If you ever encounter a British engineer of a certain age, the chances are that even if they use a modern DMM they’ll have a big boxy multimeter in their possession. This is the famous Avo 8, in its day the analogue multimeter to have. Of course it wasn’t the only AVO product, and [Thomas Scherrer OZ2CPU] is here with another black box sporting an AVO logo. This one’s an AC bridge, one of a series of models manufactured from the 1930s through to the late 1940s, and he treats us to a teardown and restoration of it.
Most readers will probably be familiar with the operation of a DC Wheatstone Bridge in which two resistances can be compared, and an AC bridge is the same idea but using an AC source. A component under test is attached to one set of terminals while one with a known value is put on the other, and the device can then be adjusted for a minimum reading on its meter to achieve a state of balance. The amount by which it is adjusted can then be used as a measure of the difference between the two parts, and thus the value of an unknown part can be deduced.
In the case of this AVO the AC is the 50Hz (remembering that this is a British instrument) mains frequency, and the reading from the bridge is taken via a single tube amplifier to a rectifier circuit and the meter. Inside it’s a treasure trove of vintage parts with an electrolytic capacitor that looks as though it might not be original, with a selenium rectifier and a copper ..
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