is dragged down to ground, it will never develop the negative voltage needed for alignment, no matter how much signal you pump into it.

The Dynamic Swing Test (Power ON)

If the cold resistance looks good, power the receiver up to verify that the V8 and V9 tubes are actively rectifying the RF into negative DC, and that the RC timing network is holding the charge.

  1. **Set the Baseline:** Turn the SX-28 **ON**. Set the front-panel AVC switch to **ON**. Disconnect your signal generator and short the A1/A2 antenna terminals to the chassis to ensure total silence.
  2. **Read the Floor:** Connect your DMM (set to DC Volts) to the AVC line. With no signal, the line should sit very close to 0 V (you might see a tiny contact potential of -0.5 V to -1 V generated by the tube electron clouds, which is normal).
  3. **Inject a Massive Signal:** Connect your RF signal generator to the antenna terminals. Set it to a frequency you know is working well (e.g., your newly aligned 1400 kHz on Band 1) and crank the generator's RF output level high.
  4. **Read the Swing:** Watch the DMM. The AVC voltage should immediately dive deeply negative—often hitting **-10 V to -20 V** or more, depending on your generator's output power.
  5. **Check the Time Constant:** Suddenly disconnect the signal generator from the antenna terminals. The DMM should not snap instantly back to zero. It should smoothly "bleed off" back to 0 V over about a quarter to a half of a second.

The Verification Verdict

* If the line drops to -15 V with a strong signal and bleeds off smoothly when the signal is removed, your AVC network is in perfect health.
It is fully certified to act as your alignment calibration device. * If the voltage barely moves (e.g., only drops to -2 V under heavy signal), V8 or V9 is weak, the AVC IF transformer (T4) is misaligned, or a resistor in the voltage divider network has drifted high.

If this checks out, you are totally clear to use the "Starve the Receiver" method to align the RF and Mixer stages.

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Last modified: Monday, 20 Jul 2026