Re: AVO Model 7 Restoration
None of the "Traditional" Avometers, except the Model 7, needs a battery for the voltage and current ranges. The AC ranges of the Model 7 are an exception as the 1.5V cell provides a slight bias to the rectifier. See page 611 of the attachment which is an extract from "The Services Textbook of Radio, Volume 1". AC ranges will however work without the cell, but the low end readings will be inaccurate.
Since none of the AC ranges works, the most likely faults are a defective rectifier or the contacts of the cam operated switch being out of adjustment. Four germanium diodes, OA91 or OA96, for example will satisfactorily replace the original copper oxide instrument rectifier. The braided leads of the copper oxide rectifier should be considered integral to it. It is best not to unsolder them at the rectifier end as excessive heat can affect to copper oxide elements.
PMM
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