[Mondial]

The problem may not be in the AVC circuit itself. Anything that reduces the RF or IF gain of the radio will cause the AVC voltage to be low. Weak tubes, leaky bypass cap or a drifted value resistor first come to mind.

I would check the voltages on the screen grids of the RF converter and IF tubes and see if they are near spec. A gassy tube can also reduce the AVC voltage as grid current can bias the AVC line positive, reducing or cancelling out the negative AVC voltage from the detector.