This is our main character today, a multimeter that traveled across the ocean with the Flying Tigers to the front line of the anti-fascist war.
Range switch knob. All equipment back then used vacuum tubes, and some tubes required as high as 1500V, hence there is a 1500V voltage range.
Text below the range knob: signal corps U.S. Army, U.S. Army Signal Corps Test Set 1-77-J, Model Serial No.1352 Order No.30126-Phila-43, with serial and order numbers. The ‘Phila’ in the order number indicates the Pacific Theater.
Produced by Chicago Industrial Instrument CO. Chicago.
Remove the screws from the bottom panel to take off the back cover. The back cover is insulated with a layer of green shell paper.
Left Side
Right Side
Carbon solid resistors, used in pairs in series. The range switch band switch will not be disassembled as it requires soldering and is difficult to restore without damage. After all, this thing is an antique.
The wire-wound resistors for current and resistance ranges, three in total, with Dupont paper in between.
Meter Head
Selenium rectifier. Back then, there were no silicon-based semiconductors, and rectifiers were made of stacked selenium plates. This selenium stack needs adjustment; different clamping forces will change the conduction threshold, dynamic resistance, reverse avalanche voltage, etc. Once adjusted by the manufacturer, it generally should not be moved.
Finally, let’s have a shocking comparison. When connecting to a 10V voltage reference, the meter shows 9.5V (half a small division), with an error of only 5%! For most vacuum tube equipment, the error requirement is within 5%.
For a 70-year-old antique that can still be used today, and considering that the error for a multimeter is not significant, it’s hard to find the right words to describe it!
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