This TM3A is an eBay find. These vintage micro-volt meters are sometimes very affordable. The question is are they any good? This video tries to find out.
UPDATE (many thanks to my Patreon Pete for finding this):
A slightly different (newer?) version of the manual clarifies the Rin/Cin procedure by using a "screened input box" to place a series impedance into the input path. The connection from input box to meter has to be 15cm or less. The impedance consist of a 100K resistor in parallel with a 1nF cap. The procedure is to first inject a 200Hz signal into the meter set to 15uV and adjust it to read 500 on the 0--500 scale, then insert the input box with the series impedance into the path and adjust Rin to give a reading of 480. Cin is adjusted the same way but at a frequency of 20Khz
The "dun-dun-duuun" sound comes from here: freesound.org/people/copyc4t/sounds/146434/ author copyc4t
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0:00 First look
1:10 First test
4:36 Inside
5:25 Cleanup
6:58 Battery
7:31 Schematic
9:22 Calibration
15:45 All ranges test
18:12 Bandwidth
18:55 Conclusion