AVC Test and Repair

The page is heavily influenced by the Gemini AI response here.

If the AVC line is to be used as a calibration and alignment device, there needs to be a procedure to guarantee that it is working properly.

AVC Test

Equipment Setup

• Power off. Radio upside down. Discharge the filter capacitors if you think necessary
• Front panel AVC switch on
• Band switch to band 1. Tune to 1400 KHz.
• Connect an RF signal generator to the antenna terminals. Frequency 1400 KHz. Modulation 1 KHz at 80%¹. If your signal generator has an on‐off button for the output (not the power switch), you can use that in the following steps. Start with it off.
• The 1st and 2nd RF modules each have a single wire exiting the modules on the side away from the power transformers. These two wires go to a common terminal strip lug but the location of that lug varies. In any case, the junction is the AVC line. Connect negative lead of multimeter to this point and the positive lead to ground. You'll use the same connection below

The Cold Resistance Test

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. Measure the resistance to ground
2. You should see a minimum of 2 to 3 Megohms (the sum of the isolation resistors in the network)
3. 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

If the cold resistance looks good, the next step is to 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.

Important: Switch multimeter from ohms to DC volts. 50 V range should be safe.

1. Power on
2. With no signal generator output, 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. Crank the generator's RF output level to high. I used 5 V p-p which worked well
4. Turn the enable output button
5. Watch the multimeter. The AVC voltage should immediately dive deeply negative ‐ often hitting -10 V to -20 V or more, depending on your generator's output power.
6. Turn off the enable output button. The voltage 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 second².

The Verdict

If the line drops to around -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.

AVC Repair

The part of the page is heavily influenced by the Gemini AI responses here, here, here, and here.

Footnotes:

1. Modulation is optional. If it's on, you should hear the signal in the speakers. It's not part of the test
2. I was using a Simpson 260 which has a damped meter needle so the timing bit didn't work for me

Page: /Avc_test_and_repair/Avc_test_original.shtml

Last modified: Tuesday, 21 Jul 2026