Vintage Marconi Instruments Valve Voltmeter TF 428B – Missing Tube

The tube pictured behind the power transformer inside has since broke– so the unit is being sold as-is for parts. The mains power bulb is burnt out, but the transformer can be heard humming.

I have tried applying a small DC voltage to the input terminals but the meter did not move. It is being sold for parts/not working. AS-IS!